MSc Thesis, Building Technology track, TU Delft by Ioanna Christia

S.T.R.A.W.S Stackable Tubes foR Absorption in Work Spaces

Deployable sound-absorbing unit for open-plan offices

Ioanna Christia 4500075

Main mentor: Dr. ir. Marcel Bilow Second mentor: Foteini Setaki Third mentor: Dr. ir. Martin Tenpierik

TU Delft University of Technology, Department of Architecture, Urbanism and Building Sciences, Track of Building Technology July 20017

methodology

acknowledgments

I would like to sincerely thank my tutors, Marcel Bilow, Martin Tenpierik and Foteini Setaki for their excellent assistance and their willingness to help me during the whole research procedure. They involved actively in every part of the thesis and provided me plenty of knowledge not only for the scientific part but also for the more creative part of this project. I would also wish to thank all of my friends who helped me with creative discussions and especially those who contributed systematically and tirelessly for building a big model in 1:1 scale. Without them the model wouldnâ&#x20AC;&#x2122;t exist now. Last but not least I would like to thank my family for the support they provide me all those years of studying and giving me the opportunity to follow this master. So a deep thank you to all those people who involved with one way or another to the accomplishment of this thesis.

table of contents

abstract methodology problem statement research question research objectives design approach

p.06 p.07 p.08 p.08 p.09

configurations technical drawings details evaluation 3.9 design evaluation 3.10 acoustic evaluation

literature review

1. acoustics 1.1. transmision principles 1.2. absorption principle 1.3. absorbers 1.4. open-plan acoustics 1.5. geometry in resonance absorption 1.6. absorbersâ&#x20AC;&#x2122; measurements 1.7. absorption cross-section 2. product design 2.1. design pronciples 2.2 widely used acoustic solutions 2.3. influence of acoustics in manufacturing 2.4. study cases 2.5. materials 2.6. design and materials 2.7. manufacturing processes 2.8. comparison of processes 3. analysis of Helix panel design analysis 3.1. introduction 3.2. design 3.3. acoustic principle 3.4. assembly 3.5. materialisation

3.6. 3.7. 3.8.

p.15 p.16 p.17 p.19 p.20 p.24 p.28 p.31 p.32 p.33 p.34 p.36 p.37 p.38 p.42 p.45 p.46 p.47 p.48 p.49

p.50 p.51 p.52 p.53 p.54

design

acoustics 4.1 introduction 4.2 design procedure 4.3 base material selection 4.4 material tests 4.5 geometry tests 4.6 performance parameters 4.7 radius comparison 4.8 amount of resonators 4.9 cross-section absorption 4.10 broadband absorption 4.11 tube arrangements 4.12 scaled measurements 4.13 sealing measurements 4.14 influence of space between the tubes 4.15 conclusions 5.2. product 5.1 introduction 5.2 design influence 5.3 customizability 5.4 customized absorption 5.5 performance guidelines 5.6 big-scale measurement 5.7 comparison of products

p.73 p.75 p.76 p.77 p.79 p.81 p.82 p.83 p.84 p.85 p.87 p.89 p.90 p.93 p.94 p.97 p.98 p.100 p.101 p.103 p.104 p.108

5.8 comparison with Helix 5.9 assembly 5.10 manufacturing techniques 5.11 improvement of design process 5.12 transportation 5.13 technical drawings 5.14 spatial configurations 5.15 design process 5.16 1:1 scale model 5.17 conclusions

6. 7.

literature appendixes appendix A appendix B appendix C appendix D appendix E

p.109 p.110 p.112 p.113 p.114 p.115 p.117 p.118 p.119 p.121 p.122 p.126 p.128 p.132 p.133 p.134

abstract In this industrialized world, technology has invaded our life in such a level where behind every product a complex manufacturing process with high transportation costs being hidden. Designing a product itself is not enough anymore; the whole manufacturing procedure needs to be included in the design. Especially when product design meets acoustic engineering, the design should not only fulfil the acoustic and aesthetic qualities but also take into consideration the cost factor. This is translated into the wise use of material, transportation, easy assembly and maintenance. At the same time, the target group is every single person that is called to participate in the assembly procedure, with no expertise knowledge, which adds extra challenge in the design procedure. Less is more, as Mies van der Rohe stated. This research taking a step back and revaluates the product design principle for acoustic panels. Having as starting point the HELIX panel designed by Panayiotis Hadjisergis and Ioanna Christia, during Bucky Lab course of 2015 fall, the target is to keep the advantages of this panel while improving the acoustic performance and keeping the manufacturing process as simple as possible. The target of Helix panel was to provide a flexible solution regarding acoustics and assembly. The advantages of this project were further developed into the STRAWS project. The acoustic principle used is quarter wavelength tubes and combined to a cheap ordinary product, as straws, a broadband sound absorbing unitized panel is designed. Until now there are lots of products that are made for acoustic but none of them fullfil so many different design needs. This research proposes an already existing product which by implementing acoustic principles, can be applied in open-plan officess. In this way the acoustic panel will not only be suitable for a wide range of frequencies but also designed in a way that uses little amount of material, without taking up a big space and being able to be re-used in a different environment.

Since people are spending almost 90% of their time indoors, providing comfort in all levels is crucial for their well-being. Acoustic comfort specifically is absolutely essential in maintaining a high level satisfaction, moral health, better performance and productivity of users (accessed in 2017, retrieved from: http://www.cetec.com.au/services/ office-noise.html). The acoustical environment of a space is typically given little or no attention during project planning and design mainly because functionality and aesthetics are usually the primary focus of the designer. So many existing spaces lack of acoustic treatment. The sound field is highly influenced by the size of the room and the materials used on the surfaces. Especially openplan layouts, even though provide efficiency in terms of spatial configuration (Duffy 1992 in Kim and de Dear 2013) and productivity by improving the communication between employees (Brand and Smith 2005 in Kim and de Dear 2013), are widely recognized to be more disruptive (Kim and de Dear 2013). Open plan spaces tend to have a relatively long reverberation time, resulting in uncomfortable and noisy spaces. The main acoustic source of disturbance in an open-plan area is usually speech. This implies that preventing sound propagation over long distances is important. For decreasing sound levels in a room means usually the use of an acoustic product. The materials used can have different properties. One common solution is the use of porous materials. The lower the targeted frequencies, the thicker the materials need to be. The result is that a big amount of material is needed for treating low/ medium frequencies, which increases the initial cost, the manufacturing costs and transportation costs

of the product. At the same time more material means more occupied space, whether this is for storage or in the room. Especially in open-plan offices that are designed to provide the maximum useful area, too much space used for acoustics means the increase of the areaâ&#x20AC;&#x2122;s value per m2 (accessed in 2017, http://www.workspacedesign. co.uk/the-benefits-of-an-open-plan-office/ ). In terms of air quality also, porous materials tend to trap dust on their surface, which without beeing cleaned often, decreases the air quality of the room. Resonator absorbers on the other hand, make use of enclosed air volumes or vibrating surfaces. There are different types of absorbing resonators like helmholtz, quarter wavelength tubes or micro-perforated plates. Some of them are based on the acoustic principle of the massspring principle where the sheet material, a membrane or an air plug acts like the mass and the air cavity acts like a spring. Others though, like quarter-wavelength tubes and microperforated panels, dissipate energy due to thermalviscous losses, which resemble to the behaviour of porous materials. According to each resonance principle, different type of geometry influences the absorption as well as the needed amount of surface (Cox & Dâ&#x20AC;&#x2122;Antonio, 2002). In order to absorb low and medium range frequencies, a bigger air cavity is needed. This means that the occupied space increases as the targeted frequencies decrease in value, and the already mentioned problems arise once more. At the same time resonators achieve their maximum efficiency within a limited frequency range. The contemporary way of living includes often changes of environment. Especially in bussiness sector, changing offices is very likely to happen. This means that with the new space,

new demands come up. In terms of acoustics, most of the solutions preffered till now are acoustic products that need permanent mounting (glued carpets of fabrics), or fixed mounting on a wall or ceiling, have fixed dimensions, with no posibility of adjusting in new acoustic or spatial needs and usually come in big, massive pieces. All these characteristics are obstacles in any kind of change. So in case of environment change, some of the products can be re-used but with low efficiency and most of them are either thrown away or stored for a future use. In both cases the cost is high, since new products need to be bought. The old products, if thrown away, rarely are recycled due to the chemical alteration from mounting method, so it is a waste of material and money. In case they are stored, they take up too much space, that is also translated in a unworthy investment. In terms of production, acoustic products are usually made of combinations of many different parts which adds extra cost to the manufacturing and assembly procedure. Especially when the parts have big dimensions and weight, cost increases. Materials like wood and metal are widely used in acoustic productsâ&#x20AC;&#x2122; design, either for their acoustic performance or for building a support frame, that need the contribution of specialists during the transportation and assembly of the panels. As a result, the procedure takes a lot of time and the price of the panel increases. In a few words, acoustic products of the market lack of spaceefficient design for high acoustic absorption performance, targeting low-medium frequencies, combined to economic manufacturing, which provide flexible acoustic solutions.

methodology

_ PROBLEM STATEMENT

methodology

_RESEARCH QUESTION

â&#x20AC;˘ In which way does the geometry of a sound absorbing unit increase sound absorption in open-plan offices while decreasing manufacturing costs and responding in different acoustic needs?

_SUB-QUESTIONS Acoustics 1. What are the advantages of a geometry-based acoustic principle? (p.21) 2. How does the geometry of the voids affect the sound absorption? (p.18-20) 3. Which geometry-based sound absorbers do exist and how do geometric variables affect their sound absorption coefficient? (p.18-20) 4. How does the chosen acoustic principle affect the design process? (p.32) 5. How can broadband absorbers be created from narrowband units? (p.84)

Product design 6. What are the possible materials that can be used? (p.34) 7. How does the material and manufacturing technique affect the sound absorption? (p.74) 8. How much can the users intervene in the customaziability of the product? (p.94) 9. Which are the selling points of the new product? (p.98) 10. Which manufacturing process is suitable for mass production? (p.101) 11. Which are the limits of the chosen manufacturing process?

_BACKGROUND QUESTIONS

_RESEARCH OBJECTIVES

12. How can the absorption coefficient of acoustic panels be predicted? (p.23) 13. Which manufacturing process is suitable for prototyping? (p.101)

The main objective of this research is to examine the influence of the geometry design of a sound absorbing unit on the manufacturing process. Specifically, the design focuses on the production of two sound absorbing units that aim at frequencies between 200Hz â&#x20AC;&#x201C; 1000 Hz, that usually prevail in open-plan spaces. The study aims to apply an acoustic principle based on the geometry of the unit so as to offer better acoustic quality than the widely used absorbing panels by saving space and material. The maximum thickness of the units is intended to be at most 1/10 of the wavelength of the targeted frequency. Decreasing the cost of manufacturing is a main target of the design, so sheetformed materials will be used. However, the type of material and possible manufacturing techniques will be researched in terms of acoustic behaviour, assembly and cost. In the end, after simulations and small testing measurements, the prototype in scale 1:1 will be measured in order to examine how similar the results are to the measurements.

The goals of the design are to create an acoustic solution that complies with the following guidelines: lightness, lowcost, self- upported, low-medium frequencies, reusability, flexibility and aesthetic value. The starting point is the common project with Panayiotis Hadjisergis of Bucky Lab course during fall semester of 2015. Helix panel will be presented in terms of product design and acoustic design approach. measured in 1:1 scale. Measurements of the panel are also to be taken in an reflective room with appropriate dimension. Methodology steps are going to be followed based on ISO 354:2003 for measurement of sound absorption in a reverberation room. The boundary conditions of the room are to be presented as well as the equipment used. In case the results do not show the expected behaviour, there are three options: a) if there was a mistake in the installation of the panel or the equipment, measurements will be repeated in the same room b) if the room is inappropriate, measurements will be conducted in another room

The results will be used at the second part where the design concepts are defined. The target is to propose two units, each one aiming for a different range of frequencies. In order to result in the final two designs, a variety of design proposals, with different geometries, are going to be tested by simulations. The investigation of form by simulations will last two weeks unless a satisfactory combination of acoustic performance of at least absorption class C, according to EN ISO 11654, is achieved and d thickness of unit ≥ λ/10, at the same frequency (Absorption Classifications according to EN ISO 11654 can be found in Appendix A). When one of these limits is reached three concepts that performed the best are going to be translated in sample testings and measured in the impedance tube twice, with

different materials. The results are going to be compared with the simulations and the design that performed the best is going to be further analysed. At the same time the most promising design will be tested in 1:1 scale so as to improve the assembly procedure. The area of the sample will be defined from the rooms’ volume. The first choice was to use a reverberation chamber. Since though the time and budget were limited, a smaller model, with1.76m2 absorptive surface is made, and the measurements took place in a smaller space, PD Test Lab, at the faculty of Architecture in TU Delft. During the third part, the gathered results from the acoustic measurements and the assembly tests are going to be translated into the design of a prototype panel. The next step includes the construction of the prototype panel and acoustic measurements in 1:1 scale. The last part includes different spatial configurations based on different scenarios and the evaluation of the product. Finally, the whole process is be presented, explaining the outcomes and the potentials of this type of sound absorbing acoustic panel.

1.0

absorption class A

0.8

absorption class B 0.6

absorption class C

0.4

absorption class D 0.2

absorption class E 125

250

500

1000

Frequency (Hz)

2000

4000

Absorption Classifications according to EN ISO 11654

methodology

The design approach mainly consists of 4 parts: literature study, design proposal, prototyping and evaluation of the product. The first part of the thesis aims on investigating existing literature on acoustics and manufacturing techniques. The basic principles of acoustics are going to be presented as well as different types of absorbers and the influence of geometry in resonant absorption. Also different possibilities of measuring absorbers are included in the research. The next part includes investigation into possible materials that can be used for the design combined to suitable sheet manufacturing processes. In a few words this is a design by research project, which will incorporate the results from the literature study into a design proposal.

c) if there is no room available with the specific characteristics, then the measurements will not be based on the ISO standards but will be qualitave. This means that two measurements will take place, one with the panel and one with a reflective surface, within the same boundary conditions, so as to compare the results. More information on the methodology can be found in Helix’s measurements chapter.

Absorption coefficient

_DESIGN APPROACH

methodology DESIGN APPROACH THEORY literature study

simulation with matlab measuremment in the impedance tube evaluation

acoustic principle possible materials manufacturing

Helixâ&#x20AC;&#x2122;s measurements

NO range of materials and geometries

time left >2 weeks

YES

SCALE 1:1

NO absorption class <A

YES

maximum thickness â&#x2030;¤0.15m

methodology evaluation impedance tube

evaluation manufacturing

YES 3 concepts’ development

remaining time ≤2weeks

YES measurement ≤2 times

SMALLER SCALE

prototype and configurations

final concept

SCALE 1:1

October

methodology

November

December

January

acoustic principles

May

April

June

4 weeks

sheet manufacturing

4 weeks 3 weeks

existing case studies

MEASUREMENTS OF HELIX PANEL

March

20th

TOPIC CHOICE

LITTERATURE REVIEW

February

1st measurements

REPORT

2 weeks 1week

P2 MEASUREMENTS OF HELIX PANEL

2nd measurements design options simulations

DESIGN PROPOSAL

1st impendence tube samples 2nd impendence tube samples scale 1:1 testings

REPORT

2 weeks 2 weeks

1 week 1 week

1 week 1 week 1 week 1 week 2 weeks 1week

P3 PROTOTYPE PANEL

design improvement construction measurements

2 weeks 2 weeks 2 weeks 1 week

REPORT

P4 CONFIGURATIONS REPORT

spatial configurations presentation

2 weeks 2 weeks 2 weeks

acoustics

1.1 TRANSMISION PRINCIPLES

Fig.1: absorption principle

Sound is caused by a source emitting energy in the form of a vibration. Areas of high and low pressure move outwards creating a form of longitudinal wave (a wave which vibrates in the direction of travel). The amplitude and frequency of the sound wave depends on what the source is and the amount of energy supplied outwards (27). The sound that human ears receive is the result of the direct sound directly from the source combined with the indirect reflections from the surrounding surfaces and objects. The acoustic quality of a room is determined by the direct sound and the reflections from the walls, ceiling and floor. The aim of this thesis is to offer a solution for open plan spaces that provides acoustic and visual comfort to the users. The sound that strikes a surface can be transmitted, absorbed or reflected. The decay of sound after a sound source has stopped is called reverberation and is a key feature in room acoustics (Kuttruff, 2000). In large sound

Fig.2: reflection principle

production rooms (concert halls, theatres etc) specular reflection and diffuse reflection are the primary acoustic tools. On the other hand in sound reproduction rooms (recording studios, home theaters), absorption and diffuse reflection are more significant than specular reflection. In noise control cases (schools, factories etc) absorption is the main acoustic tool since the aim is to reduce reverberance and sound level. (Cox & D’Antonio, 2002) In order to provide acoustic quality in a room, absorbers reflectors and diffusers can be used. The percentage of each one’s use depends on whether reverberation and/ or sound level needs to be decreased. When an echo or coloration problem needs to be reduced but without decreasing the sound energy, diffusers are to be preferred. Concert halls are a good example of mainly use of diffusers. Places where intelligibility is important, like lecture theaters, a balance between diffusers and absorbers

Fig.3: diffusion principle

should be reached. Absorption as an only solution might provoke a quite silent and uncomfortable room, so usually a combination of absorbers and diffusers is used. Based on psychoacoustical research, in critical listening rooms, diffusers are preferred to be used on side/rear walls and ceiling whereas absorption is more effective on the front wall and in corner locations (Cox & D’Antonio, 2002). In order to treat low frequencies, both absorbers and diffusers need considerable depth. However, most of times the space provided for acoustics needs to be limited due to space constraints and cost. Resonant absorbers reduce sound level in a space efficient manner, especially when placed in high pressure corner locations. Since the target of the research is to decrease sound levels in open plan offices, the further analysis will include only absorbers.

acoustics

1.2 ABSORPTION PRINCIPLE

The emission of sound particles can be simulated with the bouncing balls on a billiard table. After an emitted sound, a graph of pressure versus time shows the response at a receiver position: the first peak is the direct sound that reaches the receiver and then a series of decaying reflections arrive. The reverberation time T60 measures the time needed for the sound pressure level to decay by 60 dB when a sound stops. Sabine showed that the reverberation time could be calculated from the room volume and absorption by :

T60= 55.3V/(cA)

(Eq. 1)

where: V is the room volume (m3) c the speed of sound (m/s) A the total absorption of all room surfaces (m2)

A=Sa

(Eq. 2)

x dB direct sound initial time delay

where: S is the total surface area of the room (m2) a is the average absorption coefficient of the room

sound pressure level (dB)

For very large spaces the sound absorption of the air influences the reverberation time. More specifically, A of eq.1 is replaced by A+4mV. The recommended reverberation time for open-plan offices should be 0.5 s (for furnished spaces) ((2017), accessed in 2017, retrieved from: http:// www.nvbv.org/handboek-bouwfysische-kwaliteit-gebouwen/)) The sound absorption properties of a material can be measured by its sound absorption coefficient. The sound absorption coefficient of a material has a value between 0 and 1, with 0 representing no absorption and total reflection, and 1 representing total absorption of all the incident sound. The sound absorption coefficient varies according to the frequency of sound. There are three basic categories of sound absorbers: porous materials commonly formed of matted or spun fibers; panel (membrane) absorbers having an impervious surface mounted over an airspace; and resonators created by holes or slots connected to an enclosed volume of trapped air. The efficiency of each type of sound absorber is highly influenced by the mounting method employed.

first reflection

early reflections

reverberant tail

x-60 dB

RT60 time (s)

Fig.4: reverberation time graph

acoustics

1.3 ABSORBERS

Fig.5: Porous material, retrieved from: www.123rf.com

Fig.6: Panel resonator, retrieved from:

Fig.7: Helmoltz resonator, retrieved from:

_POROUS ABSORBERS

_PANEL RESONATORS

_HELMHOLTZ RESONATORS

A porous absorbing material is a solid that contains cavities, or openings so that sound waves are able to enter through them and transformed into heat by friction. Porous absorbers are the most commonly used sound absorbing materials, including carpet, draperies, spray-applied cellulose, aerated plaster, fibrous mineral wool and glass fiber, open-cell foam, and felted or cast porous ceiling tile. Porosity of a material measures the void spaces in it, and is a fraction of the volume of voids over the total volume, with a value between 0 and 1. For conventional sound absorbing materials the volume porosity is usually 0.95 <Ω < 0.99 (Eerden F. 2000). Thickness plays an important role in sound absorption by porous materials. Thicker materials generally provide more low frequency absorption but porous absorbers are usually used for medium and high frequencies. Fabric applied directly to a hard, massive substrate such as plaster or gypsum board does not make an efficient sound absorber due to the very thin layer of fiber. Porous absorption is most effective when it is placed at a quarter wavelength from a room boundary, where the particle velocity is maximum.

Typically, panel absorbers are non-rigid, non-porous materials which are placed over an airspace that vibrates in a flexural mode. They are mass spring systems with damping to provide absorption at the resonant frequency of the system. The mass might come in the form of thin wood paneling over framing, lightweight impervious ceilings and floors, glazing and other large surfaces capable of resonating in response to sound. Their absorption is based on the geometry and structural vibration properties of the materials. Panel absorbers are usually most efficient at absorbing low frequencies.

In the case of a helmoltz resonator, the geometry is very important for the range of frequencies. The resonator consists of a volume of air which via an opening (neck) communicates with the outside air. When the opening is hit by a sound wave, a variable pressure is created at the entrance of a neck. The mass of the air in the neck is vibrating due to the excess pressure and the volume of air in the cavity. At a certain frequency of the sound wave, there occurs resonance on. Resonators typically are used for low-medium frequencies and are characterised by narrow frequency range (Cox & D’Antonio, 2002).

c f= 2π

p md

where: f= frequency (Hz) c= speed of sound (m/s) p= density of air (kg/m3) m= acoustic mass per unit area of the panel (kg/m3) d= cavity depth (m)

A VL

where: f= frequency (Hz) c= speed of sound (m/s) L= length of neck (m) V= volume of the void (m3) A= cross sectional area of the neck (m2)

acoustics Fig.8: Tube resonators, retrieved from:

Fig.9: Micro-perforated panels, retrieved from:

Fig.10: Sonic crystals, retrieved from:

_TUBE RESONATORS

_MICRO PERFORATED PANELS

_SONIC CRYSTALS

The resonance of a tube of air depends on the length of the tube, its shape, and whether it has closed or open ends. The tube resonates at many frequencies or nodes. Its lowest resonance (called its fundamental frequency) occurs at the same frequency as a closed tube of half its length. An open tube will resonate if there is a displacement antinode at each open end. These displacement antinodes are places where there is a maximum movement of air in and out of the ends of the tube. By convention a rigid cylinder that is open at both ends is referred to as an “open” cylinder, whereas a rigid cylinder that is open at one end and has a rigid surface at the other end is referred to as a “closed” cylinder. The absorption is realised by the decrease of energy based on thermoviscous losses inside the tube.

A Micro Perforated Panel (MPP) is a device made for absorbing sound. It consists of a thin flat plate, that can be made of several materials, with small holes punched in it. An MPP is normally 0.5 - 2 mm thick, with 0.5 to 2% of the plate’s surface is perforated, depending on the need and the characteristics of the environment, while the hole diameter is usually less than 1 millimeter, typically 0.05 to 0.5 mm. Sound absorption is achieved by converting energy into heat, the friction between the air in motion and the surface of the MPP dissipates the acoustical energy. This means that there is no need for porous materials between the perforated sheet and the reflective surface behind it. These absorbers can be more robust than porous absorbers as well as having a rather surprising ability not to get clogged up even in very dusty environments. In order to increase the low-medium frequency absorption, the device can be curved, tilted or shaped to provide redirection or diffusion at mid- and high frequencies. Double layers can also be used to broaden the absorption bandwidth. It is also possible to use transparent material and suspend the panels in front of glazing. The surfaces are transparent when looked at from straight on, but at oblique angles the holes become more apparent, although the surface is still translucent.

Sonic crystals are defined as periodic distributions of sound scatterers in a fluid or air background (Dowling,1992). They have been proposed as structures for attenuating and filtering sound waves because of their acoustic bandgaps The latter exhibit spectral gaps two orders of magnitude smaller than the wavelength of sound. The spectral gaps prevent the transmission of waves at prescribed frequencies. The frequency can be tuned to desired parameters by varying the size and geometry of the metamaterial. Kock and Harvey back in 1949, were later revisited and expanded by Cervera and coworkers. These authors developed an acoustic lens for airborne sound by using a cluster of rigid rods with external lenticular shape. The lensing behavior was understood to result from the effective properties of the cluster that, at low frequencies, behaves like a homogeneous fluid with some given effective mass density and bulk modulus. In fact, it has been demonstrated that sonic crystals, with hexagonal and square symmetries, behaves like isotropic fluids whose effective parameters simply depend on the lattice filling fraction (J. SánchezDehesa et al 2009 New J. Phys. 11 013039)

nodes

antinodes

1.4 OPEN-PLAN ACOUSTICS woman’s voice

20100 200 300 400 500

1000

2000

3000

4000

acoustics

man’s voice

5000

Fig.11: human voice frequency range

_CONCLUSIONS Since Le Corbusier’s definition of ‘‘plan libre’’ as one of the five points for a new architecture (Curtis 1986) the open-plan design principle has influenced a lot modern and contemporary architecture. ‘‘Plan libre’’ refers to an open plan with non load-bearing walls dividing the interior space. Open-plan layouts have been widely applied in the design of working and learning spaces of iconic buildings of the 20th century, as spatial solutions to the reformative programs in the structure of labor and education after the 70’s. Particularly in the case of office layout, the open-plan has been an innovative solution, efficient both in terms of spatial allocation, for allowing increased net usable area, higher occupancy density and flexibility of reconfiguration and beneficial to productivity by facilitating communication and interaction between coworkers. However, post occupancy evaluation studies of open-plan office spaces proved that, this layout is quite disruptive due to uncontrollable noise and loss of privacy. Noise, especially human speech, is one of the main sources of annoyance among the occupants of openplan offices (Hongisto et al., 2004, Beaman, 2005, Wang and Bowden, 2006, Helenius et al., 2007 and Rashid and Zimring, 2008).

The range of human voice varies from 150 Hz up to almost 4000 Hz. This is quite a wide range, including low, medium and high frequencies. In terms of space, providing more private spaces with better acoustic and visual comfort would help the employees to remain concentrated, without losing contact from their colleagues. At the same time, open plan spaces are so widely used as they provide maximum net usable area. So an acoustic solution shouldn’t be contradictory to this approach. Except for the acoustic performance, a space used from a big number of persons needs to provide good air quality. Materials that can capture dust are a constant source of air pollution, even for infections as maintenance is sometimes inefficient and there is a possibility of moisture development.

Each type of absorbers has different characteristics that makes them suitable for different acoustic approaches. As mentioned before, this reasearch focuses on designing an acoustic panel that absorbs low-medium frequencies by decreasing the costs of production. More precisely, the range of aimed frequencies varies between 200Hz - 1000 Hz, because the main source of noise (human voice) is in this range and the frequencies above 1000 Hz can be easily treated by porous absorbers. At the same time the physical attributes of the materials are a key factor. Having as a target a lightweight and lowcost unit means that many most of the presented absorbers are not going to be further investigated. More specifically, helmholtz resonators and quarter wavelength tubes will be presented more in depth as they fulfill the conditions of simple manufacturing and not necessarily stiff and heavy materials. Porous absorbers are only examined here in combination with a resonator. This combination could increase the frequency range, so it might be used.

acoustics

1.5 GEOMETRY IN RESONANCE ABSORPTION

_HELMHOLTZ RESONATORS Geometry plays an important role in the absorption efficiency of an acoustic panel. Especially for resonators that use their geometry to determine the peak frequency. The classic example of a resonator absorber is the Helmholtz resonator, which has the shape of a bottle. The resonant frequency is determined by the size of the opening, the length of the neck and the volume of the void. The sound eneregy is attenuated by the thermal-viscous losses in the tube. Resonant absorbers usually provide a narrow bandwidth of absorption, which means that in order to cover a wide bandwidth, a series of absorbers with different frequency-range target is required. It is possible though to increase the bandwidth of the resonator by adding a porous absorber behind the neck (or the neck itself).

c f= 2Ď&#x20AC;

A VL

(Eq. 3)

where: f= frequency (Hz) c= speed of sound (m/s) L= length of neck (m) V= volume of the void (m3) A= cross sectional area of the neck (m2)

By a quick analysis of the formula it is obvious that the smaller the volume and the cross-sectional area of the neck get, the lower the frequencies that are in the aiming range. On the other hand, for targeting low frequencies a big volume and a big neck length are demanded. Except the traditional form of a helmholtz resonator, researches have been realised on the shape of the neck and how it influences the absorption of the resonator.Both theoretical deduction and the experimental results indicate that an increase of the highest sound absorption coefficient attainable, of a Helmholtz resonator can be obtained by tapering the neck. The deeper the tapering length, the better the improvement of sound absorption will be (Tang, 2005). Moreover, substantial increase in the sound absorption of the resonator can be achieved by tapering the neck to a depth more than 50% of the untapered neck length.

Fig.12: helmholtz resonator geometry

acoustics Fig.13: quarter-wavelength tube section

_TUBE RESONATORS The working principle of sound absorption in tube resonators is based on the resonance of the air. The tubes have an open and a closed end and usually have a uniform shape allong their length. A sound wave entering one of these tubes will travel to the closed end of the tube and be reflected back to the pipe where it arrives opposite in phase to the oncoming wave in the pipe. This interference between the two waves results in attenuation. Tube resonators are also called quarter-wavelength resonators, since the maximum absorption takes place when a quarter, and odd multiples of a quarter, of the acoustic wavelength are equal to the resonator length. the optimal tube length can be determined easily from Eq. (4) for the frequency tuning.

As mentioned before, the resonator length determines the main frequencies at which sound is absorbed, and the resonator radius and the porosity of the panel determine the height and the width of the absorption peaks. Due to inlet effects the effective length of the resonator is larger than the actual length. Therefore an end correction is added to the geometrical length of the tube. This effect has been studied extensively for different configurations of the entrance of the tube. The end correction depends on the local geometry at the entrance and ending of the tube. The effective length Leff is the geometrical length L increased by a small increment d (Rayleigh 1945). The increment d for a single tube with the opening in an infinite baffle is equal to:

If the tube is open on both sides, the end correction is applied on both sides (Eerden F. 2000). For a perforated panel that is equally distributed by a distance the end correction for each side is: d= 8R (1-0.44R/ ) 3Ď&#x20AC;

(Eq. 6)

with: >2R

This means that Leff= L+d. The final form of the equation is: (2n-1) fres= 4Leff

(Eq. 4)

8R 3Ď&#x20AC;

(Eq. 5)

where: R is the radius of the tube (m).

fres=

(2n-1)c 8R 4(L+ 3Ď&#x20AC;

)

(Eq. 7)

NAtube Awall

(Eq. 8)

where: Awall is the total area of the wall (m2) N is the number of identical tubes Atube the cross-sectional area of every tube (m2)

The dimensions of the cross-section of a resonator determines amongst others the ratio of the viscous and inertial effects and thus affects the absorption coefficient. A comparison is made for single cylindrical tubes with different radii. The porosity and the length for the different tubes are tuned in such a way that the absorption coefficient is maximised at the same frequency. (Eerden F. 2000).

0.8

Fig.16: quarter-wavelength tubes tubes

0.6 0.4 0.2 0 1000

1200

1400

1600

1800

2000

Frequency (Hz) Fig.14: absorption diagram of quarter-wavelength tube with end correction, retrieved from:

Absorption coefficient

ÎŠ=

Absorption coefficient

acoustics The goal is to create a sound absorbing wall with an normalised impedance equal or close to 1. The impedance of the wall can be related to the impedance at the entrance of a single tube by assuming that the waves are plane at a short distance from the wall. (Eerden F. 2000). The surface perforation ratio of the wall is defined as:

Length=L Length=L+d

Fig.17: quarterwavelength tubes with different radii

radius=0.01m radius=0.005m radius=0.001m radius=0.0005m

0.8 0.6 0.4 0.2 0 1000

1200

1400

1600

Frequency (Hz) Fig.15: absorption diagram of quarter-wavelength tubes with different radii

Fig.18: configurations of single and coupled tubes

TUBE RESONATORS co/4L

POROUS ABSORPTION co/2L

3co/4L

0.8

60 mm

0.6

50 mm

0.4

0.2

40 mm

0 0

500

1000

1500

2000

2500

f (Hz)

1000

1500

2000

2500

f (Hz)

Fig.21: example of quarter and half-wavelength resonator absorption diagram

25 mm

0 0 100

160

250

400

630

1000

1600

2500

f (Hz)

Fig.22: example of porous absorption diagram

transparent or transluscent materials

volume for low-medium frequencies

accurate assembly

broadband range

low-medium frequencies

Fig.20: example of helmholtz resonator absorption diagram

0 500

acoustics

HELMHOLTZ RESONATOR

acoustics

CYLINDRICAL

TRIANGULAR

_CONCLUSIONS Both principles can be used for the design proposal. In terms of acoustics, both resonators have the disadvantage of absorbing sound in a small frequency band around the resonance frequencies. For broadband sound absorption, resonators with different lengths and radii can be combined though, as shown in Figure 10. A helmholtz resonator has higher damping capacity than quarter-wave resonator which means that requires less resonators for optimal damping (Chae H. Sohn, Ju H. Park, 2011). However, a single helmoltz resonator needs a large closed void for low-medium frequencies. This means that takes up bigger space than quarter-wavelength tubes. Especially if quarterwave-length tubes with different radii and geometrical configurations are combined, it is possible to provide the same absorption in less occupied space. At the same time the assembly procedure needs to be completely accurate, with no cuts or openings on the surfaces. Taking into consideration that people with no expert knoweledge will do the assembly, designing an

airtight void by interlocking connections and no glue or any kind of sealing used is almost impossible. On the other hand, prefabricated tubes can be easily found and provide the accuracy needed for acoustic performance. Considering all the above-mentioned factors, quarterwavelength tubes seem to be more promising in terms of research and design posibilities. At the same time having a tube-based acoustic principle can offer interesting combinations in terms of materials used as different materials for tubes and shell of the unit can be combined. Finally, there is a personal fascination in investigating quarter-wavelength tubes, so further investigation will be applied in the quarter-wavelength principle.

RECTANGULAR

LAYER

Fig.19: quarter-wavelength various configurations

acoustics

1.6 ABSORBERS’ MEASUREMENTS

IMPEDANCE / STANDING WAVE TUBE Using the standing wave tube it is possible to measure both normal incidence absorption coefficient and surface impedance. This method provides well defined and controlled conditions. This method has the advantage of only needing small samples (a few centimeters in diameter) and a relatively simple device. The results are quite reliable there is no need of a special chamber. The drawback of this method is when the absorption from the small sample is not representative of the behaviour of a large sample, as would happen with some resonant absorbers. For this reason, the method is most used with porous absorbers. A loudspeaker generates plane wave propagation in the impedance tube and the plane wave propagates down the tube before reflecting from the sample. A standing wave is set up within the tube. The impedance of the sample alters how sound is reflected and, by measuring the resulting standing wave, it is possible to calculate the normal incidence absorption coefficient and surface impedance of the sample. This is such a common technique in acoustics that it has been included in international standards (ISO 10534 part 1-2). The necessity for plane wave propagation imposes many limitations. The most important ones are:

• The highest frequency, fu, that can be measured in a tube is then determined by

fu=c/2d

(Eq. 9)

where: d is the tube diameter or maximum width (m) c the speed of sound (m/s)

mic 1 loudspeaker

Fig.23: impedance tube

mic 2

helmholtz panel

movable rear panel

porous layer

This limitation means that to cover a wide frequency range, several different impedance tubes of different diameter or width are required.

• The experimental detail that is most critical is the

•

requirement for the sample to be cut and mounted correctly. It is vital that the sample fits precisely into the tube. Any gaps around the edge must be filled and sealed, otherwise the gaps will allow absorption by the edge of the sample and the measured absorption will be too high. It is also important that the rear of the absorber is properly terminated. Air gaps between the absorber and the backing plate will lead to excess absorption being measured, unless of course it is planned to mount the absorber with an air gap, in which case the measurement would be correct. The impedance tube is not often preferred for extended reaction absorbers, unless the impedance tube happens to coincidentally be the same size as the extended reaction device.

STANDING WAVE METHOD This method measures the reflection coefficient by determining the ratio between maximum and minimuM pressures. These pressures are measured by a probe microphone on a moving trolley. However, the method provides quite limited results since only one frequency at a time is possible to be measured.

TRANSFER FUNCTION METHOD In this method the pressure at two points of the tube is measured, by using two microphones in the impedance tube. It allows the measurement of all frequencies between a defined range as well as calculate the reflection, impedance and absorption coefficient. The sample should fit exactly at the edge of the tube. Any gaps could alterate the measurements.

acoustics TWO MICROPHONES FREE FIELD METHOD The two-microphone free field method allows what impedance tube is unable to measure: oblique incidence measurement. However, the test method needs a large sample and an anechoic or hemi-anechoic space for the measurement. Both requirements make it quite difficult as a method. It is of most use for measuring porous absorbers. The method can be thought of as an extension of the transfer function, impedance tube method. The technique is most straightforward for homogeneous, isotropic materials. A large sample of absorbent is irradiated by a loudspeaker a long way from the surface. The measurement can be done in an anechoic or hemi-anechoic chamber. It is assumed that plane waves are incident on the surface. Furthermore, for large isotropic, homogeneous samples, it can be assumed that the reflected sound is also a plane wave. Some practical details need consideration like the diffraction from the edges at low frequencies cause the reflected

wave to no longer be planar, and so the simple theories no longer apply. A rough lower frequency limit is when half a wavelength fits across the smallest sample dimension. Consequently, samples are typically several square metres in area. When large samples are not available, one solution is to bring the source close to the surface, so the edge waves become less significant. For non-isotropic or non-planar surfaces, it is still possible to carry out a free field measurement using methods similar to the already mentioned. To measure the absorption in the periodic case, requires the measurement using more than two microphone positions, The multi-microphone method is very sensitive to evanescent waves, so very accurate microphone positioning is needed. The microphone must be in a distance where it doesnâ&#x20AC;&#x2122;t measure the evanescent waves but also gets no diffraction from the sample edges. Fig.24: two microphones free field method

acoustics REVERBERATION CHAMBER METHOD In most applications, the sound will be incident on an absorptive material from a multitude of incident angles at once. A quick and efficient way to do this is afforded by the reverberation chamber method. The random incidence absorption coefficient is the parameter used most in the design of spaces to specify the absorption performance of materials. So while the random incidence absorption coefficient is mainly needed for room design instead of validating prediction models. The reverberation time of a room is dependent on the total absorption in the room. Consequently, by measuring the reverberation time of a room before and after a sample of absorbent is introduced, it is possible to calculate the random incidence absorption coefficient. The main characteristics of the reverberation rooms are: i) Adequate volume ii) Suitable shape or diffusing elements or both iii) Suitably small sound absorption over the frequency of interest and iv) Sufficiently low background noise levels. According to ISO 354:2003, the volume of the reverberation room should be 200m3, preferably with non parallel wall surfaces. The specimen on the other side should be between 10m2-12m2, for a roomâ&#x20AC;&#x2122;s volume of 200m3. A diffuse field can be roughly defined as requiring the reflected sound energy to be the same across the whole room and

EVALUATION the energy to be propagating evenly in all directions. To achieve this, reverberation chambers often use diffusers in the volume of the room, and the chamber walls are often skewed. Despite these measures, a diffuse field is not completely achieved, and consequently the reverberation time is position dependent. For this reason, it is normal to use multiple source and receiver positions and to average the results to reduce the effect of nondiffuseness. The source is normally placed in the corner of a room, pointing into the corner. Receivers should be at least 1 m from the room boundaries, room diffusers and the sample, and should be chosen to obtain a diverse sampling of the room volume. Even with all these measures, the measured absorption coefficients are often inaccurate at low frequencies due to modal effects. More detailed information can be found on chapter HELIX, Acoustic Evaluation, p......

Taking into consideration all these methods, it is clear that different scale measurement needs different method. The reverberation chamber test requires large sample sizes and a specialist test room, and is usually expensive to undertake. It also only gives absorption coefficients; the impedance cannot be measured. Consequently, the testing of an absorptive unit will first include impedance tube measurements, to build up an understanding of the material properties and geometry on small samples and then the prototype will be tasted in a reverberation chamber.

mic 1 speaker mic 2 Fig.25: reverberation chamber

acoustics

1.7 ABSORPTION CROSS-SECTION

Each resonator tube has a limit in the area that can contribute in sound absorption. More specifically, the absorption cross section refers to ‘‘the surface area, that is perpendicular to the direction of sound incidence, through which, in the undisturbed sound wave, the same sound power would flow through as the sound power absorbed by the resonator’’ (Beranek and Ver, 2006, p. 266) Amax=

(Eq. 10)

4π

where: Amax= absorption cross-section (m2) = the wavelength of the resonance frequency of the 0 resonator (m)

By simplifying the equation: r=

(Eq. 12)

2π

where: r is the radius of the circular absorption cross-section (m). So the maximum distance between two tubes that aim at the same frequency is: d=2r

(Eq. 13)

We can consider the absorption area as a circular-shaped area with A=πr2 So combined with the equation 10

πr2=

4π

(Eq. 11)

Fig.26: absorption cross section diagram

product design

In order to make a succesful product, it needs to fill in a gap in the existing market. As it is shown in figure 21, the design pronciples put the limits into the selection of materials and accordingliy the selection of the appropriate manufacturing techniques. In order to make a more realistic approach, the final product will be compared to some existing acoustic products, that follow in the next page. These products were selected according to the most competitive criteria (price, weight, performace and design). Acoustic Wall Covering is designed to absorb sound and reduce reverberated noise. The tiles have a self-adhesive backing but it is recommended to apply a contact adhesive to the wall first for maximum adhesion. This material is chosen because it can be put almost in every space, it is quite light and one of the simplest solutions within a relatively normal price. The Cutting Wedge 2000 creates seamless absorptive walls by reducing unwanted reflections. The thickness can be quickly increased by adding layers instead of buying different and incompatible products. This product offers a very good absorption, especially when three-layers are combined. The price is also quite competitive and is quite light. However it needs a surface to be mountained with glue, so it is not possible to be re-used in the future. The very good absorption with the combination of relatively low price were the reasons for chosing this product. The Versipanel is constructed with acoustical ribbed fabric and heavy foam cores. It can be used as space divider as it meets the floor directly from end to end, keeping sound confined. Constructed of multiple small panels, the VersiPanel may be arranged different shapes and even be extended using the attached magnets. Versipanel is chosen for its free-standing ability and the possibility of changing shapes. All the products seem to have one or two selling points, but there is nothing that combines all of them. In addition, a space divider should let the light travel as well as provide privacy to the users. In addition, considering the guidelines of figure 21, there is obviously plenty of space for improvements.

product design

2.1 DESIGN PRINCIPLES

acoustic performance low cost elegant appearence + visual contect lightweight structure easy assembly fast mass production recyclability decreased thickness Fig.27: design principles from the most important to the least

ACOUSTIC WALL COVERING

CUTTING WEDGE 2000- ACOUSTICAL FOAM

1.2

0.8

0.6 Series1 0.4

0.8 Series1

0.6

Series2 Series3

0.4

0.2

100

125

160

200

250

315

400

500

630

800 1000 1250 1600 2000 2500 3150 4000 5000

Panel size

60x60x1 cm

Adhesive price/m

Thickness

3.3 €/m

10 m -12 mm

0.8

0.6 Series1 0.4

0.2

100

125

160

200

250

315

400

500

630

800 1000 1250 1600 2000 2500 3150 4000 5000

100

125

160

200

250

Frequency (Hz)

Fig.28: absorption coefficient of acoustic wall covering

Absorption coefficient

1.2

0.2

Panel size

2.5x31x31 cm

Adhesive price/m

Thickness

1.6 kg/m3

Mass

Total price /m2

80 €/m2

Total price /m2

3.3 €/m

315

400

500

630

800 1000 1250 1600 2000 2500 3150 4000 5000

Frequency (Hz)

Fig.29: absorption coefficient of cutting wedge

Mass

VERSIPANEL ACOUSTICAL ROOM DIVIDER

1.2

Absorption coefficient

product design

2.2 WIDELY USED ACOUSTIC SOLUTIONS

25mm - 40mm - 55 mm 32 kg/m3 50-150 €/m2

Fig.30: absorption coefficient of versipanel divider

Panel size

10x120x240 cm

Adhesive price/m

Thickness

100mm

Mass Total price /m2

125 €/m2

2.3 INFLUENCE OF ACOUSTICS IN MANUFACTURING

Hollow

Solid

Flat

Parallel Features Complex

Cut-outs

No cut-outs

product design

Sheet

Prismatic

Transverse Features

MATERIAL

The design proposal has direct connection to the chosen acoustic principle. The acoustic principle is based on the use of tubes. The quarter-wavelength tubes need to be closed from one side while the half-wavelength tubes are open from both sides. This information translated into manufacturing means that three separate investigations need to take place, the tubes, the way to seal them and the supporting structure. The main target is to save material by making a light, self-supported and low-cost product. These are the main critiria in order to chose the appropriate material and manufacturing technique. As far as the tubes are concerned, for the construction of the prototype, prefabricated tubes can be used, while for mass production the tubes needed can be produced with custom dimensions. Considering that the design addresses to a product, the aesthetic value is quite important. The product is going to be used in an interior environment, as a room divider. This means that it needs to provide the users with private space as well as to keep the visual contact. For this reason transparent/transluscent elements will be further investigated. Last but not least, one of the aims for the design is to be able to re-use the product in a different space. For this reason and kind of permanent or chemical adhesive wonâ&#x20AC;&#x2122;t be included in the design process. This is important for the design process as the chosen material and the procedure should support the necessary tolerance for integrated connections.

SHAPE

Dished

Axisymmetric

NonAxisymmetric

Simple

Ceramics and glasses

Metals and alloys

Composites, foams, honeycombs, natural materials

Plastics and elastomers

Fig.31: material form selection

2.4 STUDY CASES ACOUSTIC BRICK WALL

JOXTROP

product design

NOMAD SPACE DIVIDER

Source: Brittany, 11/11/2009, Retrieved from: http://blog.wantist.com

Retrieved from: http://acadia.org/papers/FYW2G2

Retrieved from: http://www.ikea.com

• modular and self-supported

• acoustic properties based on depth

• assembly procedure

MIO’s Nomad is a modular, free-standing system that requires no tools to assemble, or for that matter disassemble and build again. It’s sculptural, decorative, and pleasing to look at. It’s versatile functionally and limited mostly by your imagination. Nomad can divide a workplace or create a room. It can go up for a party and come down when it’s over. Put a tabletop on it and you’ve got yourself a table. Made from recycled, double-wall cardboard, and colored with vegetable based inks, the tiles can be dusted and cleaned with a damp cloth. The Nomad system is, unabashedly, an impermanent solution. It’s temporary— as are so many situations in our lives. So thankfully, it’s recyclable. MIO’s Nomad System is sold in module packs of 24, one color per pack, for $56. Assembled that’s about 4.5ft×4.5ft.

The project focuses on robotically fabricated acoustic wall system which functions as an acoustic zoning element for office spaces. The aims were the investigation of alternate design and fabrication based on sound diffusion principle and the development of mass produced individually adaptable walls. The targeted frequency range was 125 Hz to 4000 Hz. For the diffuse reflection of various frequencies, depths of 20 cm and 10 cm were applied. The most important criteria for the choice of robotically driven assembly was the demand for a high degree of automation, by an autonomous and fast process with minimal human intervention. Acrylonitrile butadiene styrene (ABS), was selected for its vast color palette and it’s excellent processability. The quantifiable measurements were done in a 5.8 x 6.8 x 3.0m large room with flat walls furnished with tables and chairs and equipped with an absorbing ceiling. The results showed a clear reduction of the reverberation time which is an indirect proof of the efficient scattering.

This is one of the IKEA’s products that demand self assembly. The lamp shade is made from 80% of paper, which folds flat. So it takes less space in transport and the buyers need to assemble it themselves, following simple steps. The design is kept extremely simple, providing an also very low final price (around 4euros). Flat sheet, after being cut and engraved, is folded by hand. It is kept in shape by slot joints so no glue or tape is needed.

product design Retrieved from: http://inhabitat.com/pomadas-recycled-cardboard-furniture-gives-scrap-tubes-a-second-life/

Retrieved from: https://pietheineek.nl/en/product/tube-furniture

Retrieved from: http://www.rael-sanfratello.com/?p=1466

• tube-assembly

• transparent tube design

We’ve featured cardboard furniture here at Inhabitat loads of times before, but we love how this set of chaise lounges, tables and chairs by Antonela Dada y Bruno Sala gives recycled cardboard tubes a second chance to escape the landfill. Cardboard is light, foldable, easily recycled, biodegradable and most importantly, a very cheap and readily available material. The cardboard furniture is made from 30 cm-wide cardboard tubes, which are cut and sanded before they slide into frames made from plywood or recycled OSB. After the tubes are glued together, the product is covered with a clear hydro lacquer that preserves each piece.

Dutch designer Piet Hein Eek‘s reclaimed steel tube furniture is a simple design that activates negative space by joining hollow tubes. The old pipes used to construct these pieces were salvaged by the designer last year while he was renovating an older building into his new studio. The metal tubes are connected with screws on each side and create a range of furniture objects, including an armchair, bar chairs, small tables and a pipe bench.

SOL Grotto is a small shelter made from 1,368 glass rods once destined for use in Solyndra solar panels. The project is located in the Berkeley Botanical Garden in the California Native section. The tube’s original role as a light concentrating element is extended to transmit cool air into the space via the Venturi effect, to amplify sounds from the adjacent waterfall via the vibrations of the tubes cantilevering over the creek, and to create distorted views of the garden. The glass tubes are illuminated electric-blue naturally from the direct and ambient light that is conducted through the glass causing each tube to change in intensity throughout the day. The cylindrical shape of the rods absorb more light, especially at dawn and twilight, than their flat predecessors.

2.5 MATERIALS HYBRIDS: COMPOSITES, FOAMS, HONEYCOMBS, NATURAL MATERIALS

METALS AND ALLOYS

PLASTICS AND ELASTOMERS

price transparency flexibility

product design

lightness

CERAMICS AND GLASSES

Fig.32: material families for frame

There are four categories of materials: ceramics and glasses, hybrids (composites, foams, honey-combs, natural materials), metals and alloys, and polymers (plastics and elastomers). According to the design needs, these material families are compared in terms of lightness, flexibility, transparency and price, as shown on the table above. Ceramics and glasses are inorganic, nonmetallic materials consisting of metallic and nonmetallic elements. They have high strength bonds that give them a wide range of engineering applications. Regarding the project, due to their high transparency and aesthetic value they could be used for tubes. However, this means that the price of the product will be increased. Hybrid material family includes a wide variety of materials; composites are materials made from two or more constituent materials with different physical or chemical properties. When combined they produce a material with quite different characteristics which might be strength or lightness. Foam is called a material in a lightweight cellular form resulting from introduction of gas bubbles during

manufacture (accessed in 2017, retrieved from :https://www. merriam-webster.com/dictionary/foam). Foams are known for their lightness and widely used in acoustics for their absorption properties and their low price. For this project though, foam materials donâ&#x20AC;&#x2122;t comply with tube resonators, since non-porous materials are preferred, and neither provide the necessary stiffness for the frame. Honeycomb materials are commonly made by layering a honeycomb material between two thin layers that provide strength in tension. This behaved like a plate-like assembly. Honeycomb materials are widely used where flat or slightly curved surfaces are needed and their high strength is valuable. Their stiffness can be used for the frame structure. Natural materials include different kinds of wooden or wood-like materials that can easily be light, cheap and in flat-sheet form. According to their thickness they can be used for the frame as well as for the tubes. Metals and alloys are chemical elements which are characterised by their strong bonding. At the same time they have high mass while they lack of flexibility. They can

be used for both frame or tube manufacturing. However, due to their high mass and cost they are not preferred to be further investigated. Plastics and elastomers consist of any of a wide range of synthetic or semi-synthetic organic compounds that are malleable and can be molded into solid objects. Plastics are typically organic polymers of high molecular mass. They are usually synthetic, most commonly derived from petrochemicals, but many are made from renewable materials such as polylactic acid from corn or cellulosics from cotton linters. Elastomers are polymers with viscoelasticity, low Youngâ&#x20AC;&#x2122;s modulus and high failure strain. They are usually light and can provide transparency or tranluscency and combined with their low cost they can be used for both frame and tube manufacturing. The materials are chosen by the use of each element. In the following page it is shown how the design is divided in parts and which types of materials can be used respectively.

2.6 DESIGN AND MATERIALS TUBES

FRAME

product design

SEALING

tubes (plastics & elastomers)

tubes+connections (plastics & elastomers)

cap for each tube (plastics & elastomers)

each tube sealed (no extra material) folded frame (plastics or natural materials)

interlocking modular frame (plastics or natural materials)

modular wire frame (metal)

cap for group of same straws (plastics & elastomers)

rigid plate (plastics or natural materials)

2.7 MANUFACTURING PROCESSES

INJECTION MOLDING

POLYMER EXTRUSION

In INJECTION MOLDING the polymer after being molted is injected into a cold steel mold under high pressure. The polumer solidifies under pressure and then the molding is ejected. The capital and tooling costs are very high, so the process is mainly used for large volume production. The production rate though is quite high for small moldings. Complex shapes are possible though it might increase tooling costs. The uses are quite varied, from containers to tool handles and plumbing fittings. The leftovers are recyclable but as the injection blow molding, any malfunctions can be very hazardous. This might be an expensive process for staw manufacturing but for small connections is quite suitable.

POLYMER EXTRUSION is a process where powder or granule of polymer is processed by a rotating screw through heating chamber and after being melt is forced through a shaped die opening. The extruded product is cooled as it leaves the die. The advantage of the process is the relatively low tooling costs but high capital costs. The output usally needs further processing like cutting to size or remelting and injection molding. Used mainly for polymers with prismatic shapes. The typical uses include rods, channels, pipes, tubes, seals, window frames etc. Like the previous processes there is dust exposure during the resin formulation and dangerous in case of thermostatic controller malfunctions. This is the most appropriate way for staw manufacturing, and can massively produce straws of any dimension and colour.

product design

INJECTION BLOW MOLDING

In INJECTION BLOW MOLDING, a hollow preform is injection-molded over a mandrel that forms the hollow shape. The hot preform is transfered by the mandrel to the blow molding die, which also functions as the blow nozzle. The polymer is blowed by under pressured air through the mandrel against the mold walls where it cools and freezes. The mold then is opened and the part removed. The process produces no waste material. The injection blow molding process offers better control over the thickness and the weight of the finished part. The process is most competitive for production of small bottles. The tooling cost ranges according to the moldâ&#x20AC;&#x2122;s geometry. The waste material can be recycled but malfunctions in the thermostatic controller can be extremelly hazardous. For manufacturing straws this is not the optimum process.

PUNCHING, BLANKING, PERFORATING, NIBBLING

LASER CUTTER

product design

WATER JET CUTTER

A comparison of the most appropriate manufacturing processes for the project was made, based on CES software. In first place the secondary shaping processes appropriate for cutting were chosen. The range of materials includes Polymers and hybrids in a flat sheet form. The result was thirteen manufacturing processes, from which the five most appropriate are presented. Water jet cutting is a process where filtered water is pumped at high pressure through a narrow sapphire nozzle, emerging with a velocity almost 3 times the speed of sound. The kinetic energy has enough power to slice relatively soft materials as plastics, thin composites, paper, leather and food stuffs. It has quite good cutting speed (0.05-0.5 m/s, which depends on the material) and good tolenace. The equipment costs are medium while tooling costs and labor intensity are low. The typical uses are those of cutting paper, cardboard and other packaging materials.

In punching a circular shaped hole is punched through the sheet by a hardened die. During perforating an array of punches that acts as one unit creates in once a pattern of holes. In blanking a punch with the desirded shape is created by a shaped disk. In nibbling, a small hardened punch cuts or trims the sheet to give the desired shape. This process has and average tolerance and could be considered as relatively expensive, compared to the other manufacturing procedures, since it has relative high equipment cost and medium relative tooling cost. The labor intensity on the other hand is low. The typical uses include mainly metal plastic or card sheets, fiberboard, cork, wood and cardboard

In laser cutter a beam of monochromatic light creates a local melting. The cutting speed of the beam is quite good providing an average tolerance (0.03-0.001 m). The cost in general can be considered slightly high as the equipment cost is very high but the tooling costs are medium and the labor intensity low. On the other hand the provess is quite fast and allows automation. Laser cutter is typicaly used for cutting composites and polymer sheets although it can be used for cutting fabrics and textiles as well.

product design

DIE CUTTING

FUSED DEPOSITION MODELLING

THREE ROLL PUSH BENDING

press creasing rule steel rule die creasing channel

plywood substracts ejection rubber

pressure rolls

sheet material press

bend die

wire

Die cutting is a manufacturing process used to generate large numbers of the same shape by using a die to shear webs of low-strength materials, such as rubber, fiber, cloth, paper, corrugated fiberboard, paperboard, plastics, foam and sheet metal. The typical uses of die cutting include the production of labels, corrugated boxes, and envelopes. Die cutting can be done on either flatbed or rotary presses. The main difference between them is that the flatbed is not as fast but the tools are cheaper. Dies used in rotary die cutting are either solid engraved dies, adjustable dies, or magnetic plate tooling. Engraved dies have a much higher tolerance and are machined out of a solid steel bar normally made out of tool steel. Adjustable dies have easy removable blades that can be replaced with others, either due to wear or for cutting a different material. Magnetic plate has a cylinder with magnets placed in it, and an engraved metal plate is attached on the base cylinder, held onto it by the force of the magnets.

Fused deposition modelling (FDM) is an additive manufacturing technique suitable for use in an office environment. A fine stream of molten material (usually thermoplastic) is deposited by a two-axis heated extrusion head. Semi-liquid thermoplastic material is extruded and then deposited one layer at a time starting at the base. This builds the model vertically on a fixtureless base. Successive layers adhere together throughthermal fusion. The FDM process requires no post-production UV curing, enabling multiple versions of a part to be created within a short time frame. This is the most widely used additive manufacturing method for home and office use. However, the low cost is valid for a small production only (1-10 units). The build envelope ranges from 127x127x127mm to 914x610x914 mm. The materials used are non toxic and the cost comes about 0.40 euros/cm2.

The process of THREE ROLL PUSH BENDING involves mechanical force to push stock material pipe or tubing against a die, forcing the pipe or tube to conform to the shape of the die. Often, stock tubing is held firmly in place while the end is rotated and rolled around the die. The Three-Roll Push Bending (TRPB) is the most commonly used freeform-bending process to manufacture bending geometries consisting of several plane bending curves. Nevertheless, a 3D-shaping is possible. The profile is guided between bending-roll and supporting-roll(s), while being pushed through the tools. The position of the formingroll defines the bending radius. The bending point is the tangent-point between tube and bending-roll. To change the bending plane, the pusher rotates the tube around its longitudinal axis. The process is very flexible since with a unique tool set, several bending radii can be obtained.

HEATING SEALER

converter: transforms electrical energy to mechanical vibration

booster: amplifies vibration and serves as a mount power supply: converts standard 50/60 Hz electrical energy to 30000 cycles per second

VACUUM FORMING involves the heating of a thermoplastic sheet to itâ&#x20AC;&#x2122;s softening point and then by subtracting the air the sheet is forced against the mold. The process can handle large range of sizes. The advantages include low tooling costs, low capital costs, high production rates. However, the raw material is more expensive, the leftovers canâ&#x20AC;&#x2122;t be recycled directly and the process might be labor intensive. The shape capability is limited to simple shapes. Openings and holes are not possible without adittional processes. The typical uses include trays, packaging, bath tubs, drink cups etc. For forming a hexagonal cap vacuum forming is quite a fast procedure and quite accurate. The disadvantage in terms of prototyping is that the raw material is quite expensive.

product design

VACUUM FORMING

anvil: supports tube during sealing

horn: applies ultrasonic vibration to tube 60Hz

Heat sealing is the process of sealing one thermoplastic to another similar thermoplastic using heat and pressure. The direct contact method of heat sealing utilizes a constantly heated die or sealing bar to apply heat to a specific contact area or path to seal or weld the thermoplastics together. Heat sealing is used for many applications, including heat seal connectors, thermally activated adhesives, film media, plastic ports or foil sealing. Common applications for the heat sealing process: Laminate foils and films often are heat sealed over the top of thermoplastic medical trays, plates, bottles and containers to seal and/or prevent contamination for medical test devices, sample collection trays and containers used for food products. Medical and fluid bags used in the medical, bioengineering and food industries. Fluid bags are made out of a multitude of varying materials such as foils, filter media, thermoplastics and laminates.

CONCLUSIONS These manufacturing processes are compared in terms of cost, speed, tolerance, engraving possibility and whether they are good for prototype or mass production. For each part of the product a different process is more suitable. For strawsâ&#x20AC;&#x2122; manufacturing, polymer extrusion is the most common way for mass production. For the frame, water jet cutting, die cutting and die cutting can support a fast production line with low cost. For the cap, according to the design there different manufacturing processes. This decision is mainly based on the final design. The manufacturing technique used for the prototype might be different from the one that ideally would be more appropriate, since the availability of nearby machines is important and it is also dependent on the number of units produced.

2.8 COMPARISON OF PROCESSES part of the product

processes

product design

Injection blow molding Injection molding

Polymer extrusion

Water jet Punching, perforating, blanking, nibbling Laser cutting

Die cutting Fused Deposition Modelling Three Roll Push Bending Vacuum Forming Heating Sealer Fig.33: comparison of manufacturing processes

cost

speed

tolerance

envraving

prototype

mass production

Helix panel

product Helix design panel

3.1 INTRODUCTION

The goal of this project was to improve the acoustic comfort of the office of Environmental Technology & Design, in Faculty of Architecture, in Delft University of Tecnology. Helix panel can be placed either in the middle of the room, also acting as a room divider and providing the users with more personal space, or in front of the surrounding walls. Since it is possible to place the panel in the room, the size of the units and visual contact in the room were taken under consideration as well as the aesthetic quality it offers from both front and back sides. Helix was the result of a team project realised by Panayiotis Hadjisergis and Ioanna Christia.

Helix panel

3.2 DESIGN

_ DESIGN APPROACH The design approach includes the proposal of an acoustic panel made units with same shape but different openings and depth. This would allow them not only to have a nice visual effect but also easily mass produced. The proposal includes two hexagonal units, with edge of 10 cm each and lengths of 21cm and 19cm. Each one is made of 0.4mm MDF. Honeycomb shape is convenient as it can distribute the loads of the units efficiently and eases the stacking. Inside the MDF shell there is a cardboard surface providing the necessary stiffness for stacking. Both materials are light and strong enough to make a self-bearing structure. The main concept is to keep the manufacturing as simple as possible, with simple materials, avoiding glue and nails. Both front and back side take form by overlapping triangle surfaces. The back side is flat and due to an extra folding there is space for the connections between the modules. For this purpose mettalic clips are used, two of them for each side. The front part is perforated using 3 sizes of holes (3mm, 2mm, 1.5mm diameter). The percentage of the perforated area is 12.5%. For the frame construction we made 12 modules, 8 of them with small neck diameter. The connections at the back side provided enough support for keeping the units together even though each unitâ&#x20AC;&#x2122;s weight is about 1.3 kg. According to the roomâ&#x20AC;&#x2122;s needs, it is be possible to use different combinations of units so as target the frequencies that cause the more noise.

_ ACOUSTIC PRINCIPLES

product Helix design panel

3.3 ACOUSTIC PRINCIPLE

Two principles were combined in this project: helmholtz resonators and porous absorbers. The frequencies we target are the ones of human speech, which start at 85-180 Hz for males and 165-255 Hz for females (±20 Hz) . The dimensions of the units were defined in order to target at 150Hz (short unit) and 250Hz (long unit). The front part of the unit is folded towards inside, in order to increase the surface of the sound incidence. The surface is going to be perforated so as to allow the sound waves to reach the absorptive material at the inside. Felt of 5mm is used as porous absorber, shaped as a hexagon so as to fit in the inside surface of the front part of the units. At the same time, the folding of the front side works as extension of the neck. According to the research ‘‘Sound Absorbing Acoustic Horns’’, a horn shaped neck behaves much better than a rectangular one. Helmholtz resonator aims for certain high frequencies and absorber aims in lower ones. So the combinations offers better acoustic results in a wider spectrum of frequencies. The impedance tube is also used for measurements that clearly showed that the combination of materials provided a smoother curve, with a wider range of frequencies. For more information check Acoustic Evaluation on page 64.

3.4 ASSEMBLY

Helix panel

7x Fig.34: assembly of Helix, source: panayiotis hadjisergis, ioanna christia

_ MATERIALISATION In order to provide acoustic comfort in a room, a considerable number of units would be required. The idea of redeveloping the module as a flat-pack design rose primarily in order to counteract transportation inefficiencies. Adopting flatpack design benefits the design a lot: • Minimization of space required when transported as a flat sheet • Ability for high speed production of units from flat sheets through shearing manufacturing processes necessary since even for one application, a high number of modules is needed • The module is simple enough so that it can be designed such that it is assembled by the user, further minimizing costs • The possibility of re-use since modules can be assembled when needed, and disassembled and moved to another space or stored.

product Helix design panel

3.5 MATERIALISATION

The priorities were therefore aligned towards the ability to flatten all components of the module. To maintain the components ability to be assembled, disassembled and reassembled, as well as to maintain simplicity of the design no glue or any kind of non-reversible connection would be used. The joints were therefore incorporated in the design. A main challenge of focusing on flat pack design of such a stackable module would be structural integrity. The bottom modules are expected to be able to withstand pressure from a minimum of 10 units, and hopefully up to as much as 20. Different materials were tested including cardboard, corrugated cardboard and polypropelene. In the prototype MDF is used, as it’s eco-friendly, cheap and gives a more ‘‘cosy’’ feeling. However, there is an alternative proposal with polypropylene of 0.8mm which is more resistant as a material, and doesn’t melt easily in a full-blown fire.

Helix panel

3.6 CONFIGURATIONS

Fig.35: spatial arrangement of HELIX, source: panayiotis hadjisergis, ioanna christia

product Helix design panel

3.7 TECHNICAL DRAWINGS

source: panayiotis hadjisergis, ioanna christia

1. 2. 3. 4. 5. source: panayiotis hadjisergis, ioanna christia

Red lines show the engraving path. Blue lines show the cutting path. Red dashed line means different direction folding. Perforation percentage: 12.5% The diameter of the circle is slightly bigger that the neckâ&#x20AC;&#x2122;s diameter, as the sides being folded they create smaller opening than the one designed. 6. The curveâ&#x20AC;&#x2122;s shape provides the necessary stability, with the minimun surface overlapping during folding. 7. Keeping this point at the center of hexagon gave the angle of 60o. 8. The folding surfaces are for keeping felt in the right position. 9. Opening for holding the neck 10. Strengthen the support 11. The small length is the perimeter of the neck cylinder (1.5cm diameter)

3.8 DETAILS Detail

Detail

Helix panel

Detail

172

1 2

100

200

210

Back elevation elevation, 1:4

Section Section(small (smallmadule) module), 1:2

All modules align on the back side All modules align on and are connected by split pins the back side and are (paperboard modules, shown), or connected bymodules). split pins. snap buttons (PP The connections are removable and The connections are reusable. removable and reusable.

Connection detail, detail 4:1 Connection 2

Front Front corner corner detail detail, 4:1

Front Front elevation elevation(small (smallmadule) module), 1:4

The twisting action of the shell The twisting action proďŹ le, the pressure by 4 of the shell profile, overlayed leafs and the cardboard the pressure four support, keep the neckby cylinder in place with no adhesives. The overlayed MDF sheets cardboard also maintains the felt and against the the cardboard pushed perforated support keep surface. the

neck in place without any adhesives. The cardoard also maintains the felt pushed against the perforated surface.

The cardboard support is wedged into the corner of the module, keeping The cardboard support is wedged the felt corner in place well into the of the as module, keeping the felt in placestructural as well as as providing providing structural support. support. 1

Cardboard support, 1.5 1.5mm cardboard support, mm

Paperboard, 1mm cardboard, 1mm

felt layer, 2.52.5mm mm Felt layer,

split Split pin,pin, 8mm 8mmdiameter width

Perforations, Ă&#x2DC;1.5/2.5/5mm perforations O1.5/2.5/5mm

Crease crease

Overlayed leafs (no perforation) overlayed area

hole split Split pin pin hole

2 1 8

Neck detail detail, 2:1

source: panayiotis hadjisergis, ioanna christia

As far as the design is concerned, the total impression is that it was quite successful. The folding procedure was quite easy and fast, most of the parts could stay in place and the units were easily stacked. At the same time the initial cost is kept relatively low. For the prototype construction, each unit cost about 5-6 euros. This means that if there is a mass production the price per unit will be even lower. For a 1m2 of surface, 39 units are needed, which means a maximum price of 200 euro/m2, regarding the prototype scale. Regarding the size of production, the price can drop even 97%. The stability of the panel was good enough. Taking into account structural mechanics’ calculations, the maximum amount of stackable pieces was 12, which means create a division of about 2 m high. For a higher structure another material for shell or an optimised version of the inner support with a symmetrical design, to distribute the forces in a better way could be a proposal.

Volume Classification

Volume (units)

Investment (€)

Prototypes

1-100

Short run

500-1000

Mid run

However, even if there are many successful details, there is always room for improvements. First of all, the neck’s design could be improved by including a detail that helps it stay in position. This could be easily done by slightly increasing the thickness of the tube in the entrance. The lower units have also the tension to be slightly deformed. This problem could be solved with a possible extra base that would either keep them together. The base should be also modular, with easy assembly, able to follow the shape of the panels. A rectangular mdf that can fold in a half haxagon, connected in the same way with the units could be a solution. At the same time, one possible problem is that even though both sides are designed to be elegant, in fact the absorption takes place only from one side. This means that if the units are used as a space divider, they need to be put in both sides, without any existing posibility for connection between them.

Tooling (€)

5mm felt/m2 (€)

Production Price/m2 (€)

150-450 TOTAL

2.0-5.0

150-450

1.15-1.75 /unit

200-300

1.0-2.0

45-70

2500-5000

0.50-0.85 /unit

300-500

1.0-2.0

20-35

Mid-large run

10000-25000

0.25-0.45 /unit

300-500

1.0-2.0

10-20

Large run

50000-10000

0.08-0.15 /unit

500-850

1.0-2.0

4-6

Fig.36: tooling cost of folding cartons

product Helix design panel

3.9 DESIGN EVALUATION

Retrieved from: http://howtobuypackaging.com/tooling-costs-for-packaging/ (2016)

Helix panel

3.10 ACOUSTIC EVALUATION

_METHODOLOGY OF MEASUREMENTS

_VOLUME OF REVERBERATION ROOM

_TEST SPECIMENTS

The test method is defined in ISO 354:2003 Acoustics - Measurement of sound absorption in a reverberation room. The method is used for testing plane absorbers i.e. flat areas of sound absorptive material such as carpeting or acoustic tiling, or for testing discrete sound absorbers, i.e. pads, baffles, chairs or free-standing screens. The method requires a diffuse reverberant sound field in a reverberation room. The average reverberation time is measured in the empty room when the sound is switched off. The test specimen or test items are then placed in the room and the reverberation time is measured again. Because of the sound absorption, the reverberation time is now shorter. From these two reverberation times, the equivalent sound absorption area of the test specimen, AT, is calculated by using Sabineâ&#x20AC;&#x2122;s equation.

The volume of the reverberation room shall be at least 150 m3. For new constructions, the volume is recommended to be at least 200 m3. When the volume V of the room differs from 200 m3, the given values of absorption areas shall be multiplied by (V/200 m3)2/3. The shape of the reverberation room shall be such that the following condition is fulfilled:

The test specimen shall have an area between 10 m2 and 12 m2. If the volume V of the room is greater than 200 m3, the upper limit for the test specimen area shall be increased by the factor (V/200 m3)2/3. The area to be chosen depends on the room volume and on the absorption capability of the test specimen. The larger the room, the larger the test area should be. The test specimen shall be of rectangular shape with a ratio of width to length of between 0,7 and 1.

Imax < 1,9 V 1/3

(Eq. 14)

where: Imax is the length of the longest straight line which fits within the boundary of the room (e.g. in a rectangular room it is the major diagonal), in meters; V is the volume of the room, in cubic metres. In order to achieve a uniform distribution of natural frequencies, mainly in the low-frequency bands, no two room dimensions shall be in the ratio of small whole numbers.

_MOUNTING OF SPECIMENTS â&#x20AC;˘ The test specimen is mounted or placed directly against a room surface, such as the floor of the reverberation room. Adhesives or mechanical fasteners that do not leave a thin air space may be used to hold the test specimen in place during the test, if required.

•

are butted together to form the test specimen, it may be necessary to cover the joints between the adjacent pieces with material that is not sound absorbing. The reason for covering the joints is to prevent the side edges of the individual pieces from absorbing sound. The perimeter edge of the test specimen shall be sealed or covered to prevent the edges from absorbing sound. If the edges of the test specimen are exposed when the material is normally installed in an actual application, then the edges of the test specimen shall not be sealed or covered during a test. If the edges are not covered, the area of the edges shall be included in calculating the test specimen area. The perimeter edges of the test specimen may be sealed or covered with an acoustically reflective frame. A frame of 1,0 mm thick steel, 12,5 mm thick gypsum board or 12,5 mm wood (minimum thicknesses) may be used. The frame shall be tightly butted to the specimen and sealed to the room surface. The exposed face of the frame shall be flush with the surface of the specimen.

_TTEMPERATURE AND RELATIVE HUMIDITY The room where the measurements are performed should be under conditions of temperature and relative humidity that are almost the same. In any case, the relative humidity in the room shall be at least 30 % and max. 90 % and the

product Helix design panel

• If two or more pieces of material (or separate panels)

temperature shall be at least 15 °C during the whole test.

_EQUIVALENT ABSORPTION AREA

_MICROPHONES

The equivalent sound absorption area of the empty reverberation room, A1, in square metres, shall be calculated using the following formula. The same formula can be used for calculating the equivalent sound absorption area with the panel.

The directivity characteristic of the microphones used for the measurement shall be omnidirectional. The measurements shall be made with different microphone positions which are at least 1,5 m apart, 2 m from any sound source and 1 m from any room surface and the test specimen. The minimum number of microphone positions shall be three,

_SOUND SOURCE The sound in the reverberation room shall be generated by a sound source with an omnidirectional radiation pattern. Different sound source positions which are at least 3 m apart shall be used. The minimum number of sound source positions shall be two.

_REVERBERATION TIME The reverberation time of the room in each frequency band is expressed by the arithmetic mean of the total number of reverberation time measurements made in that frequency band. The mean reverberation times of the room in each frequency band without and with the test specimen, T1 and T2 respectively, shall be calculated and expressed using at least two decimal places.

A1=

55,3V -4Vm 1 cT1

(Eq. 15)

where V is the volume of the empty reverberation room (m3) c is the propagation speed of sound in air (m/s) T1 is the reverberation time of the empty reverberation room (s) m1 is the power attenuation coefficient, (m-1) using the climatic conditions that have been present in the empty reverberation room during the measurement. The value of m has the following values

Temp 20

125

250

500

1000

2000

4000

0.445

1.32

2.73

4.66

9.86

29.7

0.615

1.42

2.52

5.01

14.1

48.5

Helix panel

Fig.37: equipment for reverberation room measurements

BOUNDARY CONDITIONS The demandings of the needed room was to have reflective walls, floor, ceiling (not a false one), with the condition of the longest straight line Imax < 1,9 V 1/3 being fulfilled. Since the equipment was quite heavy, the first measurements took place in a restroom on the ground floor of Architecture building of TU Delft. The dimensions of the room are:

(4) measurements took place; 2 with position 1 of the microphone, with and without the panel (1a and 1b) and another 2 with position 2 of the microphone, with and without the panel (2a and 2b). However, there is a limit for low frequenciesâ&#x20AC;&#x2122; measurements, according to the space dimensions.

2,53m x 1.6m x 3.05m= 12.35m3 Imax < 1,9 V 1/3= 4.4m

f=2000

and I= 4.27<4.4 m, so the condition is valid In order to have more valid results, 2 different source positions and 3 different microphone positions were examined in the room, with and without the panel. The source was first put in the middle of the room, and four

f= 2000

T60 V

(Eq. 16)

1.6 = 700 Hz 12.2

T60=reverberation time (s) V=volume of the room (m2)

This means that a big amount of frequencies (0Hz-700Hz) wonâ&#x20AC;&#x2122;t have reliable measurement. The minimum hardware required is a PC with a soundcard, a sound source, and a microphone connected to the actual soundcard line input. Typical soundcard functions are a line input, a line output and gain controls. In this case, a Norsonic Nor276 omni-directional source dodecahedron speaker is used as sound source for creating omni-directional sound field. A Norsonic Nor140 sound analyser is used for receiving the sound, connected to a Norsonic Nor280 power amplifier.

case 1 1.00

Absorption coefficient

0.8

microphone

speaker

product Helix design panel

0.75 m

0.80 0.60 0.40 0.20 0.00 -0.20

125

250

500

1000

2000

4000

2000

4000

-0.40

Frequency (Hz) empty room, microphone 1

panel, microphone 1

case 2 1.00

Absorption coefficient

2.50 m

0.8

1.60 m

0.80 0.60 0.40 0.20 0.00 -0.20

125

250

500

1000

-0.40

Frequency (Hz) empty room, microphone 2

panel, microphone 2

case 3

1.80 m 0.75 m

1.80 m

Absorption coefficient

1.00

2.50 m

Helix panel

1.60 m

0.40 0.20

125

250

500 1000 2000 Frequency (Hz)

4000

new source panel, microphone 2

case 4 1.00

Absorption coefficient

1.6

0.60

0.00

1.6 5m

new source empty room, microphone 2

0.80

0.80 0.60 0.40 0.20 0.00

125 new source empty room, microphone 1

new source panel, microphone 1

250

500 1000 2000 Frequency (Hz)

4000

The results presented show the absorption coefficient for the change of 5 to 25 dB. The graphs of reverberation time for the change of 1 to 11dB, 5 to 25dB, 5 to 35 dB and 5 to 45dB are included in the Appendix B. The results were not expected, as not only they don’t show any decrease in reverberance time, but also in cases 1, 2 the reverberance time is even higher. As was mentioned before though, the testing conditions were not appropriate. Possible influences on the results follow:

• in cases 1,2 the distance between the speaker and the

microphone was too small • in cases 1,2 also the microphone was too close to the boundary walls • between panel and wall there was a slight gap, which might have created unwanted vibrations • in cases 1,2 there were reflective objects less than 1m distance • during all the measurements there was too much exterior sound combined to the slightly open door • the frame of the panel might have provoked unwanted sound reflections • too small surface for the volume of the room; As mentioned in ISO 354, the test specimen should have an area between 10 m2 and 12 m2. If the volume V of the room is greater than 200 m3, the upper limit for the test specimen area shall be increased by the factor (V/200 m3)2/3. In our case the room volume (V1) is smaller than 200m3 so the specimen’s area is decreased by the same factor 10*(V/200)2/3 = 10* (12.2/200)2/3 = 10*0.155 = 1.86 m2 The specimen has only 0.3 m2. So it doesn’t reach the lower limit. Since the results are not valid, and the space used was too noisy and small, these are the alternative options: • redo the measurements in a more appropriate space

• do another series of measurements based on quality • construct a scaled impedance tube where a unit can fit

The impedance tube has the advantages of beeing more accurate than the other methods, doesn’t need a specific place to use it, the already existing equipment of the original impedance tube, that can be used and relatively simple construction. However, it is limited to plane wave propagation but due to the simplicity and the repeatability af the measurements is going to be further developped.

_CONSTRUCTION OF SCALED IMPEDANCE TUBE For this method an impedance tube, two microphone locations and a digital frequency analysis system for the determination of the sound absorption coefficient of sound absorbers for normal sound incidence are needed. Plane waves are generated by a noise source, and the decomposition of the interference field is achieved by measuring the acoustic pressures at two fixed locations. The materials used and the construction of the tube are based on the ISO 10534-2:2001

_ CONSTRUCTION OF THE IMPEDANCE TUBE The device is a tube with a test sample holder at one end and a sound source at the other. Microphone ports are usually located at two or three locations along the wall of the tube. The impedance tube should have a straight shape with a uniform cross-section and with rigid, smooth, reflective walls without holes or slits (except for the microphone positions). The walls should have the necessary mass and thickness so that they are not prone to vibrations by the sound signal and don’t show vibration resonances in the working frequency range of the tube. The side wall thickness should be about 10 % of the diameter of the tube. Tube walls made of wood should be sealed by a smooth adhesive finish to ensure air tightness. The shape of the cross-section of the tube can be circular or rectangular (if rectangular, then preferably square). If rectangular tubes are composed of plates, care

shall be taken that there are no air leaks (e.g. by sealing with adhesives or with a finish)so as to be sound and vibration isolated against external noise or vibration.

_ WORKING FREQUENCY RANGE The working frequency range is: fl < f < fu where: fl is the lower working frequency of the tube; f is the operating frequency; fu is the upper working frequency of the tube. The working frequency (fl) is limited by the accuracy of the signal processing equipment and fu is chosen to avoid the occurrence of non-plane wave mode propagation. The condition for fu is: d < 0,58 lu and fu·d < 0,58 c0

product Helix design panel

_ANALYSIS OF THE RESULTS

(Eq. 17)

for circular tubes with the inside diameter d in metres and fu in hertz. d < 0,5 λu and fu·d < 0,50 c0 (Eq. 18) The spacing s in metres between the microphones shall be chosen so that (Eq. 19) fu·s < 0,45 c0 lu= the wavelength of fu (m) c0= speed of sound=340,29 (m/s) The lower frequency limit is dependent on the spacing between the microphones and the accuracy of the analysis system but the microphone spacing should exceed 5% of the wavelength corresponding to the lower aimed frequency, provided that the requirements of “(Eq. 19)”are satisfied. A larger spacing between the microphones increases the accuracy of the measurements.

Helix panel

_ LENGTH OF IMPEDANCE TUBE The tube should be long enough to cause plane wave development between the source and the sample. The loudspeaker will also produce non-plane modes besides the plane wave which will die out within a distance of about three tube diameters for frequencies below the lower cut-off frequency of the first higher mode. For this reason microphones should be located preferably in a distance mentioned before but in any case no closer than one diameter. Test samples will also cause proximity distortions to the acoustic field and the following recommendation is given for the minimum spacing between microphone and sample, depending upon the sample type:

of the tube. A small recess is often necessary as shown in figure 1; the recess should be kept small and be identical for both microphone mountings. The microphone grid shall be sealed tight to the microphone housing and there shall be a sealing between the microphone and the mounting hole.

sealing

• non-structured: ½ diameter • semi-lateral structured: 1 diameter • strongly asymmetrical: 2 diameters microphone

_ MICROPHONES Identical microphones should be used in each location. When side-wall-mounted microphones are used, the diameter of the microphones shall be small compared to c0/fu.

_ MICROPHONES’ POSITIONS For side-wall mounting, it is recommended to use microphones of the pressure type. When side-wallmounted microphones are used, each microphone shall be mounted with the diaphragm flush with the interior surface

Fig.38: mounting detail of the microphone

_ TEST SPECIMEN MOUNTING The test sample holder is either integrated into the impedance tube or is a separate unit which is tightly fixed to one end of the tube during the measurement. The length of the sample holder shall be large enough to install test objects with air spaces behind them as required. If the sample holder is a separate unit, it shall comply in its interior dimensions with the impedance tube to within ±0,2 %. The mounting of the tube shall be tight, without insertion of

elastic gaskets (vaseline is recommended for sealing). For rectangular tubes, it is recommended to integrate the sample holder into the impedance tube and to make the installation section of the tube accessible by a removable cover for mounting the test sample. The contact surfaces of this removable cover with the tube shall be carefully finished and the use of a sealant is recommended in order to avoid small leaks. The back plate of the sample holder shall be rigid and shall be fixed tightly to the tube since it serves as a rigid termination in many measurements.

_ SPEAKER A membrane loudspeaker (or a pressure chamber loudspeaker for high frequencies with a horn as a transmission element to the impedance tube) should be located at the opposite end of the tube from the test sample holder. The surface of the loudspeaker membrane shall cover at least two-thirds of the cross-sectional area of the impedance tube. The loudspeaker axis may be either coaxial with the tube, or inclined, or connected to the tube by an elbow. The loudspeaker shall be contained in an insulating box in order to avoid airborne flanking transmission to the microphones. Elastic vibration insulation shall be applied between the impedance tube and the frame of the loudspeaker as well as to the loudspeaker box (preferably between the impedance tube and the transmission element also) in order to avoid structure-borne sound excitation of the impedance tube.

product Helix design panel

The unit that needs to be measured has a hexagonal shape, with 100 mm length of each side and and inscribed circle’s diameter of 173mm. The needed wall thickness is about 10% of the diameter. For this reason betonplex of 18mm is used, so as to provide also the smooth and reflective surface needed. According to “(Eq. 17)” fud<197.2 where d=0.18m so: the maximum value of fu=1095Hz Using fu in “(Eq. 19)” we have: s<0.45c0/fu so: the maximum value of s=0.14 m The distance from the closer microphone to the speaker will be two diameters: 2x0.18 = 0.36m. The impedance tube consists of three parts, one part for the testing unit, one for the speaker and the main tube. For a speaker is used a Pioneer TS-G1723i 17cm 3-Way Coaxial Cone Speaker 240W which transmits from 26 to 28,000 Hz. The rest of the setup is used from the 4206 Brüel & Kjær impedance tube (2x Type 4187 1/4’’ pressure-field microphones, Behringer UCA222 audio interface Power Amplifier and Type3160-A-042: Generator, 4/2-ch. Input/Output Module LAN-XI 51.2kHz). At the connection of the parts insulation has been put in order to make it sound proof. The microphones are put in place and kept stable by two 3d print brackets which are side-wall mounted. At the next page the construction drawings follow.

Helix panel

_ CONSTRUCTION DRAWINGS

product Helix design panel

_ 3D PRINT DRAWINGS

plan view

section

Helix panel

_ RESEARCH OBJECTIVE OF TESTINGS

the same, but all holes have the same size now. Felt of 0.5 cm thickness is used and neck’s diameter is 1.5 cm.

During these measurements there were three questions: for the targeted frequencies

• if the calculated volume of helmholtz combined by the •

use of felt increases the range of frequencies and how much how the different openings of the necks influence the range of frequencies

Regarding the calculation-part, according to the volume of the tube, the testings with the small neck diameter should target about 130 Hz and 235 Hz for the wider neck diameter models. Each physical model was put at the same postion, so the volume created in the tube was the same.

_ SAMPLES’ CHARACTERISTICS There are 7 samples which are combined in pairs of two. During the first three measurements the perforation ratio is changed in order to find out which combination of percentage perforation and hole diameter performs better. 1. The first sample is perforated in a combination of different holes of 2.5, 1.5 and 0,5 mm radius. The perforation surface is 12.5% and behind the surface is felt of 5 mm. thickness. The neck’s diameter is 15 mm. 2. For the second sample the percentage of perforation is

3. The third sample has the same hole diameter as sample 2 but bigger perforation percentage per 1.4 times. Felt of 0.5 cm is used and the neck’s diameter is 1.5 cm. The next 3 samples have the same perforation percentage and the same hole size. The changing parameters are the neck’s diameter, the existence or not of felt, the existence or not of the holes and the combination of fabric. Each one is compared to sample 2. 4. The fourth sample has a neck of 30 mm diameter and felt of 5 mm thickness.

1. 12.5% open, combination of holes

5. At the fifth sample the neck has a 15 mm diameter, covered by a very thin elastic fabric, acting like a ‘‘sieve’’ at the inside part of the neck. 6. The sixth sample has no felt inside, the neck’s diameter is 1.5 cm. 7. The seventh sample has no perforation and no felt in order to find out how the unit works as a helmholtz resonator only. The neck’s diameter is 1.5 cm. This sample is also compared to sample 2. The measurements of each sample follow, as well as the analysis of the results.

Sample 1

Absorption Coefficeint

• which perforation has the better acoustic performance,

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

12.5% ratio, combination of holes

50 150 250 350 450 550 650 750

Frequency (Hz)

product Helix design panel 2. 12.5% open, same holes’ size

3. 17.5% open, same holes’ size as 2

Frequency (Hz)

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

17.5% ratio, one size holes

50 150 250 350 450 550 650 750

Frequency (Hz)

Absorption Coefficient

12.5% ratio, one size holes

50 150 250 350 450 550 650 750

Sample 4

Sample 3

Sample 2 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

4. 12.5% open, same holes’ size as 2, neck’s diameter 30mm

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

12.5% ratio, 30mm diameter

50 150 250 350 450 550 650 750

Frequency (Hz)

Helix panel 5. neck’s diameter 15mm + fabric

6. neck’s diameter 15mm + no felt

Frequency (Hz)

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

no felt

50 150 250 350 450 550 650 750

Frequency (Hz)

Absorption Coefficient

fabric

50 150 250 350 450 550 650 750

Sample 7

Sample 6

Sample 5 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

7. neck’s diameter 15mm + no felt+no perforation

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

no holes

50 150 250 350 450 550 650 750

Frequency (Hz)

Graph 2 tube (new foam Calibration of impedance 10.5 cm)

1.0

0.9

0.8 ...new.foa m.gap.14. 5

0.6 0.5 0.4 0.3

sample 2 new.foam

.800.front.. .new.foa m.front

0.2 0.1 0.0

200

300

400

500

600

700

800

Graph 2 compares empty measurements in the two impedance tubes. The tubes were calibrated with the new foam, at the back of the tubes. It seems that the the circular

CALIBRATION

In order to examine the impact of the calibration, 4 measurements were realised in the hexagonal tube. In the first measurement (Graph 1) the foam of 6.5 is measured twice in the same position, with two different calibrations. It seems that the different calibrations do not affect the results. So the outcome of graph 1 is that the position of the foam during calibration is not that important.

0.6 0.5 0.4

sample 4 new.foam.. .empty

0.3 0.2 0.1 0.0

200

300

400

500

600

700

800

Frequency (Hz) sample 3: circular tube, calibration with original foam at the back of the tube

13.5 cm 20 cm

sample 2: hexagonal tube, calibration with new foam with 13.5cm of gap

sample 4: hexagonal tube, calibration with new foam at the back of the tube

13.5 cm

21 cm

MEASUREMENT

original foam, thickness: 2.5cm

0.7

Frequency (Hz) sample 1: hexagonal tube, calibration with new foam at the back of the tube

new foam, thickness: 6.5cm

sample 3 orig.foam. nogap...e mpty.meas urement.2 0cm.more

0.8

sample 1 new.foam

0.7

Absorption Coefficient

Like with the normal impedance tube, calibration of the equipment is part of the measurement procedure. In the case of the hexagonal tube, there was no material included for the calibration, so foam of 6.5 cm is used for this purpose. For the specific geometry of the tube, the range of frequencies is smaller the the original one. As a result, it is regulated from the settings that the maximum frequency is 800Hz. The proper way to perform the calibration is to remove any air behind the foam. This is possible in the normal impedance tube, as the depth of the tube is adjustable. However, in the hexagonal one this is not possible. There are two available foams, the original one, used for the calibration of the normal impedance tube and the new foam that has unknown material properties and is used for the calibration on the hexagonal tube.

1 with 14.5 cm Measurement ofGraph new foam gap

product Helix design panel

_ CALIBRATION OF IMPEDANCE TUBE

measurement: hexagonal tube, calibration with new foam with 13.5cm of gap

measurement: empty

13.5 cm

21 cm

Helix panel

tube has almost zero absorption when it is empty. On the other hand, the hexagonal tube has a curve with the same shape but with higher values, increased around 0.1. This means that the measurements in the impedance tube have slightly higher values. For more information look at Appendix C. In order to make a more spherical approach, the same material needs to be measured in both tubes (Graph 3). The new foam is put in both tubes with the same distance from the back of 13cm. By comparing the measurements of the same porous material in both impedance tubes, it seems that the shape of the curve has similar shape the negative peaks happen in different frequencies (300Hz and 400Hz). As a result, the measurements from 200Hz - 400Hz are not reliable. However, the rest of the values match relatively well.

_ CONCLUSIONS According to the realised measurements, the hexagonal tube is generally reliable. The calibration process doesnâ&#x20AC;&#x2122;t seem to influence significantly the results. However, the measurements might have slight higher the values than what they are supposed to have according to theory. Also, based on the comparison of the tubes, (first graph in Appendix C) it seems that the measurement of lower frequencies (200Hz- 400Hz) might not be very accurate. From 400Hz and higher the measurement is quite reliable and can be used for estimating the performance of the samples. The positioning of the sample is also important, since it there is gap behind it, there will be probably increased absorption values in low frequencies.

Fig.40: calibration foam in the hexagonal impedance tube

DESIGN QUESTION

product Helix design panel

Do the units target the frequencies that they were designed for? Does felt improve the acoustic performance of the units?

DESCRIPTION OF THE TESTING

RESULTS

Short Unit

Absorption coefficient

Two hexagonal units with different volume and neck dimesions are tested in the hexagonal impedance tube. The short unit has 21 cm of length and 1.5cm of neck diameter. The longer is 23 cm long with a neck of 3cm wide. The targeted frequencies were calculated at 150Hz (short unit) and 250Hz (long unit), based on helmholtz theory. The front side is perforated with felt on the inside which definitely is combined to the absorption behaviour of the unit. Using the equation e f=54* dD with e=perforation ratio=0.06, d=the thickness of the perforated plate=0.0004m and D=the thickness of the cavity behind the plate=0.2m, the peak frequency is calculated at 1478Hz. Which is above the frequency range measured. The short unit was also tested with and without felt. When put in the tube the units are sealed so as they exactly fit in the tube and no leak is created.

• the curves seem to have an expected shape even though the peaks happen in higher frequencies than what was

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Short unit Short unit no felt

calculated Possible reason: the calculations were made by hand using the formula for helmoltz resonators. The shape of the unit doesn’t really clarify how the neck should be considered. At the same time the material is too thin so it might participate in the absorption. the longer unit has a narrower frequency range but almost the same absorption peak. • results from 50Hz-150Hz are not considered valid since the frequencies are so low and therefore can easily be disturbed • felt is improving a lot the absorption of the unit as not only increases the absorption values but also makes the curve more smooth

Frequency (Hz)

Long unit

•

frequency and the values of the measurements might be slightly lower. This might be cause from the material properties of the resonator or from measurement inaccuracies felt significantly improves the absorption performance of the unit

SUGGESTIONS It would be interesting to make measurements of materials with known properties so as to estimate the performance of the tube. Assuming that the results are relatively reliable, a proposal for the design would include a different relationship between the two units’ sizes, so as to provide more broadband frequency range.

Absorption coefficient

CONCLUSIONS

• it seems that the performance of both units is good enough, even though the peak is not exactly at the expected

150 250 350 450 550 650 750

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Long unit

150 250 350 450 550 650 750

Frequency (Hz)

Helix panel

Fig.41: small unit in hexagonal impedance tube

Design (Acoustics Part)

4.1 INTRODUCTION

Matlab

Impedance Tube

comparison of materials

Design

ACOUSTICS

Design

valid form of measurements

comparison of geometries number of straws

radius of straws

absorption performance (class) Fig.39: design process

easy assembly

PRODUCT DESIGN

A successful product except for being cheap and good-looking, needs to have first of all good performance. In order to achieve this, the first part of the design concerns acoustic measurements which will provide some restrictions for the product design. The method that is going to be followed includes simulation with matlab software and comparison to physical models, measured in the impedance tube. The target is by this procedure to have some conclusions on materials and geometries and how they affect the absorption performance and the design procedure. MATLAB (matrix laboratory) is a multi-paradigm numerical computing environment and fourth-generation programming language. Developed by MathWorks, MATLAB is optimized for solving engineering and scientific problems. The matrix-based MATLAB language is the worldâ&#x20AC;&#x2122;s most natural way to express computational mathematics. Built-in graphics make it easy to visualize and gain insights from data ((2017). accessed in 2017, Retrieved from https://www.mathworks.com/products/ matlab.html). In order to take some design decisions, the measurements were organized in a way that could be could be compared in groups and give answers to the following questions.

low cost

production techniques (prototype and mass production)

choice of materials

PROTOTYPE PANEL Fig.42: measurementsâ&#x20AC;&#x2122; methodology

Design

BASE MATERIAL

Which material should be used as a base? Does the thickness of the plastic straw affect the absorption?

MATERIAL

Does cardboard have similar absorption behaviour to plastic? Can coupled tubes be used in order to decrease the thickness of the unit? Does corrugated straw have similar absorptive behaviour to the rigid one?

GEOMETRY

Does its shape affect its performance? Does the inner radius of the tube influence the absorption performance? Does the increase of the number of resonators improve the absorption coefficient as much as the matlab simulation?

OPEN/CLOSE RATIO

Does the distance between same length tubes should be regulated by the cross section absorption? Can broadband absorption be achieved by combining narrowband units? Does the distance between same-length straws affect their acoustic performance?

MINIMUM DISTANCE

Does the arrangement of the buckets affect the acoustic performance? Does the distribution in single units have better acoustic performance than the buckets? Does the use of the hexagonal tube influence the results? Does the arrangement of the buckets affect the acoustic performance in the hexagonal tube?

BIGGER SCALE MEASUREMENTS

Does the plastic cap influences the acoustic performance of the tubes? Is it possible to heat seal the tube without significant change of the acoustic absorption?

SEALING METHODS

Design

4.2 DESIGN PROCEDURE

What mostly influences the design in terms of acoustics is the choice of acoustic principle, materials and geometries. Using the quarterwavelength tubesâ&#x20AC;&#x2122; theory translated in a matlab script, (check appendix D) originally coded by F. Setaki and M. Tenpierik and based on the theory of viscothermal wave propagation in prismatic tubes by Zwikker and Kosten (1949), simulations of various quarterwavelength tubes were realised. The aim is to simulate the samples that will be measured in the impedance tube, so as to be able to compare the results and reach to quite valid conclusions. The simulations/testings are organised in terms of different tube materials and geometries. Each tube is put in the middle of a base, as shown in Figure 39, trying to make the front side as flat as possible. The base is slightly bigger from the inner diameter of the impedance tube (100mm the diameter of the tube, 103mm the diameter of the base) so as to fit exactly at the opening and be in vertical position. In addition, the plate is sealed with blu tack material, so as to make the base as stable as possible and to prevent any air leaks. The aim is to have results as close to theory as possible, in order to be able to control the behaviour of the whole design during the next steps

impedance tube blu tack

base

base tube

tube

impedance tube

Fig.43: impedance tube arrangement

4.3 BASE MATERIAL SELECTION

DESIGN QUESTION

Graph Cardboard base1masses 1.00 0.90

DESCRIPTION OF THE TESTING Two samples with the same straws and different base mass are compared to the matlab simulation performed for a quarter wavelength tube with the same dimensions. The top graph shows the results of 3mm cardboard and 3mm cardboard plus modelling clay. These results lead to the next measurements where the same material is used, with different thickness (MDF 4mm, 8mm). In this chapter only different masses of the base are compared. In order to reach in this result though, more measurements were performed.

Absorption Coeeficient

Design

Which material should be used as a base?

0.80 664

0.60

grey cardboard 2mm

0.50 662

0.40 0.30

664

0.20

grey cardboard 2mm + clay

0.10 0.00

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

RESULTS

• low base mass influences the values of the results • low base mass causes increased absorption in lower frequencies

Matlab simulation

0.70

Tested straw

length: 125mm material: plastic thickness: 0.10mm

Possible reason: the base might vibrate during the lower frequencies and increases the absorption range. • as the mass of the base is increased, the results are more similar to the matlab simulation and as a result more predictable

inner radius: 6.5 mm

Graph Plywood base 2 thickness 1 0.9

• as the mass of the base increases, the less influence it has on the results • MDF of minimum 8mm thickness will be used for the base in the next measurents • the results show good agreement with the matlab script for quarterwavelength tubes

0.8

Absorption Coeeficient

CONCLUSIONS

0.7 0.6 0.5 0.4 0.3

664 662

672

4mm plywood 8mm plywood

0.2 0.1 0 200 300 400 500 600 700 800 900 1000

Frequency (Hz)

matlab simulation

4.4 MATERIAL TESTS

DESIGN QUESTION

Thick andGraph thin plastic 3 straws 1.0

Does the thickness of the plastic straw affect the absorption?

0.9

Two samples with the same dimensions and different material thicknesses are compared to the matlab simulation performed for this radius and length. The base is made of 12mm of MDF.

0.7 matlab simulation

0.6 0.5

Sample thick 2 plastic

0.4 0.3

Design

DESCRIPTION OF THE TESTING

Absorption Coefficient

0.8

thin plastic Sample 1

0.2 0.1 0.0 200 300 400 500 600 700 800 900 1000

Frequency (Hz)

RESULTS

• generally, the curves seem to follow the same path • matlab simulation has a slightly higher absorption peak from the samples.

Possible reason: inaccuracies during the cutting of the straws or the precision of the straws’ geometry • both samples have slightly higher peak frequency Possible reason: the length of the straws might be decreased due to the sealing (6Hz-7Hz of difference = 1,5mm-2mm of different length) • sample 2 has higher absorption performance in average and a small peak around 550Hz. Possible reason: due to the low absorption value the measurement is more vulnerable to inaccuracies

Sample 1 length: 125 mm

Sample 2 length: 125 mm

material: plastic

thickness: 0.05mm

thickness: 0.15mm

inner radius: 3.25 mm

CONCLUSIONS

• the difference in the thickness of the materials doesn’t seem to influence the acoustic performance.

• both materials can be used for the design. However, in order to keep the cost and the weight as low as possible, the thinner plastic is preferred.

DESIGN QUESTION

4 plastic straws CardboardGraph and thin

Does cardboard have similar absorption behaviour to plastic?

1.0

Design

0.9

DESCRIPTION OF THE TESTING Two samples with the same characteristics and different material are compared to the matlab simulation performed for this radius and length. The base is again made of 12mm of MDF.

Absorption Coefficient

0.8 0.7 0.6

0.3

Matlab matlab simulation simulation Sample 3 cardboard

0.2

Sample 4 rigid plastic

0.5 0.4

0.1 0.0 200 300 400 500 600 700 800 900 1000

RESULTS

Frequency (Hz)

• generally, the curves seem to follow the same pattern • sample 4 has slightly higher absorption values from the matlab simulation

Possible reason: the straw might be too thin and it vibrates, there might be a gap of air between the MDF plates or there might be inaccuracies of the tube dimensions • sample 4 has also slightly higher peak frequency Possible reason: the length of the straw might be decreased due to inaccuracy in cutting process or because of the sealing (18Hz of difference = 5mm of different length) • sample 3 has slightly lower peak frequency than matlab simulation Possible reason: the length of the straw might be slightly longer (12Hz of difference = 3mm of bigger length)

CONCLUSIONS

• both materials can be used for the design. However, the plastic straw seems to have slightly better performance whereas the one from cardboard is more sustainable but more expensive and harder to manufacture. So the plastic straw is going to be further developped.

Sample 3

length: 143 mm

Sample 4

length: 143 mm

material: cardboard

material: plastic

thickness: 0.10mm

inner radius: 2.25 mm

4.5 GEOMETRY TESTS

DESIGN QUESTION

Graph 5 Coupled tubes configurations 1.0

Can coupled tubes be used in order to decrease the thickness of the unit?

0.9

DESCRIPTION OF THE TESTING The samples are compared in two ways. First in terms of ratio between the length of the two different straws (samples 5,7,8) and then in terms of positioning the thin straw in the wider one (samples 5,6).MDF of 12 mm is used for the base.

0.7 0.6 0.5

sample 54

0.4

sample 65

0.3

sample 76

0.2

sample 87

0.1 0.0

Design

Absorption Coefficient

0.8

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

RESULTS Sample 8 length1: 50mm

length2: 43mm

length2: 71.5mm

length2: 93mm

material: plastic

thickness: 0.10mm

inner radius1: 2.25 mm

inner radius2: 3.25 mm

143 mm

length2: 43mm

50 mm

• the results are not fully trustworthy as there is lack of

Sample 7 length1: 71.5mm

143 mm 71.5 mm

CONCLUSIONS

Sample 6 length1: 100mm

103 mm 100 mm

absorption but sample 6 targets higher frequency Possible reason: putting a straw into another decreases the total length which might cause the higher frequency target • by comparing samples 5,7,8 seems that as the length of the wider tube decreases, so does the absorption • according to F.van der Eerden, 2000, two peaks would be expected, however there is only one peak. Possible reason: the ratio between the tubes diameters might not be the optimum

Sample 5 length1: 100mm

143 mm 100 mm

• it seems that samples 5,6 have almost the same

litterature that could help in analysing them

• the coupled tubes geometry seems not to reduce the total length while aiming the same frequency, so it won’t be further developed

DESIGN QUESTION

6 Rigid andGraph corrugated straws 1.0

DESCRIPTION OF THE TESTING There are two samples that have plastic straws with the same radius and length and different geometry. They are both compared to the matlab simulation.The base is from 12mm of MDF.

0.9 0.8

Absorption Coefficient

Design

Does corrugated straw have similar absorptive behaviour to the rigid one? Does its shape affect its performance?

0.7 0.6 0.5

matlab simulation sample 9 corrugated plastic sample 10 rigid plastic

0.4 0.3 0.2 0.1 0.0 200 300 400 500 600 700 800 900 1000

RESULTS

Frequency (Hz)

• The rigid straw has measured almost the same absorption as calculated with matlab simulation, just

in slightly higher frequency. Possible reason: the sealing or the cutting process decreased slightly the length (16Hz difference means 4mm of difference) • the corrugated straw aims at higher frequency of 64Hz more. This difference translated in length is 14 mm. Possible reason: the shaping process decreased the length of the straw and combined with the sealing, the total length of the straw decreased this much

CONCLUSIONS

• corrugated straw behaves the same as the rigid one • the rigid straw is prefered to the corrugated one since the lack of accuracy during the shaping of the last one might influence a lot the acoustic performance.

Sample 9

length: 143 mm

Sample 10 length1: 50mm

material: plastic

thickness: 0.10mm

inner radius2: 2.7 mm

4.6 PERFORMANCE PARAMETERS

Design

L = 0.143 length of tube (m) n = 1 number of resonators rsample = 0.05 radius of cylindrical test sample (m)

R=7mm R=6mm R=5mm R=4mm R=3mm R=2mm

Fig.44: influence of radius in the absorption coefficient

Aiming at the wanted frequency and making the absorption curve as high and wide as possible are the two factors that need to be controlled. The targeted frequency is easily controlled by the length of the tube. On the other side, the peak value and the width of the curve can be controlled by either chosing the appropriate radius or the appropriate number of resonators. In terms of product design, increasing the diameter of the tube means that less tubes are needed in total, which decreases the amount of material used and makes the assembly easier. However during this period of time it was quite hard to find straws with diameter more than 7mm, within also the budget range. So in order to proceed with the construction of a prototype, it was decided to use the straws with the biggest diameter (7mm) I found massively sold and increase the amount of tubes. Also by increasing the number of straws there is a small advantage of getting a slightly wider absorption curve.

Regardless of the already mentioned approach, I wanted to investigate whether the wider straws have similar behaviour to the matlab simulation for wider radii and whether many resonators with the same length can create the same absorption. So, the next measurements compare straws with different radii (samples 11, 12) and different number of resonators (samples 13, 14).

L = 0.143 length of tube (m) R = 0.003 radius of resonator (m) rsample = 0.05 radius of cylindrical test sample (m)

n=6 n=5 n=4 n=3 n=2 n=1

Fig.45: influence of the amount of resonators in the absorption coefficient

4.7 RADIUS COMPARISON

Radius of 7 0.48 Graph

DESCRIPTION OF THE TESTING There are two samples that have plastic straws with different radius and same length and material. They are both compared to matlab simulation for tubes with the same dimensions. The base is from 12mm of MDF.

0.9

0.8

0.7 0.6 0.5

matlab

0.4

sample green 11 straw

0.3 0.2 0.1 0

RESULTS

• Sample 11 has almost the same absorption with matlab simulation, just a slightly higher frequency. Possible reason: the sealing or the cutting process decreased slightly the length (16Hz difference means 1.5mm of difference) • The same happens with sample 12, since there is a slight difference in frequency (2 Hz). Possible reason: the sealing or the cutting process decreased slightly the length of the straw

CONCLUSIONS

• for the specific length and ratio of surface/resonator

area it seems that the wider straws offer better acoustic performance with one resonator. Considering the time limit though and the availability of materials, this option is not going to be further investigated.

Radius of 0.55 Graph 8

200 300 400 500 600 700 800 900 1000

Absorption Coefficient

Does the inner radius of the tube influence the absorption performance? Absorption Coefficient

Design

DESIGN QUESTION

0.7 0.6 0.5 0.4

matlab

0.3

sample 12 pink straw

0.2 0.1 0

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

Frequency (Hz) Sample 11 length: 143 mm material: plastic thickness: 0.10mm inner radius: 4.8 mm

Sample 12 length1: 143mm material: plastic thickness: 0.10mm inner radius: 5.5 mm

4.8 AMOUNT OF RESONATORS

DESIGN QUESTION Different Graph number9of resonators

Does the increase of the number of resonators improve the absorption coefficient as much as the matlab simulation?

1.0

Design

0.9

DESCRIPTION OF THE TESTING There are two samples with the same straws. The first measurement is with one straw and in every next measurement a straw is added. The base is again made of 12mm of MDF.

Absorption Coefficient

0.8 0.7 0.6 matlab 13 1

0.5 0.4

matlab 14 matlab 4

0.3

sample 14 number 4

0.2

RESULTS

0.0

200 300 400 500 600 700 800 900 1000

• generally, the curves seem to follow the same pattern • the values are in general slightly higher than in simulation

Possible reason: the straw might be too thin and it vibrates or there is a gap of air between the MDF plates • sample 14 has much higher peak absorption coefficient value • the measurements have also a wider absorption curve Possible reason: the lengths of the straws might be slightly different, so the frequency range is slightly wider

sample 13 number 1

660 Hz

0.1

Frequency (Hz)

Sample 13 length: 125 mm material: plastic

Sample 14 length: 125 mm material: plastic

inner radius: 3.25 mm

number of resonators: 1

number of resonators: 4

CONCLUSIONS

• using more resonators of the same length in order to increase the absorption is possible • the distance between them will be also further investigated

4.9 CROSS-SECTION ABSORPTION Graph 10cross-section Absorption

DESIGN QUESTION

DESCRIPTION OF THE TESTING According to the literature about the absorption cross-section (p. 28) each resonator has a limit in the area that contributes to sound absorption. Three samples are compared in terms of the distance between the tubes. Sample 15 has exactly the same distance as the maximum distance resonators can have. In sample 16 the distance is bigger than that.The base is once more made of 12mm of MDF. The matlab simulation is performed for only one resonator.

Absorption Coefficient

Design

Should the distance between the tubes of the same length be regulated by the absorption cross section?

1 0.9 0.8 0.7 sample 16 61..6.5.pl astic 2>d

0.6 0.5 0.4 0.3

sample 15 61..6.5.pl astic 2=d

0.2 0.1 0 500 600 700 800 900 1000 1100 1200 1300 1400 1500

Frequency (Hz)

RESULTS

• when the distance was increased (sample 16) there was a slight decrease of the absorption value but still within the inaccuracy of the measurement

CONCLUSIONS

• the change in the distance between the tubes doesn’t seem to influence significantly the absorption performanc

• the cross-section absorption theory won’t be used for the design process

Sample 15 lengths: 61.5 mm

Sample 16 lengths: 61.5mm

material: plastic

inner radius: 2.25 mm

thickness: 0.10 mm

distance: 80 mm

distance: 90 mm

4.10 BROADBAND ABSORPTION

Design

Length of tubes= 0.213m, 0.189m, 0.170m Radius of the resonators = 0.0035m Radius of the total sample area = 0.05m Number of resonators= 8

Fig.46: interval of 25Hz

One of the basic targets of the project is to create a broadband absorption proposal. The absorption curve of each quarterwavelength tube is quite narrow. In order to create a wider absorption range, narrowband units are combined in a way that create a broadband absorptive surface. With the specific acoustic principle and the diameter of the straws (7mm) it seems that in order to keep a high average absorption curve the strawsâ&#x20AC;&#x2122; lengths need to change every 50 Hz. In this way the fluctuations of the absorption curve will stll have values higher than 0.6. achieve a smooth absorption curve. Decreasing more the inverval though in 25 Hz it is possible to achieve a smooth absorption curve with values over 0.9. The result of the combination of resonators with different lengths is shown in the figure with the blue dashed line.

So the following measurements deal with the amount of resonators put in the impedance tube and observe whether they behave accordingly to the matlab simulation for the same specifications. In further research the distribution of intervals could be made in octave band. A band is defined when has the width of an octave and the upper band frequency is twice the lower band frequency. Right now the intervals are equally distributed every 25Hz or 50Hz. This means that for the whole frequency range the resonators are distributed in the same way. If though the distribution is based on octave band, more resonators will be used in lower frequencies, where they might be needed more, and less in higher frequencies.

Length of tubes= 0.2m, 0.189m, 0.18m Radius of the resonators = 0.0035m Radius of the total sample area = 0.05m Number of resonators= 8

Fig.47: interval of 50Hz

Graph Graph1111 0.9

Can broadband absorption be achieved by combining narrowband units?

0.7

DESCRIPTION OF THE TESTING Six straws with different lengths are placed in the same testing so as to be compared to the matlab simulation performed for this number of resonators and radius and length. The base is again made of 12mm of MDF.

0.8

Absorption Coefficient

Design

1.0

DESIGN QUESTION

0.6 0.5

matlab

0.4 sample 17 broadband2

0.3 0.2 0.1 0.0

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

RESULTS

• most of the peaks are almost on the same frequencies as the matlab simulation • the peak of 700Hz is missing from the absorption curve

Possible reason: the tube aiming that frequency might have inaccuracies in its geometry • there is a small peak around 550Hz Possible reason/; inaccuracies during the measurement might create this effect

Sample 17 lengths: 115, 110, 105, 100, 95, 90 mm material: plastic inner radius: 3.5 mm thickness: 0.10 mm number of resonators: 6

CONCLUSIONS

• combining straws with different lengths can lead to the production of a broadband aborption unit, so this is going to be further investigated

• the appeared peak around 550Hz is not reliable and is neglected

4.11 TUBE ARRANGEMENTS Different Graph bucket12 distribution

DESIGN QUESTION 1

Does the distance between same-length straws affect their acoustic performance? Does the arrangement of the buckets affect the acoustic performance?

Two samples with buckets of straws of same length are compared to the matlab simulation. The frequency range is 400 Hz - 950 Hz, by 50 Hz of increase. The base is from 2mm of cardboard and extra layer of clay, in order to provide the necessary mass.

matalab simulation

0.7 0.6

sample 18 450-1100Hz quarterwavelength

0.5 0.4

sample 19 450-1100Hz New arrangement

0.3 0.2 0.1 0

Design

0.8

Absorption Coefficient

DESCRIPTION OF THE TESTING

0.9

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

RESULTS

• The peaks mostly happen at the same frequencies as the matlab simulation. However, there are

some slight differentiations Possible reason: the sealing or the cutting process decreased slightly the length of the straws • the arrangement of the straws affects the performance with a non predictable way Possible reason: the distance between straws with almost the same length might cause disturbances in the absorption • the lower frequencies (around 300Hz) there is a peak that doesn’t comply with the matlab simulation Possible reason: the base is from cardboard and even though there is clay for providing mass, it might have taken part in the vibration

Sample 18 arrangement: groups of 7

Sample 19 arrangement: groups of 7

material: plastic

inner radius: 3.5 mm

thickness: 0.10 mm

CONCLUSIONS

• it seems that even if the straws with the same length are very close to each other, the results are quite • •

good. The distance between the same length straws might though influence the performance, so this is further investigated with the following samples. the distribution between the buckets seems to influence the results, which means that the acoustic performance might have unexpected values. the arrangement in buckets though can be used in the design, because it makes the assembly easier. So it is going to be further developped.

Bucket andGraph random 13distribution

DESIGN QUESTION 1

Does the distribution in single units have better acoustic performance than the buckets?

0.9

DESCRIPTION OF THE TESTING Two samples are compared, one having the same straws arranged in buckets and the other one having them randomly distributed. The frequency range is 400Hz - 950Hz, by 50Hz of increase. The base in both cases is made of 2mm of cardboard and clay.

Absorption Coefficient

Design

0.8

matalab simulation

0.7 0.6

sample 19 450-1100Hz quarterwavelength sample 20 450-1100Hz evenly distributed

0.5 0.4 0.3 0.2 0.1 0

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

RESULTS

• the performance of sample 20 is clearly better than sample 18 and even from matlab simulation • peaks are mostly at the same frequencies coming from matlab • the curves have smoother fluctuations than the matlab curve

CONCLUSIONS

• even though sample 20 performs much better, it won’t be further developped as in terms of product design is nowhere close to modularity

• the random distribution might also have unexpected results

Sample 19 arrangement: groups of 7 Sample 20 arrangement: single units material: plastic

material: plastic

inner radius: 3.5 mm

thickness: 0.10 mm

4.12 SCALED MEASUREMENTS

DESIGN QUESTION

Comparison of sample's position Graph 14

Does the use of the hexagonal tube influence the results?

1.0

0.5 0.4

• the absorption curve doesn’t show any significant peaks at the expected frrequencies,

Possible reason: the straws might have small differences in lengths so the absorption curve doesn’t have many fluctuations or there is the possibility of • in the lower frequencies (200Hz-500Hz) there is also almost maximum absorption even though there are no resonators Possible reason: there are gaps between the straws so the gap behing the straws acts as helmholtz resonator. The geometry of the tube might also influence the results.

matlab matlab simulati on

0.3 0.2

800

750

700

650

600

550

500

450

400

350

300

0.1 0.0

RESULTS

sample back221

0.6

Design

0.7

250

Two samples with buckets of straws of the same length are compared to the matlab simulation. The frequency range is 500 Hz - 800 Hz, by 50 Hz of increase. The base is from 8mm of MDF. The hexagonal tube is used in order to test a bigger sample.

0.8

200

DESCRIPTION OF THE TESTING

Absorption Coefficient

0.9

Frequency (Hz)

Sample 21 arrangement: buckets material: plastic inner radius: 3.5 mm thickness: 0.10 mm number of resonators: 61

CONCLUSIONS

• the hexagonal tube probably influences the quality of the results. Especially in the lower frequencies (200Hz-400Hz) the results might not be valid due to calibration process (p. 67)

• it seems that the groups of straws provide optimum absorption performance, within a broadband range, even though the values are much better than the matlab simulation

• the arrangement in buckets will eventually be used in the final design

4.13 SEALING MEASUREMENTS

DESIGN QUESTION Comparison of plastic Graph 15 caps

Does the plastic cap influence the acoustic performance of the tubes?

Design

1.00

0.60

• the performance of both samples is almost the same • there aren’t once more any significant peaks • the curves have smoother fluctuations than matlab curve

possible reason : the material is light so it takes part in the absorption, the straws might have slight length differences, so the result is smoother or the space between the straws acts as resonator • in lower frequencies (around 350Hz) there is still the curve of much higher values than the matlab simulation possible reason : the base is made of MDF of 8mm and some air might be trapped between the plates that vibrates in lower frequencies. The groups of straws are not sealed between them, so there are slight gaps that might act as necks for a helmoltz resonator (which is the space behind the sample). At last, something occured with the settings during the calibration process might have affected the results.

CONCLUSIONS

• the absorption in lower frequencies (200Hz- 400Hz) are not considered very reliable • the plastic cap doesn’t seem to influence significantly the results, so it can be used in the design process

matlab matlab simulatio n plastic 22 sample cap

0.50 0.40 0.30 0.20

800

750

700

650

600

550

500

450

400

350

300

0.10 0.00

RESULTS

sample back2 21

0.70

250

The two samples compared have different sealing methods and at the frequency range is 400Hz - 950Hz, by 50Hz of increase. The base in both cases is made of 4mm of MDF.

0.80

200

DESCRIPTION OF THE TESTING

Absorption Coefficient

0.90

Frequency (Hz)

sample 21 arrangement: buckets

Sample 22 arrangement: buckets

material: plastic

inner radius: 3.5 mm

thickness: 0.10 mm

number of resonators: 61

Design

DESIGN QUESTION

Graph 16 Sealed straw

Is it possible to heat seal the tube without significant change of the acoustic absorption?

DESCRIPTION OF THE TESTING Two samples are sealed by making one edge flat. Since a heat sealer was not available, the one straw is sealed by glue and tape and for the other one only tape is used. The base is from 12mm of MDF.

Absorption Coefficient

Design

1 0.9 0.8 0.6

• the frequency targeted is the same in all cases • sample 25, which has no glue has lower performance

Possible reason: the tape is quite light and might didn’t seal tight the straw • saple 26 on the other hand has higher absorption value than matlab simulation Possible reason: the glue made stiffer the sealing of the tube or the tapering of the tube created more friction

CONCLUSIONS

• it seems that the heat sealing can be used in the design as it can behave pretty well under the condition that it is properly sealed and there is no leakage, so it should have similar result to sample 28

sample sealed 23 straw 6.5 125mm

0.5 0.4 0.3

sample sealed 24 straw 6.5 125mm new2

0.2 0.1 0

RESULTS

matlab sealed straw

0.7

200 300 400 500 600 700 800 900 1000

Frequency (Hz) Sample 23 length: 125 mm

Sample 24 length: 125 mm

material: plastic

inner radius: 2.75 mm

thickness: 0.10 mm

sealing: tape

sealing: glue and tape

Design

4.14 INFLUENCE OF SPACE BETWEEN THE TUBES

calculation a new, bigger radius of 3.8 mm is used (left figure). The graph on the right bottom shows that the influence is not that important. For the halfwavelength tubes, the procedure is the same, gaps just increase slightly the absorption surface which increases slightly the absorption values.

1 0.9 0.8

Absorption Coefficient

The space between the straws behaves as tube resonators as well. When quarter wavelength tubes are sealed separetely at the back side, the space formed between the straws, as it has both open ends, behaves as halfwavelength resonators. They have the same length as the straws (the straws are quarterwavelength) so the frequency targeted by the gaps is 2 times higher. This means that the design is not influenced by that. However, when the unit has quarterwavelength tubes and is sealed together, the gaps between the straws (red outline at the graphs above) behave as resonators as well. The area of absorption measured till now is only the area of the straw (R=3,5 mm). In order to figure out whether they influence the total absorption, the gap in the between is divided in every strawâ&#x20AC;&#x2122;s area (middle figure). So for the

0.7

no gaps

0.6 0.5 0.4

with gaps

0.3 0.2 0.1 0

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

4.15 CONCLUSIONS

BASE MATERIAL

Which material should be used as a base?

Design

• MDF min. 8mm

MATERIAL

Does the thickness of the plastic straw affect the absorption?

• thin plastic straw

Does cardboard have similar absorption behaviour to plastic?

GEOMETRY

Can coupled tubes be used in order to decrease the thickness of the unit?

• rigid plastic straw

Does corrugated straw have similar absorptive behaviour to the rigid one? Does its shape affect its performance?

OPEN/CLOSE RATIO

Does the inner radius of the tube influence the absorption performance?

• 7mm diameter • 223 straws per unit

Does the increase of the number of resonators improves absorption coefficient as much as in matlab simulation?

Does the distance between same length tubes should be regulated from cross section absorption?

MINIMUM DISTANCE

• no minimum distance is kept

Can broadband absorption be achieved by combining narrowband units? Does the distance between same-length straws affect their acoustic performance? Does the arrangement of the buckets affect the acoustic performance? Does the distribution in single units has better acoustic performance than the buckets? Does the use of the hexagonal tube influence the results? Does the arrangement of the buckets affect the acoustic performance in the hexagonal tube? Does the plastic cap influences the acoustic performance of the tubes? Is it possible to heat seal the tube without significant change of the acoustic absorption?

BIGGER SCALE MEASUREMENTS • the approach works in 2 different scales, so should work on 1:1 scale

SEALING METHODS

• plastic cap & heat sealer: both good in acoustics

Design (Product Part)

Design

5.1 INTRODUCTION

TIMELINE

PRINCIPLES FROM ACOUSTIC MEASUREMENTS The conclusions withdrawn from the acoustic measurements are going to be used for the development of the new product. Different designs were developped during the whole process. Considering though the amount of resonators needed for a good acoustic performance, the final design is based on exactly this characteristic. The straws form each unit and they work as a solid mass, able to carry also its own weight. The straws used are white - transluscent of 3.5mm radius

and 24cm of total length. They are chosen because of the transluscency that they provide, as well as the slightly bigger dimensions that could be found in a low price (1000 pieces for 4 euros). It was already mentioned that for achieving broadband absorption, different lengths of straws are needed for each frequency. This means that in terms of spatial configurations, the absorptive surface extends in 3 dimensions. In terms of acoustis, this means that there is no longer normal incidence

but random incidence. This means that the performance will probably be slightly better than what is measured. At the same, the surface of the absorber might have scattering behaviour.

Design

5.2 DESIGN INFLUENCE

25cm Fig.48: different lengths of straws

DESIGN IN DIFFERENT SCALES The approach of the measurements was to test how the product behaves in different sizes. The normal impedance tube and the hexagonal were both used in order to achieve this. The first design concept was to have in the same hexagonal unit straws with different lengths (Fig.37). However, in order to facilitate the assembly and the sealing of the straws, each unit will have the same straws. The acoustic principle stays the same in both cases, so the choice concerns clearly the assembly part.

Fig.49: small scale

Fig.50: bigger scale

The acoustic results are based on the ratio of open/closed surfaces. The resonators are put at a sample area, to which the total area of each type of resonators is compared. Till now the sample area is considered the circle that has the same surface as the hexagons in use. Since the units are placed in the room in reality, the final design proposal takes into consideration different sizes of walls and the influence in the density of the resonators. The final design includes 19 different units. Each unit has same length straws that aim at the same frequency. The range of frequencies is from 350Hz to 900Hz raised by 50 Hz. The reason behind this pattern is to try make sure that the units are mostly equally distributed in the space, which offers better acoustic performance than putting all the same units close. However, the different colours in Figure 39 mark only the type of the resonator that is put in the specific place. This means that with the same pattern different combinations can be achieved. In order to make an interesting visual a combination of quarter wavelength and half wavelength tubes are used for the final proposal. The space between the straws behaves as tube resonators as well. Quarter wavelength tubes are sealed separetely at the back side. This means that the space formed between the straws, as it has both open ends, behaves as halfwavelength resonators. They have the same length as the straws (the straws are quarterwavelength) so the frequency targeted by the gaps is 2 times higher. This means that the design is not influenced by that. For the halfwavelength tubes, the gaps just increase the number of the tubes which means that they increase slightly the absorption values. For more information look at Appendix....

Design

DESIGN IN SPACE

1/4 wavelength tubes, open side 1/4 wavelength tubes, closed side 1/2 wavelength tubes

Fig.51: 1:1 scale

Design

5.3 CUSTOMIZABILITY

One of the big advantages of this product is the possibility of adjusting to different spatial and acoustic needs. The users can can have multiple variations of the panel according to their needs. The options they have are:

• • • • •

frequency range acoustic performance different geometries area of application (in front of a wall or space divider) colour of straws

According to the needs of the user, the pattern can change. Figure 40 is an example of pattern created by the specific range of frequencies and absorption needs. In case though that the frequency range changes, the pattern can adjust, like figures 41-43.

100

Fig.52: pattern of repetition

5.4 CUSTOMIZED ABSORPTION

Fig.53: front side, 300Hz - 750Hz

Fig.54: both sides, 300Hz - 500Hz

1/4 wavelength tubes, open side

1/4 wavelength tubes, closed side

1/2 wavelength tubes

Indicative Performance

Absorption Coefficient

0.6 0.4 0.2

0.8 0.6 0.4 0.2 0

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

Absorption Coefficient

0.8

Fig.55: front side 600Hz-1000Hz

1/4 wavelength tubes, open side

Indicative Performance Absorption Coefficient

According to different acoustic needs of each space, it is possible to have different configurations. The three figures on the right are examples of proposals accoring to the placement of the panel and the acoustic performance. The targeted frequencies change every 25 Hz, in order to decrease the fluctuations of the curves. Quarterwavelength tubes are sometimes put from both sides (open and closed) so as to make sure that the panel can be used from both sides. Figure 53 shows a combination of units that can be put in fron of a wall and has quite a broadband absorption. This means that the maximum thickness of the surface is almost 30 cm. Quarterwavelength tubes are used in combination with halfwavelength tubes in order to create a more interesting visual effect. The maximum thickness of the combination in figure 54 is the same. However the range of frequencies is much narrower but it can work from both sides. Since the target is low frequencies, only quarter wavelength tubes are used so as to keep the thickness as low as possible. The last one, figure 43, targets medium and high frequencies. Both surfaces have absorptive behaviour and the maximum thickness is 15 cm in this case. In this case only quartewavelength tubes are used, so as to keep the thickness as low as possible.

Design

ACOUSTIC VARIATIONS

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

0.8 0.6 0.4 0.2 0

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

101

Design SAME PATTERN - DIFFERENT SOLUTION These two figures are based on the same pattern of figure 39. This means that they target the same range of frequences, but offer two different combination of units. This is an example of having multiple posibilities even when having a specific pattern that is followed. The maximum thickness is 24cm and the acoustic principles used are quarterwavelength tubes for frequencies 350Hz-650Hz and halfwavelength tubes for frequencies 700Hz-850Hz. The combination of the two principles gives the advantage of two different visual effects, as the quarterwavelength tubes are closed from one side and the halfwavelength tubes are open from both sides.

SPATIAL GUIDELINES

Fig.56: arrangement 1

Fig.57: arrangement 2

1/4 wavelength tubes, front side

1/4 wavelength tubes, back side

1/2 wavelength tubes

Indicative Performance

102

Absorption Coefficient

The needs of the absorption performance vary according to the use of space or the noise produced. The users are free to chose ‘‘how much’’ absorption they want according to the amount of money they want also to spend. On figure 46 it is shown how different amounts of units influence the absorption coefficient. In order to achieve Absorption 2 in every m2 of wall surface 3 units per frequency should be placed.

Absorption Coefficient

1 0.8 0.6 0.4 0.2 0

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

0.8 0.6 0.4 0.2 0

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

Design

5.5 PERFORMANCE GUIDELINES

4.00 m 1 0.9 0.8

3.00 m

Absorption Coefficient

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 200

300

400

500

600

700

800

900

1000

Frequency (Hz)

absorpion 1= 45 units = 165- 330 € absorpion 2 = 35 units = 130- 260 € Fig.58: absorption surface according to performance needs

absorpion 3 = 25 units = 95- 190 €

103

Design

5.6 BIG-SCALE MEASUREMENT

Fig.59: PD Test Lab, source: Jeroen van Veen

In order to make a measurement in bigger scale and in a real space, a budget is given to the project from TU Delft to make a bigger model. However, since the time and budget were limited, the surface made is 0.88m2, including 95 units. Since the surface is absorbing from both sides, it means that calculated surface is 1.76m2. Each one has 217 straws and aims a specific frequency. In total, the frequency range targeted is 675Hz-1125Hz with interval of 25Hz. In order to facilitate the manufacturing process, which is completed by hand, the principle chosen is the half-wavelength tubes, that doesn’t need sealing. Moreover, an aim of the measurements is to decrease possible inaccuracies during measurement, so the frequency range examined is medium and high frequencies (675Hz-1125Hz). As it was mentioned in previous chapter, the room

104

should be small with reflective surfaces. From the already conducted research for Helix project, it seemed that is really hard to find an appropriate room to use for measurement. So it was decided to pay more attention to a space that has a small size and possible future use. PD Test Lab, that was recently constructed, fullfills these criteria and even though the walls are made from OSB (which is not very reflective) will be used as a reverberation chamber and a possible study case for further research. The Delft Product Development (PD) Test Lab was constructed in the courtyard of the Faculty of Architecture and the Built Environment. It is an example of innovative construction that will provide a home for research into new building innovations in the future. Students and researchers can use it for everyday working practice based on the

principle ‘learning through making’. The production of the space goes along with the idea of innovation in this project, so right now it will be used as a reverberation chamber but from the analysis of the results it can be shown whether it could be a real study case (the difference is that in a proper study case the analysis of the space would come first and then the appropriate product would be developped. In this case however, the product is already made and PD Lab is

Design 2.80m 2.73m 2.73m

2.73m

Fig.60: schematic section of PD Test Lab, source: Jeroen van Veen

2.73m

Fig.61: schematic section of PD Test Lab, source: Jeroen van Veen

Fig.62: measurement equipment in PD Test Lab

used for the measurement. The dimensions of the PD Lab are 3.15m x 5.77m and 5.45m of maximum height. The whole volume of the space is 35.14 m3. The procedure of the measurements is the same as the one performed in the restroom: the equipment is put in the space and it is measured with and without the absorbing panel. In order to have as much as possible reliable results, 12 measurements took place, with 2 different speaker positions and 3 different microphone positions. Each of these measurements was realised with and without the absorbing panel. According to the size of the space and an average reverberation time from the measurements, it seems that the results from 250Hz and more are reliable.

f=2000

f= 2000

T60 V

(Eq.16)

0.53 = 250 Hz 35.14

T60=reverberation time (s) V=volume of the room (m2)

105

1.10 m 0.55 m 0.60 m

2.87 m

4.00 3.00

2.00

1.00

0.00

1.28 m

125

250

500

1000 2000 4000

-1.00

0.86 m

1.28 m 0.86 m

1.28 m

0.60 m

Graph 1 5.00 Absorption Coefficient

1.10 m

2.00 m

Design

Frequency (Hz)

speaker microphone sample

Asample in this case is double the surface of the sample, since it works as an absorber from both sides a=

Afilled- Aempty Asample

(Asample =2x0.88=1.76m ). 2

V V A= 0.16 t=0.16 t A t= reverberation time (s) V=volume of space (m3) A=absorption of surface (m2 sabins) From the reverberation time of the empty space (tempty) the Aempty is calculated. Respectively, for tfilled the Afilled is calculated. Finally the absorption coefficient of the surface is given by the formula:

106

Both graphs show the absorption coefficient calculated in every case for the change of 5dB to25dB. Values from 0Hz to 250Hz are not taken into account. The absorption values should be between 0 (for the lowest) and 1 (for the maximum absorption)

â&#x20AC;˘ From a first analysis of the results it seems that the

graphs donâ&#x20AC;&#x2122;t have the smooth curve with some peaks at the targeted frequencies that would be expected

Graph 1 (grid from 0-1) 1.00

Absorption Coefficient

The equipment used for the measurements is the same as the measurements realised in the restroom (check chapter 3.10, p.54). For every pair of measurements (with and without the panel) the absorption coefficient of the surface is calculated. The Sabine formula is used:

0.80 0.60

0.40

0.20

0.00 -0.20

125

250

500

1000

-0.40

Frequency (Hz)

2000

4000

Graph 2

0.55 m

• it seems that the positioning of the equipment migh

have influenced the results since there is an attic in the room and a big wooden table • the reverberation time in the empty space was already too low and this might be a reason for the low values in some cases • in cases 2a and 1a the absorption values are quite high almost at the expected frequency range. This might be explained due to the close distance of the microphone and the absorption panel. , of the sample is also small for the specific • the surface space. According to ISO 354 the ideal specimen should have 10m2 to 12m2 for 200m3 of space. In this case the space has 35.14m3 volume so the specimen’s area should decrease by the factor (V/200 m3)2/3. 10*(V/200)2/3 = 10* (35.14/200)2/3 = 3.14 m2 The specimen has only 1.76 m2 of surface so it doesn’t reach the lower limit.

3.00

2.00

1.00

0.00 0.80 m

125 0.86 m

2.87 m 0.97 m

1.28 m 0.70 m 0.80 m

0.97 m

0.86 m

0.80 m

0.86 m

1.28 m

0.80 m

1.28 m

0.60 m

4.00

Design

1.10 m

Absorption Coefficient

5.00

As a result, based on these measurements it is not possible to have very valid results. The small surface of the panel has an important defect in the accuracy of the measurements. The type of the room also is not very helpful, since the walls are not very reflective and the reverberation time is already quite low. There are also some surfaces (like the attic or the big table) that affect the measurements. There was also no possibility of being outside during the measurements, so my presence in the room also affected the absorption values. However, it seems that the absorption panel decreases the reverberation time of the room, but it is not clear yet at which level. As it was mentioned before, the optimum way would be to have the necessary surface of at least 3.14 m2 of absorptive surface which could be tested in a real reverberation room. However, based on all the previous simulations on matlab software, the results are quite encouraging and the product should work in 1:1 scale as well.

250

500

1000

2000

4000

-1.00

0.97 m

Frequency (Hz)

Graph 2 (grid from 0-1) 1.00 Absorption Coefficient

2.00 m

0.80 0.60

0.40

0.20

0.00 -0.20

125

250

500

1000

2000

4000

-0.40

Frequency (Hz)

107

5.7 COMPARISON OF PRODUCTS PRODUCTS

PERFORMANCE

PRICE

MASS

80 €/m2

1.6 kg/m3

THICKNESS CUSTOMIZABIL- RECYCLABILITY TRANSPARENCY/ FIBRELESS ITY TRANSLUCENCY

Wall covering

Fig.63: Acoustic wall covering, retrieved from: www.eqacoustics.com/

Absorption Coefficient

Design

1 0.8 0.6

1 cm

0.4 0.2 0

315

400 500 630 Frequency (Hz)

800 1000

Wedge

wedge,

retrieved

from: www.eqacoustics.com/

Fig.65: Versipanel room divider, retrieved

Absorption Coefficient

Fig.64: Acoustic

Absorption Coefficient

1 0.8

layer 1 layer 2 layer 3

0.6 0.4

50-150 €/m2 35-100 kg/m3 2.5/4/5.5 cm

0.2 0

315 400 500 630 800 1000 Frequency (Hz)

Versipanel

1 0.8 0.6 0.4

125 €/m2

30-35 kg/m3

10 cm

20-40 €/m2

50 kg/m3

13-24 cm

0.2 0

315

from: www.versare.com/

STRAWS

400

500 630 800 Frequency (Hz)

1000

Indicative Performance

Absorption Coefficient

Fig.66: STRAWS PROJECT

108

0.8 0.6 0.4 0.2 0

200 300 400 500 600 700 800 900 1000

Frequency (Hz)

The graphs present indicative types of curves. The real performance might be slightly different

Design

5.8 COMPARISON WITH HELIX

PRODUCTS

Fig.68: HELIX

Fig.67: STRAWS STRAWS Indicative Performance

Short unit HELIX

In terms of product design both products are compared to what they can offer. Regarding their performance, both can provide broadband absorption. In lower frequencies HELIX might be more effective in terms of less thickness needed. The straws though act as a dense unit and will have bigger stacking limit than HELIX. It seems though that as a total result straws are more durable than HELIX and can provide a transluscent effect as they are absorbing from both sides. On the other hand, HELIX has more detailed design, thing that makes the assembly a bit harder. To sum up, both options have advantages and disadvantages but have a good absorption performance which means that both can work as acoustic products.

Absorption coefficient

PERFORMANCE

Absorption Coefficient

1 0.8 0.6 0.4 0.2 0

200 300 400 500 600 700 800 900 1000

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Short unit

150 250 350 450 550 650 750

Frequency (Hz)

PRICE

20-40 €/m2

20-50 €/m2

MASS

50 kg/m3

50 -60 kg/m3

STACKING LIMIT CUSTOMIZABILITY RECYCLABILITY TRANSPARENCY/ TRANSLUCENCY FIBRELESS FLAMABILITY BOTH SIDES ABSORPTION

109

Design

5.9 ASSEMBLY

40 split pins

19 packets of 217straws

19 mdf stripes

110

Design

_ASSEMBLY PROCEDURE The assembly procedure of one absorptive surface consists of two steps: one is realised by experts, right after the manufacturing process (steps shown on the page on the left) and the other takes place at the application space from the users. During the first step, a big hexagon of 19 units is assembled by the MDF stripes and the metal splits. As soon as the right shape is made the straws are put in the frame, and the basic form of a 19-unit hexagon is created. In this form the panels are transported to the space of application, where the second step takes place. The big hexagons are combined, as it is shown at the illustrations. The assembly is realised in raws in order to make the connection between the units easier. In order to stabilise the whole structure, the big hexagons are connected by velcro tapes on the sides. Due to the size of

the big hexagons, there are some gaps formed (diagram 2, p.111). In order to avoid this, two combinations are going to be also provided, as shown below, one of half the unit and of two units.

These extra units offer the users more flexibility in terms of shaping and provide also a closed shape with more stability. In this way a limitless surface with many different configurations can be formed.

Fig.69: velcro tape retrieved from: cy.rsdelivers.com

111

Design

5.10 MANUFACTURING TECHNIQUES

PRODUCT PARTS straws

The prototype is usually made with the available materials and the processes that are more available and cheap for such a small production. In the case of straws, they are bought in a standard size and cut in the proper length bu hand. In mass production though they can have the dimensions needed (like a bit bigger diameter 8mm9mm and lengths according to the frequency target). A secondary process in needed though for cutting them in the right size. The sealing process takes place after cutting the straws. This process is chosen because there is no need for extra material or extra piece for the assembly. During the prototype procedure the straws were sealed manually with the help of a lighter and a hairgrip. Finally the manufacturing of the frame includes a piece of MDF or 0.4 mm that is folded and locked in shape. The easiest way for the prototype was to use the laser cutter of the university that engraves along the folded sides and cuts the connection points. During mass production though a die cutting maching would make the whole process much faster and cheaper

112

sealing

PROTOTYPING

MASS PRODUCTION

prefabricated + cut by hand

polymer extrusion

heating sealer

glued or heated with lighter manually

converter: transforms electrical energy to mechanical vibration

booster: amplifies vibration and serves as a mount

anvil: supports tube during sealing

horn: applies ultrasonic vibration to tube

power supply: converts standard 50/60 Hz electrical energy to 30000 cycles per second

frame

60Hz

die cutting

cut with laser cutter

press creasing rule steel rule die creasing channel

plywood substracts ejection rubber sheet material press

Design

5.11 IMPROVEMENT OF DESIGN PROCESS

Fig.71: honeycomb core, 7mm diameter

Fig.70: honeycomb core, 7mm diameter

Fig.72: honeycomb core, 3mm diameter

Replacing straws with honeycomb cores would be another solution for the final product that offers simpler mass production , faster assembly and better stability. This option was recently discovered, so there was no time for adjusting the existing design to this product. Honeycomb cores range from paper and card for low strength and stiffness to plastics or aluminum for high strength and stiffness, according to their use. Thermoplastic honeycombs are usually produced by extrusion and sliced into the desired thickness. They are usually used for

manufacturing of sandwich panels, so sealing from one side regarding the STRAWS project, can be quite easy. The skin is usually made of FRP, but can also be almost any sheet material with the appropriate properties, including wood, thermoplastics or sheet metals. The cellcore diameter can vary from 3mm-8mm and the common dimensions are 1220x2400mm. The thickness can also vary from 2mm - 400mm (retrieved from: https:// www.alibaba.com/product-detail/low-density-highstrength-plastic-honeycomb_277032054.html).

Fig.73: wire surrounding middle Fig.74: vertical pole fixed in the units and fixed to base

middle of each unit

Fig.75: exterior frame

Fig.76: triangular base with wire passing through the units

Using honeycomb cores instead of straws offers simpler manufacturing process, much faster assembly as well as much more stable structure. Since the product can be used for dividing space it needs some extra stability in order to prevent it from collapsing. Possible solutions are presented at figures 68-71, even though they need further research.

113

48 cm

Design

5.12 TRANSPORTATION

The maximum dimensions of a unit are 0.52mx0.48m and 0.25m of thickness. Considering an average length of the straws as 18cm means that the average weight of each big hexagon is around 1.7 kg. This means that is quite easy to transfer. There are going to be three possible options of pieces: one with a full hexagon, one with half of it and one with two pieces. Combining all of these it is possible to have a homogenous panel, without any holes and flexible enough for different spacial configurations.

52 cm Fig.78: size of the biggest unit

Fig.77: biggest unit transportation

114

Fig.79: whole hexagon

Fig.80: half hexagon

Fig.81: three double units

5.13 TECHNICAL DRAWINGS

11.00

Design

58.80

4 121.00

7 8

9 3 1

367.00

30.00 150.00

connection detail

back elevation of open unit

side elevation of open unit

scale: 4:1

scale: 1:2

1. 2. 3. 4. 5. 6.

frame in spread 10.00 30.00

scale: 1:2

121.00

front side of straws of 7mm diameter back side of straws of 7mm diameter side view of straws of 7mm diameter 0.4 MDF stripe connection of MDF stripe metal split for connection between the MDF stripes 7. side elevation of metal split 8. overlapping MDF sheets 9. metal split opened 10. engraving line for folding 11. slot for metal split

30.00 150.00

back elevation of sealed unit

side elevation of sealed unit

scale: 1:2

115

Design

front elevation of stacked units

116

side elevation of staked units scale: 1:7

Design

5.14 SPATIAL CONFIGURATIONS

Fig.82: STRAWS placed between offices

Fig.83: STRAWS placed by the wall

Fig.84: STRAWS placed as space divider

117

Design

5.15 DESIGN PROCESS

Before the final design proposal, different solutions were tested. For sealing one side of the tubes two different methods were tried: one 3d printed cap for one single straw and one vacuum formed cap for the whole unit. The first solution was quickly abandoned because it would take a huge amount of time to put all the caps one by one by hand. On the other hand, the vacuum formed cap is an easy solution that could hold the straws together. One disadvantage of this method is the need of a mold, that if is deeper than 1cm-2cm the sides need to have at least 1o - 2o of tilt. This means that the cap will not fit exactly along its whole length at the unit. Another disadvantage is that the cap form is an extra procedure with extra material and needs also secondary process to cut it in shape. This increases the material and the time of construction and in result the total cost. At the same way this type of sealing influences the acoustic performance of the unit as the gaps between the straws behave as resonators also. On the other hand it is still an elegant solution that can be used with the appropriate design. As far as the frame is concerned, one option considered was to make a frame out of straws also. The connections at the edges are 3d printed and each one connects 4 straws. This alternative seems promising but with the specific straw it was quite fragile. A stiffer material should be used, thicker plastic or even metal. In that case though the structure would be heavier and probably more expensive.

118

Fig.85: MDF interlocking frame

Fig.86: MDF interlocking frame

Fig.87: 3d printed cap

Fig.88: 3d printed connection

Fig.89: Vacuum formed cap

Fig.90: straw frame

Design

5.16 1:1SCALE MODEL

Fig.91: 1:1 scale model in PD Test Lab

119

Design Fig.92: 1:1 scale model in Faculty of Architecture, TU Delft

120

Fig.93: 1:1 scale model in Faculty of Architecture, TU Delft

_FURTHER RESEARCH The field of acoustics is very wide and often unexplored. Everyday materials combined to mass production techiques can offer interesting acoustic results with the appropriate research. A new type of acoustic device is proposed that comes in custom sizes and gives freedom to users to satisfy their own needs. Chosing a very common product, the drinking straw, helped in order to keep the production costs low. This already widely used product has a simple 3d hollow shape, and is made out of polypropelene. Both facts make the production quite fast and cheap, advantages that were desirable from the beginning of the project. At the same time, the tube geometry is one of the acoustic principles that can achieve quite high absorption with narrow range. Changing the function of the drinking straws has never been applied in acoustics before so it has a great potential for re-approaching traditional acoustic design with a different set of materials. During literature study, absorbers were further researched as the most appropriate approach for decreasing sound levels in open-plan offices. Quarterwavelength and half wavelentgth tubes were the acoustic principles further developped due to • the high absorption values that can be achieved • fast and low-cost manufacturing process • and personal fascination The next step included constant measurements in the impedance tube and comparison to the acoustics’ theory via matlab software. In this way the most suitable material

and geometrical configuration were chosed in order to proceed with the final design. In the meantime, a bigger inpedance tube was developped in order to measure the HELIX project. The new impedance tube has a hexagonal shape and is also used for measuring straws’ combinations in bigger scale. The research project continued by the design process, during which the conclusions of the previous measurements were used as well as the literature on the manufacturing techniques. For the final proposal, the most important guidelines included, among others, the absorption performance, low-cost, fast assembly. Finally, a 1:1 scale model was realised and tested in PD Test Lab in the Faculty of Architecture, in TU Delft. The first indications show that the panel indeed decreases reverberation time in the room. However, it seems still that the panel’s surface was too small to have valid results. During the manufacturing of the panel it seemed that the assembly wasn’t that easy as it was expected. It demanded a lot of time, accuracy and many working hands. At the same time, the stability of the whole panel is a crucial matter. The result though is more than interesting. From the design aspect, the visual effect created combines a semi-transparent material with direct viewing through it, when standing right in front.

Further research could be applied in decreasing the total thickness of the unit, as by using rigid straws the thickness is at 1/4 of the wavelength of the targeted frequency. This means that for frequencies around 200Hz the thickness of the unit is 42cm. By using more flexible piping this could be achieved, but probably with higher cost.

Design

5.17 CONCLUSIONS

Taking into consideration the geometry of the panel which has different lengths, it is fair to assume that scattering of the sound waves is also produced. As a result, it would be useful to further research on how the arrangements of the straws or the size of the units influence the scattering and if there are any limits to the design guidelines. It would be also very interesting to apply the new product in a study case. In this way, it would be an application of the theory into a specific problem. Furthermore, in order to simplify the manufacturing procedure plastic honeycomb core could be used for improving the whole design process. In terms of assembly, using this material will facilitate a lot the whole procedure while the acoustic performance will probably be the same. Combining the acoustic performance with another principle, like light and visual comfort, makes the proposal even more interesting. Last but not least, performing measurements in 1:1 scale would be very interesting. For this purpose a surface of 10m2-12m2 is needed according to ISO standards, and this was the reason the measurements was not performed during this research project.

121

Literature

_LITERATURE 1.

5. 6.

Cox, T. J., & D’Antonio, P. (2009). Acoustic absorbers and diffusers: Theory, design and application (2nd ed.). London: Taylor & Francis. Cross,N. (2008).Engineering design methods: Strategies for product design (4th ed.). Hoboken, N.J.: Wiley. Hannink M. (2007)Acoustic Resonators for the Reduction of Sound Radiation and Transmission, PhD thesis, University of Twente, Enschede, The Netherlands Hishinuma, Kazuo (2009). Heat Sealing Technology and Engineering for Packaging (1st ed.). Lancaster, PA: DEStech Publications, Inc. p. 267. ISBN 978-1-932078-85-5. Retrieved 27 February 2015. ISO 266:1997, Acoustics Preferred frequencies ISO 9613-1:1993, Acoustics Attenuation of sound during propagation outdoors Part : Calculation of the absorption of sound by the atmosphere ISO 10534-2:1998, Acoustics - Determination of sound absorption coefficient and impedance in impedances tubes- Part2: Transfer-function method IEC 61260, Electroacoustics Octave-band

and fractional-octave-band filters Kuttruff, H. (2009) Room Acoustics, (5th ed.), Spon Press, Oxon, UK 10. Kuttruff, H. (2000) Room Acoustics, (4th ed.), Spon Press, Oxon, UK 11. Kuttruff, H. (1979). Room Hcoustics (2d ed.). London: Applied Science Publishers. 12. N. W. Larsen, E. R. Thomson and A. C. Gade, “Variable low-frequency absorber for multipurpose concert halls”, Proc. Forum Acusticum, 616 (2005). 13. Setaki F., Tenpierik M., Turrin M and van Timmeren A (2012) ‘Acoustic absorbers by additive manufacturing’, Masters’ thesis, TU Delft University of Technology, Delft, Netherlands 14. Templeton, D., & Saunders, D. (1987). Acoustic design. London: Architectural Press. 15. Van Meel, J.J. (2000): The European office: Office design and national context, doctoral thesis 16. Van Veen, J. (2016) : ‘PD_LAB: A File-toFactory envelope’: Masters’ thesis, TU Delft University of Technology, Delft, Netherlands 17. Zwikker, C., & Kosten, C. W. (1949). Sound absorbing materials. New York: Elsevier Pub.

_ PAPERS

18. Vyzoviti

S., Remy N.,: Acoustically Efficient Origami Based Partitions for Open Plan Spaces 19. Carolina Reich Marcon Passero, Paulo Henrique Trombetta Zannin (2012) Assessment of Acoustic Quality in Classrooms Based on Measurements, Perception and Noise Control, Chapter 10 of book ‘‘Noise Control, Reduction and Cancellation Solutions in Engineering’’ 20. Richard W. Harrison, Walter M. Madigosky, Basil Vassos, (1986) Sound Absorbing Acoustic Horns, _ ARTICLES 21. A. A. A Selamet, Circular concentric Helmholtz

resonators, Journal of the Acoustical Society of America, Acoustical Society of America 1997 22. Chae H. Sohn, Ju H. Park, A comparative study on acoustic damping induced by half-wave, quarter-wave, and Helmholtz resonators, in Aerospace Science and Technology 15(8):606-614 · December 2011 23. J. P. Dowling, “Sonic band structure in fluids with periodic density variations,” J. Acoustical

Society of America (5):2539-2543 Âˇ May 1992. S.K., On helmholtz resonators with tapered necks, 2005, Journal of Sound and Vibration, Volume 279, Issues 3-5, pages 1085-1096

24. Tang

36. http://blog.wantist.com/2009/11/you-move-

me-said-the-wall/ 37. http://howtobuypackaging.com/how-much-

does-packaging-cost/ 38. https://acoustic-design.wikispaces.com/ 39. https://www.mathworks.com/products/matlab.

html _WEBSITES 25. http://www.acoustics.com/open_office.asp 26. http://www.google.nl/patents/US3396812?

hl=nl&dq=quarter+wave+tubes#v=onepage &q=quarter%20wave%20tubes&f=false [last visit: 11/06/2012] 27. h t t p s : / / w f t p r i n t a m . w i k i s p a c e s . c o m / Die+cutting 28. http://www.eqacoustics.com/productspage/acoustic-treatment/acoustic-panelling 29. http://www.acousticsfirst.com/acousticalfoam-cutting-wedge-2000.htm 30. http://adamdealva.com/ 31. http://www.ikea.com 32. https://en.wikibooks.org/wiki/Engineering_ Acoustics/Outdoor_Sound_Propagation 33. http://www.sciencemadesimple.co.uk/ activity-blogs/sound 34. http://muranoacoustics.com.au/microperforated-acoustic-timber-panels/ 35. http://www.engineeringtoolbox.com/dry-airproperties-d_973.html

40. http://www.cetec.com.au/services/office-noise.

html 41. http://www.workspacedesign.co.uk/the-

benefits-of-an-open-plan-office/ 42. h t t p s : / / w w w . a l i b a b a . c o m / p r o d u c t detail/low-density-high-strength-plastichoneycomb_277032054.html

product Helix design panel

Appendix

125

Appendix A

_STACKING LIMIT OF HELIX/HAND CALCULATIONS

_STACKING LIMIT OF SUPPORT A Support B

Support A l=0.1m Shell cross section Support A - Cardboard beam made up of a continuous Ushape Support B - Back cap of the box, made of polypropylene, similarly to the rest of the shell Shell - Continuous flat mdf sheet In order to analyze the beam it is broken into two parts.

126

h=0.02m b=0.001m

The standard equations of beam deflection/rotation are used, derive an expression of q as a function of M1 based on the assumption that the two rotations φ1 and φ2 are equal (the corner rotates while its angle remains constant):

Appendix A Only one half of the beam is considered as it is symmetric, and the formulas used account for this symmetry.

Support A in reality, is a continuous cardboard beam in the shape shown below, supported at its two edges. Since it is a continuous beam, the two 120° corners are assumed to remain at this angle, while the whole system deforms. The goal is to calculate the maximum load q the beam can take before yielding. This will be used to derive the maximum limit for allowed stacked modules. Using the densities for PP, cardboard and felt, and their volumes, a unit’s mass M = 1.55kg. Yield stress of cardboard σy = 30MPa accounting for a safety factor of 2, σy = 15MPa is used

Mmax should be less than My. Using My, and the ratio of reaction forces found in the previous section, max number of stacked modules can be found

Mmax should be less than My. Using My, and the ratio of reaction forces found in the previous section, max number of stacked modules can be found The calculations show that 13 modules (including base module) should be the maximum number of modules that can be stacked. This number is within our acceptable range. This calculation is only based on the ability of the cardboard support to not yield, however it is assumed to be the most susceptible part of the module therefore the method should be valid. structural analysis, source: panayiotis hadjisergis, ioanna christia

127

1.60 m

Graph 1(1->11)

0.8 0

Appendix B

_EXTRA MEASUREMENTS OF HELIX PANEL IN THE REVERBERATION ROOM

microphone

Graph 1

10.00

9.00

8.00

Time (s)

2.50 m

0.75 m

speaker

6.00 5.00

full room mic1 0

4.00

7.00

Time (s)

empty room mic1 0

7.00

5.00

1a empty measurement room mic1

4.00

3.00

2.00

1.00

0.00

full room 1b measurement mic1

6.00

0.00

125

250

500

1000

2000

4000

125

250

Frequency (Hz)

empty room, microphone 1

1000

2000

4000

Frequency (Hz)

panel, microphone 1

Graph 1 (1->35)

Graph 1 (1->45)

10.00

9.00

8.00

empty room mic1

6.00 5.00

full room mic1

4.00 3.00

7.00

Time (s)

7.00

5.00

3.00 2.00 1.00

250

500

1000

Frequency (Hz)

2000

4000

full room mic1

4.00

1.00 125

empty room mic1

6.00

2.00

0.00

128

500

0.00 125

250

500

1000

Frequency (Hz)

2000

4000

Graph 2

Graph 2(1->11) 10.00

10.00

9.00

8.00

Time (s)

6.00 5.00

full room mic2 0

4.00

7.00

Time (s)

empty room mic2 0

7.00

full room2b measurement mic2

6.00 5.00

2a empty measurement room mic2

4.00

3.00

2.00

1.00

0.00

125

250

500

1000

2000

4000

125

250

Frequency (Hz)

500

1000

2000

4000

Frequency (Hz)

panel, microphone 2

Graph 2 (1->35)

Graph 2 (1->45)

10.00

9.00

8.00

empty room mic2

6.00 5.00

full room mic2

4.00 3.00

7.00

Time (s)

7.00

Time (s)

empty room, microphone 2

Appendix B

5.00

full room mic2

4.00 3.00

2.00

1.00

0.00

empty room mic2

6.00

0.00 125

250

500

1000

Frequency (Hz)

2000

4000

125

250

500

1000

2000

4000

Frequency (Hz)

129

Graph 3 (1->11)

Graph 3

10.00

9.00

8.00

empty room new source mic2 0 full room new source mic2 0

Time (s)

1.80 m

2.50 m

7.00 6.00 5.00 4.00

0.75 m

3.00

8.00 6.00 5.00 3.00 2.00

1.00

0.00 250

500

1000

2000

empty 3a room new measurement source mic2

4.00

2.00

125

full room 3b new source measurement mic2

7.00

Time (s)

Appendix B

1.60 m

0.00

4000

125

250

Frequency (Hz)

new source empty room, microphone 2

1000

2000

4000

Frequency (Hz)

new source panel, microphone 2

Graph 3 (1->45)

Graph 3 (1->35) 10.00

10.00

9.00

9.00 8.00

8.00

Time (s)

6.00 5.00

full room new source mic2

4.00 3.00

6.00 5.00

full room new source mic2

4.00 3.00

2.00

1.00

0.00

empty room new source mic2

7.00

Time (s)

empty room new source mic2

7.00

0.00 125

250

500

1000

Frequency (Hz)

130

500

2000

4000

125

250

500

1000

Frequency (Hz)

2000

4000

Graph 4(1->11)

10.00

9.00

empty room new source mic1 0 full room new source mic1 0

7.00

Time (s)

1.6

5m 1.6

Graph 4

10.00 8.00 6.00 5.00 4.00 3.00

8.00

full room 4b new source measurement mic1

7.00 6.00 5.00

empty 4a room new measurement source mic1

4.00 3.00

2.00

1.00

0.00

125

250

500

1000

2000

125

4000

250

Frequency (Hz)

500

1000

2000

4000

Frequency (Hz)

new source panel, microphone 1

Graph 4 (1->35)

Graph 4 (1->45)

10.00

9.00

8.00

6.00 5.00

full room new source mic1

4.00 3.00

6.00 5.00

full room new source mic1

4.00 3.00

2.00

1.00

0.00

empty room new source mic1

7.00

Time (s)

empty room new source mic1

7.00

Time (s)

new source empty room, microphone 1

Appendix B

Time (s)

0.00 125

250

500

1000

Frequency (Hz)

2000

4000

125

250

500

1000

2000

4000

Frequency (Hz)

131

Tubes' comparison

Calibration of impedance tube (new foam 10.5 cm) 1.0 0.9

0.8

0.7

sample foam gap1

0.6

130 mm

0.5 0.4

sample hexagon2

0.3

al tube

0.2 0.1 0

200

300

400

500

600

700

800

1.0

back.of.the .tube.17fro nt....new.fo am.front.si de

0.6 0.5 0.4

sample orig.foam.4

0.3

nogap...ne w.foam.10. 5.gap

0.2 0.1 200

300

400

CALIBRATION

17.5 cm 20 cm

sample 2: hexagonal tube, calibration with new foam with 13.5cm of gap

13.5 cm

21 cm

measurement: original and new foam with13.5cm gap behind it

13.5 cm 20 cm

500

600

700

800

sample orig.foam.5

0.8

nogap...or. foam.back. of.the.tube .no.gap

0.7 0.6 0.5 0.4

sample orig.foam.6

0.3 0.2 0.1 0.0

200

300

400

Frequency (Hz)

sample 1: circular tube, calibration with original foam according to guidelines

0.9

sample orig.foam.3

0.7

0.0

Calibration of impedance tube (new foam 10.5 cm)

Absorption Coefficient

1 0.9

Absorption Coefficient

Appendix C

_CALIBRATION OF HAXAGONAL TUBE

sample 3: circular tube, calibration with original foam at the back of the tube

17.5 cm 20 cm

500

600

700

800

back.of.the .tube.17fro nt....new.fo am.back.si de

Frequency (Hz)

sample 5,6: circular tube, calibration with original foam at the back of the tube

17.5 cm 20 cm

sample 4: circular tube, calibration with original foam according to guidelines

17.5 cm 20 cm

measurement: original foam with10.5cm gap behind it

10.5 cm 20 cm

sample 5: circular tube, original foam measured at the back of the tube

17.5 cm 20 cm

sample 6: circular tube, new foam measured at the back of the tube

13.5 cm

21 cm

132

20 cm 13.5 cm

Appendix D

_MATLAB SCRIPT, CODED BY FOTEINI SETAKI, MARTIN TENPIERIK

%parameters T = 293.15; %temperature (K) P = 1.013e5; %ambient pressure (Pa) RH = 50; %relative humidit,L Lj = 0.1; %length of tube (m) Rj = 0.00275; %radius of resonator (m) rsample = 0.05; %radius of cylindrical test sample (m2) n_r= 1; %number of resonators %air properties c0 = 20.05*sqrt(T); %speed of sound (m/s) p0 = P/287.058/T; %air density (kg/m3) gamma = 1.4; %ratio of specific heat mi0 = 18.27e-6*(291.15+120)/(T+120)*(T/291.15)^(3/2); %dynamic viscosity, Sutherlands Law Cp = 0.000309*T^2-0.134343*T+1017.405714; %specific heat at constant pressure of air lamda = 2.428e-2*(T/275)^(0.9); %thermal conductivity of air nu = lamda/p0/Cp; %reduced thermal conductivity phi = (n_r*pi*Rj.^2)/(pi*rsample^2); %’ratio of open to close area’ %end correction d = 0.8*(1-1.47.*phi.^(1/2)+0.47*phi.^(3/2)).*Rj; %end correction Cremer and Müller L = Lj + d; %effective length %impedance at end of tube (1/4lambda: Z1=inf) Z1=1e99; %added friction from material sigma=0; %flow resistivity of possible porous material ta=0.025; %thickness of porous material %resistance added by added porous material rm=sigma*ta; %resistance due to thin absorbing material near opening

%frequency range f = 200:2:1000; %targeted frequencies (Hz) omega = 2*pi*f; %angular frequency (rad/s) k = omega/c0; %wave number for i=1:length(Lj) %formulas mi = sqrt(omega*p0*(Rj^2)/mi0); %auxiliary parameter B = sqrt(mi0*gamma/p0/nu); %auxiliary parameter K = gamma.*P./(1+2./B./mi./sqrt(-1i).*(gamma-1).*besselj(1,B.*mi.*sqrt(-1i))./ besselj(0,B.*mi.*sqrt(-1i))); %compression modulus in the tube 2.13 rhozk = p0./(1-2./mi./sqrt(-1i).*besselj(1,mi.*sqrt(-1i))./besselj(0,mi.*sqrt(-1i))); %effective density of air in the tube kzk = omega.*sqrt(rhozk./K); %wave number in the tube Zt2= p0.*c0.*(Z1.*cosh(1i.*omega.*(L(i)).*sqrt(rhozk./K))+sqrt(rhozk.*K).*sinh(1i.*ome ga.*(L(i)).*sqrt(rhozk./K)))./(Z1.*sinh(1i.*omega.*(L(i)).*sqrt(rhozk./K))+sqrt(rhozk.*K).*c osh(1i.*omega.*(L(i)).*sqrt(rhozk./K))); Zw=Zt2./phi; %impedance at the top of an array of resonators R = (Zw-(p0*c0))./(Zw+(p0*c0)); %complex reflection coefficient a(i,:)= 1-abs(R).^2; %absorption coefficient

133

Appendix D

_UNDEVELOPPED CONCEPTS

Fig.94: first concept idea

134

Fig.95: improvement of first concept

Ioanna Christia 4500075

TU Delft University of Technology, Department of Architecture, Urbanism and Building Sciences, Track of Building Technology