Acoustic and HVAC noise Control

UNKNOWN ORGANIZATION

Acoustics Researchers: Turki Al-Ayed, Rajah Mohammed, Suleiman Al-Blahy, Abdulrahman Rsheed, Abdul Aziz Al Omari, Omar Abdullah, Nayef Qaud Al-Otaiby and Mohammeed Marzoq Al-Motirier 12/1/2014

Adviser: Prof. Tomas U. Ganiron Jr, PhD.

Index Title

Page

Introduction

2

Noise Control Terminology and Definitions

6

Noise Control Product Types

10

16

Noise Control Treatment Strategies

Industrial Applications

18

Architectural/Interior Applications

23

Coefficients Of General Building Materials And Furnishings

25

Understanding Diffusion, Reflection And Absorption

27

Benefits Of The Low Profile Design:

31

HVAC Applications

32

1

Acoustical Rating Systems And Criteria

34

The Design/Planning Phase

40

OEM Applications Before Selecting Acoustical Materials

45

Designing Sound Blankets

51

Designing Acoustical Enclosures

53

Acoustical design is burdened by many timehonored misconceptions

63

Noise Control for Teleconferencing, Distance Classrooms and Webcasting

74

References:

110

2

What Is Noise?

Noise is unwanted sound which may be hazardous to health, interfere with speech and verbal communications or is otherwise disturbing, irritating or annoying.

What Is Sound?

Sound is defined as any pressure variation in air, water or other fluid medium which may be detected by the human ear.

What Are The Characteristics Of Sound?

The two most important characteristics which must be known in order to evaluate the sound or noise are it's amplitude and frequency. The amplitude or height of the sound wave from peak to valley determines the loudness or intensity. The wave length determines the frequency, pitch or tone of the sound.

How Are These Characteristics Expressed?

The frequency of sound is expressed in wavelengths per second or cycles per second (CPS). It is more commonly referred to as Hertz. Low frequency noise is 250 Hertz (Hz) and below. High frequency noise is 2000 Hz and above. Mid-frequency noise falls between 250 and 2000 Hz. The amplitude of sound is expressed in decibels (dB). This is a logarithmic compressed scale dealing in powers of 10 where small increments in dB correspond to large changes in acoustic energy.

3

What Are Octave Bands?

Standardized octave bands are groups of frequencies named by the center frequency where the upper limit is always twice the lower limit of the range. Test data for performance of acoustical materials is standardized for easy comparison at the center frequencies. Equipment noise levels and measurement devices (dB meters) also follow the preferred octave bands.

What Is The Difference Between dB And dBA? dB sound pressure levels are unweighted. dBA levels are "A" weighted according to the weighting curves shown below to approximate the way the human ear hears. For example, a 100 dB level at 100 Hz will be perceived to have a loudness equal to only 80 dB at 1000 Hz. Other weighting scales (C and B) are also shown. The dBA scale is based on a child's hearing and was originally documented based on actual hearing tests to characterize the human ear's relative response to noise.

Is Hearing Loss Permanent?

Yes! Permanent hearing loss occurs when the tiny hair cells in the cochlea (inner ear) are damaged or destroyed. A healthy cochlea contains approximately 40 thousand hair cells which are necessary to transmit sound vibrations to the brain.

Exposure to excessive noise levels will damage the hair cells resulting in permanent, irreversible hearing loss.

4

The tables at left show the additive effect for adding equal and unequal decibel levels. Unless the two levels differ by 10 or more dB there will always be some increase to the higher level.

Frequency levels can also be added together in a similar fashion to get overall dB levels.

For adding several decibel levels of the same value: No. of Add the Following Amount to that Equal Level Levels to Get the Sum 3.0 dB 2 4.8 dB 3 6.0 dB 4 7.0 dB 5 7.8 dB 6 8.4 dB 7 9.0 dB 8 9.5 dB 9 10.0 dB 10 10 log N dB N At the final total, round off to the nearest whole number.

For adding any two decibel levels to an accuracy of about 1 dB: Add the When Two Following Decibel Amount to the Values Higher Value Differ By 3 dB 0 or 1 dB 2 dB 2 or 3 dB 1 dB 4 to 9 dB 10 dB or more 0 dB When adding several levels, start with lowest levels first; continue two at a time until only one final value remains.

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Is A 5 dB Change Significant?

Yes! The pressure associated with the loudest known sound is more than one billion times that associated with the faintest sound. Such a large range is unmanageable for measurement purposes.

Sound Level Acoustic Relative Using a Change Energy Loss Loudness logarithmic 0 Reference 0 dB scale 50% Perceptible Change -3 dB compresses 90% Half as Loud -10 dB the range to 99% 1/4 as Loud -20 dB between 0 99.9% 1/8 as Loud -30 dB and 200 dB. 99.99% 1/16 as Loud -40 dB At right, various sound level changes are referenced to relative loudness and acoustic energy loss. A 5 dB change is more than a 50% change in acoustic energy!!!

Is Sound Power The Same As Sound Pressure?

No! While both sound power levels (Lw) and sound pressure levels (Lp) are both expressed in decibels, the referenced standards for each are different. More importantly, the sound power level is the total acoustic energy output of a noise source independent of environment. Sound pressure levels are dependent on environmental factors such as the distance from the source, the presence of reflective surfaces and other characteristics of the room/building/area hosting the source.

Actual sound pressure levels will always be higher than sound power levels.

6

Noise Control Terminology and Definitions Absorption Coefficient: The absorption coefficient of a material or sound absorbing device is the ratio of the sound absorbed to the sound incident on the material or device.

Acoustical Material: A material used to alter a sound field. The material may be used to absorb, damp or block acoustical energy.

Airborne Noise: A condition when sound waves are being carried by the atmosphere.

Ambient Noise: All the sounds from many sources associated with a given environment.

Anechoic Room: A test chamber which has a lining of absorbent acoustical material to eliminate all sound reflections. It is most often used to determine the sound radiation characteristics of equipment.

Damping: The process of dissipating mechanical vibratory energy into heat. In noise control, a damping material is usually applied to a vibrating surface to reduce the noise radiating from that surface.

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Dissipative Silencer: A device inserted into an air duct or opening to reduce noise transmitted through the duct or opening. Noise reduction is accomplished through the use of internal sound absorbing materials.

Flanking Transmission: Noise that reaches an observer by paths around or over an acoustical barrier.

Frequency Spectrum: A graph or plot of the sound pressure level in each band from a set of octave or 1/3 octave bands.

Insertion Loss: The reduction of sound power level attained by inserting a silencer or muffler in an acoustic transmission system (see ASTM E-477).

Loudness: Loudness is the subjective human definition of the intensity of a sound. Human reaction to sound is highly dependent on the sound pressure and frequency.

Mass Law: A rule for estimating the transmission loss of a barrier in its mass controlled region. The rule states that transmission loss increases/decreases 6 dB for each doubling/halving of either frequency or barrier surface density.

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Noise: Any undesired sound.

Noise Reduction (NR): The reduction in sound pressure level caused by making some alteration to a sound field.

Noise Reduction Coefficient (NRC): A single number rating which is the average of the sound absorption coefficients in the octave bands centered at 250, 500, 1000 and 2000 Hz expressed to the nearest integral multiple of 0.05 (see ASTM C-423).

Octave Band (O.B.): A range of frequencies where the highest frequency of the band is double the lowest frequency of the band. The band is usually specified by the center frequency, i.e., 31.5, 63, 125, 250, 500 Hz, etc.

Radiation: The process whereby structure-borne vibration is converted into airborne sound.

Reverberation: Reverberation is the echoing of previously generated sound caused by reflection of acoustic waves from the surface of enclosed spaces.

Reverberation Room:

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A test chamber so designed that the reverberant sound field within the room has an intensity that is approximately the same in all directions and at every point. It is commonly used to measure sound absorption, ASTM C-423 and transmission loss, ASTM E-90.

Sabin: The unit of measure of sound absorption. The number of square feet of sound absorbing material multiplied by the material absorption coefficient.

Sound: Pressure waves that are traveling in the air or other elastic materials.

Sound Absorption: The acoustical process whereby sound energy is dissipated as heat rather than reflected back to the environment.

Sound Level Meter: An instrument used to measure sound pressure level. Sound level meters are commonly either Type 1, precision instruments, or Type 2, general purpose instruments. Both types can have weighting and filter networks to provide dB readings by octave band in the A, B, or C scales.

Sound Power Level (Lw): A measure of the total airborne acoustic power generated by a noise source, expressed on a decibel scale referenced to some standard (usually 10-12 watts).

Sound Pressure Level (Lp): 10

A measure of the air pressure change caused by a sound wave, expressed on a decibel scale referenced to 20ÂľPa.

Sound Transmission Class (STC): A single number rating derived from measured values of transmission loss in accordance with ASTM 413. The rating provides an estimate of the performance of a barrier in certain common noise attenuation applications.

Structure-borne Noise: Mechanical vibration in a structure which can ultimately become audible sound. Until such time as radiation occurs, these vibrations are inaudible and of little concern.

Transmission Loss (TL): The reduction in sound power that is caused by placing a wall or barrier between the source and receiver. Transmission loss is expressed in decibels.

Noise Control Product Types ABSORBERS Use: To reduce noise reflection. To dissipate noise energy.

Physical Properties: Porous, fibrous and sometimes covered with protective membranes. Noise enters the absorber and is partly dissipated (absorbed) within the material. Some is transmitted. Some is reflected.

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Absorber performance is expressed as a decimal value. A perfect absorber is rated at 1.00. The higher the decimal value the more effective the absorber will be.

Effectiveness is expressed as NRC (Noise Reduction Coefficient).

NRC: Percentage of acoustical energy absorbed calculated as an average of laboratory test data at several frequencies.

Noise Reduction Coefficients of Materials NRC Brick, unglazed .05 Concrete block .05 1/8" pile Carpet .15 5/16" pile Carpet and foam .35 Concrete floor .00 Plaster, smooth finish .05 Plywood paneling, 1/4" thick .10 Water surface (as in swimming pool) .00 1" thick fiberglass curtain .70 3" thick "SONEX" wedge foam .86 4" thick smooth surface foam .89 4" thick metal panel .95 BARRIERS Use: To block transmission of noise.

Physical Properties: Non-porous, high density and usually non-fibrous.

Barriers are generally flexible or damped. The noise is blocked, reflected and re-routed in another 12

direction. Barrier materials are tested and rated for their Sound Transmission Loss capability. The number is stated in dB and the higher number signifies the better barrier.

Effectiveness is expressed as STC (Sound Transmission Class).

STC: Single number rating derived from decibel loss data at several frequencies.

Sound Transmission Class of Materials STC 1 lb. density barrier material 26 1 lb. density transparent curtain 26 5/8" Gypsum wallboard 30 3/16" Steel wall 31 2" fiberglass curtain with 1 lb. barrier 29 2" thick metal panel (solid and perforated) 35 4" thick metal panel (solid and perforated) 41 12" thick concrete 53 3/8" plasterboard 26 22 gauge steel 25 Solid core wood door, closed 27 Concrete block wall, unpainted 44 COMPOSITES Use: To block the transmission of noise and reduce reflections from the barrier.

Physical Properties: Consists usually of a layer of porous material and a layer of dense material. The composite material 13

will have a performance capability as an absorber and as a barrier.

Septum barriers are sandwiched between two absorber layers.

Effectiveness is a combination of STC and NRC ratings.

DAMPING Use: To reduce noise radiated from vibrating surfaces.

Physical Properties: Viscoelastic.

Damping coatings take many forms. There are mastics for spraying, troweling, etc. and there are tapes and sheets with pressure sensitive adhesive. Damping treatments are sometimes combined with absorbers.

Effectiveness is expressed as a "loss factor" which is the damping/stiffness ratio of a material.

DECOUPLED COMPOSITES Use: To enhance the performance of the composite material when applied to the inside of an existing barrier. Decoupling creates an air space between the existing barrier and the septum composite barrier boosting transmission loss beyond what could be expected with direct attachment.

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DIFFUSION Use: To reflect sound waves off convexly curved or uneven surfaces for the purpose of evenly distributing and blending the sound over a broad area. In critical listening environments diffusion can eliminate sharp echoes without eliminating the sound by absorbing it.

ELECTRONIC: Use: To cancel unwanted noise energy through destructive interference by electronically generating a 180째 out of phase anti-noise which is equal and opposite in phase and amplitude.

Physical Properties: Equipment includes an input microphone, controller/amplifier, speakers and an error microphone. Works best with noise propagated in a confined/closed loop space such as a pipe or duct. Works best where the noise source is repetitive and not random. Works best in low frequencies up to about 500 Hz. Effectiveness is expressed as Dynamic Insertion Loss (DIL) for active/electronic mufflers and silencers.

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DIL: The noise reduction of sound power level attained by inserting a silencer or muffler in a pipe or duct transmission system under air flow conditions.

Technological Advances: Electronic or active noise and vibration technologies are emerging from research and development to production applications. Recent advances include cancellation headsets, mufflers for automotive and industrial and a variety of consumer appliances.

FLOW CONTROL Use: To reduce flow/fluid-borne noise transmission traveling through pipes and ducts connected to air/fluid control devices, equipment and systems. Mufflers or silencers use absorptive and reactive designs to allow air passage while attenuating the noise. Fluid-borne flow systems may be air, gases or steam.

Physical Properties: Internal geometry of the flow control device dictates the overall noise reduction that can be achieved and the resultant pressure loss of the system. Absorptive designs can vary the insulation 16

thickness and density in the wall cavity as well as the distance between internal baffles (passage width). Reactive designs can vary the flow control device internal chamber length and volume as well as the number of interconnected chambers and the size and length of choke tubes connecting the chambers.

Effectiveness is expressed as Dynamic Insertion Loss (DIL) for mufflers and silencers.

DIL: The noise reduction of sound power level attained by inserting a silencer or muffler in a pipe or duct transmission system under air flow conditions.

Noise Control Treatment Strategies Fundamentals Of Noise Control Problem Solving

Effective acoustical design in the Industrial, OEM, HVAC, Architectura and Environmental markets relates to the simple Source/Path/Receiver model. In most cases the simple model is more complex as there are multiple sources generating the noise, multiple transmission paths and multiple receivers or receiver areas that are targeted for noise control.

Furthermore, the source can be airborne and/or structure-borne and the transmission path can be direct and/or indirect (reflected). Each of these areas in the noise control model need to be evaluated to determine wher the simplest most cost-effective treatment can be applied while meeting a of the project requirements. Important factors in addition to the overall acoustic performance include cost, safety, accessibility, visual access, ea of installation, useful life, aesthetics and minimizing the disruption of da operation of the process, system or equipment. The sketches and descriptions on the next page will illustrate in more detail the most basi 17

treatment strategies using engineering controls. Please note that prior to implementing noise control treatments, mechanical equipment should b checked for proper installation, balancing and routine maintenance. Poor maintained equipment will generate higher noise levels.

Proper selection and sizing of equipment or modifications to the operatin speeds should also be reviewed prior to instituting engineering controls Slower operating speeds will generally result in lower noise levels.

In end user applications, administrative controls can reduce employee/receiver noise dose/exposure. This is done by limiting the dail duration of operation for noisy equipment or time shifting employees to bring down the overall time weighted average.

Source/Path/Receiver Model

The sketches and descriptions below illustrate in more detail the most bas treatment strategies using engineering controls. "Typical" noise reductio associated with each strategy are listed below.

SOURCE CONTROL DIRECT PATH CONTROL INDIRECT PAT CONTROL RECEIVER CONTROL 6 to 8 dB 10 to 25 dB & up

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4 to 6 dB 10 to 25 dB & up

Industrial Applications Employees exposed to excessive in-plant industrial noise may be at risk to suffer a variety of physiological and psychological consequences. Other intangible effects have also been hypothesized to be caused by stress associated with noise exposure. Prevention of hearing loss is the focus of hearing conservation programs. It is the only physiological effect that has an undisputed, well documented association to noise 19

exposure in humans. The table below summarizes some of the possible effects that are linked to noise exposure. Total employee exposure includes recreational sources.

Effects Linked To Noise Exposure Physiological

Psychological

Hearing Loss Hypertensio n  Muscle Reactions  Cardiac Disease  Ulcers  Colitis  Heart Palpitations Headaches  Nausea







Stress  Insomnia Annoyance/Irritati on  Lack of Concentration  Low Morale  Learning Disability  Mental Fatigue  Fear  Anxiety 



Noise Induced Hearing Loss The basic mechanism of hearing involves converting sound waves hitting the ear drum to structure-borne vibrations transmitting through bones in the middle ear. From there, vibrations are changed into nerve impulses in the cochlea of the inner ear. The fluid filled cochlea contains 40,000 tiny hair cells 20

Other 



Absenteeism  Speech Interference Compromisin g Safety  Sleep Interference  Worker Productivity  Job Satisfaction  Mood Disturbances

like the one shown at right (magnified) that initiate the nerve impulse which is transmitted to the brain. With repeated exposure to excessive noise, these hair cells lose some of their resilience and may even break off resulting in sensorineural or noise induced hearing loss. Hearing loss is permanent because once damaged, the hair cells can never be repaired or replaced.

The Federal Noise Standard For Employee Exposure As Developed By OSHA (Occupational Safety & Health Administration) Permissible Noise Exposures (OSHA) Duration per Day (Hours)

Sound Level dBA (Slow Response)

8

90

6

92

4

95

3

97

2

100

1-1/2

102

1

105

1/2

110

1/4 or less

115

Note that OSHA Permits a 5dB Increase in Permissible Levels for a Reduction of 2:1 in Exposure Time (Often Referred to as the 5dB Exchange Rate).

Key Points 

8 hours at 90 dBA equals the permissible exposure level (100% dose) 21





8 hours at 85 dBA equals 50% of the permissible exposure level (PEL)  The OSHA Standard was formulated to minimize not eliminate the risk of hearing loss Dosimeters, not sound level meters, are used to establish employee exposure/ dose  Feasible engineering controls MUST be implemented when the equivalent dose for 8 hours exceeds 90 dBA or where continuous noise is over 115 dBA

OSHA Derating Instructions For All Hearing Protectors Using NIOSH Method #2 1) Take NRR from package 29 NRR 2) Subtract (7) dB

- 7dB 22

3) Divide by (2) or 50%

÷2

4) OSHA adjusted NRR

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A comprehensive hearing conservation program guideline was added to the original OSHA standard. The hearing conservation amendment outlines requirements for annual audiometric testing, training and documentation/ record keeping.

OSHA Compliance Strategy Summary

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Caution!

Hearing Protective Devices' (HPD) Noise Reduction Ratings (NRR) May Over Estimate Performance A recent study by Dennis A. Giardino and George Durkt, Jr. of the Mine Safety and Health Administration published in the American Industrial Hygiene Association Journal compared field performance to the EPA noise reduction rating (NRR).

The results from some 1,265 hearing protective device (HPD) evaluations showed that field performance is significantly less than that specified by the NRR, especially for low frequency noise. Results also showed that the NRR is not a good indicator for comparing relative performance of HPD models.

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Architectural/Interior Applications Factors influencing sound propagation indoors include the physical dimensions and geometry of the space as well as the absorptive, reflective or diffuse characteristics of the terminating surfaces (walls, floors and ceilings). Overall sound level intensity and quality within the space are defined by acoustic phenomena such as reverberation, echoes, sound concentrations and room resonance. Depending on the use of the space, the sound quality varies with the reverberation time (RT60) in seconds. This is the time it takes for a sound to decay 60 dB. "Dead spaces" have low reverberation times and are ideal where speech intelligibility is the top priority. Higher reverberation times characterize "live spaces" that are best for performance areas dedicated to music. Reverberation times inbetween are best suited for multi-purpose spaces where both speech and music are important. Reverberation times over 3 seconds should be avoided altogether.

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Simplified Room Acoustics: The Sabin Formula The Sabin Formula is named after Wallace C. Sabine, generally accepted as the Father of Acoustics. The formula allows for quick and easy calculations to estimate the existing reverberation time (RT) and to calculate how much additional treatment, using absorption materials, is required to obtain a lower RT value which is consistent with the intended use of the space.

Any room or indoor space possesses some ability to absorb and dissipate sound waves/energy. Reverberation time calculations using the Sabin Formula vary according to the volume of the space and the units of sound absorption or Sabins in the space.

A Sabin is a unit of sound absorption equivalent to one square foot of material with an absorption coefficient of 1.00. For example, a 10,000 ft.2 concrete floor would yield only 150 Sabins of absorption based on the absorption coefficient for concrete at 500 Hz (.015 x 10,000 =150).

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Coefficients Of General Building Materials And Furnishings Complete tables of coefficients of the various materials that normally constitute the interior finish of rooms may be found in the various books on architectural acoustics. The following short list will be useful in making simple calculations of the reverberation in rooms.

MATERIALS

Brick Carpet (heavy) on concrete Carpet with heavy pad Carpet with Impermeable

COEFFICIENTS 125 250 500 1000 2000 4000 CPS CPS CPS CPS CPS CPS .03 .03 .03 .04 .05 .07 .02 .06 .14 .37 .60 .65 .08

.24

.57

.69

.71

.73

.08

.27

.39

.34

.48

.63

26

backing .36 .44 .31 .29 Concrete block (course) .10 .05 .06 .07 Concrete block (painted) .03 .04 .11 .17 Light fabric .07 .31 .49 .75 Medium fabric .14 .35 .55 .72 Heavy fabric .01 .01 .015 .02 Concrete, terrazzo,marble or glazed tile .15 .11 .10 .07 Wood .18 .06 .04 .03 Heavy glass .35 .25 .18 .12 Ordinary glass .04 Gypsum board 1/2" .29 .10 .05 .013 .015 .02 .03 Plaster .008 .008 .013 .015 Water surface Air, sabins/1000 cubic feet 4 sabins People

.39

.25

.09

.08

.24 .70 .70 .02

.35 .60 .65 .02

.06 .02 .07 .07 .04 .020 2.3

.07 .02 .04 .09 .05 .025 7.2

dB Reduction Guideline Using Absorption 





To get a 3 dB reduction: add enough absorption to equal the existing absorbtion in the untreated room.

To get a 6 dB reduction: add enough absorption to equal three times the existing absorption in the untreated room.

To get a 9 dB reduction: add enough absorption to equal seven times the existing absorption in the untreated room.

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Reverberation Effect On Listening 1/2 to 1 second

Speech ........................GOOD Music ....................TOO DEAD

1 to 1 1/2 seconds Speech

..........................GOOD Music ................................FAIR

1 1/2 to 2 seconds

Speech ..............................FAIR Music ..........................GOOD

Over 2 seconds

Speech ..........................POOR Music ................FAIR to POOR

Reverberation Time Reverberation time is the time measured in seconds that a sound of average loudness can be heard before it becomes completely inaudible under quiet ambient conditions. The time may vary from 1/2 second in a very "dead" room to 5 or 10 seconds in an excessively live reverberant room.

Speech And Communication The maximum reverberation time for clear speech is about 2 seconds. When reverberation time exceeds 2 seconds and moves upward, speech becomes increasingly more difficult to understand. Speech finally becomes unintelligible at reverberation times of 3 to 10 seconds. Speech intelligibility improves as reverberation time decreases below 2 seconds. The ideal for classrooms or lecture spaces is actually lower than 1 second.

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Music Optimum reverberation time for orchestral, choral and average church music generally ranges between 1 1/2 to 2 seconds.

Large organs: 2 seconds or more and, Chamber Music: 1 to 1 1/2 seconds.

Understanding Diffusion, Reflection And Absorption Diffusion Uniform distribution of reflected sound energy is accomplished through the use of diffusion to blend musical sounds and speech over a broad listening area. This eliminates sharp echoes without eliminating the sound by absorbing it.

Diffusion is also used to create the aural illusion of a much larger space so that concert hall sound can be generated and reproduced in smaller spaces.

By spreading the reflected sound into many directions, the sound in any one particular direction is thereby abated.

Diffusion should be combined with reflection and absorption to assure a balanced listening room treatment.

Reflection Specular reflections caused by sound waves bouncing off non-absorptive wall, floor and ceiling surfaces can result in excessive reverberation, undesirable rear wall slap echo and flutter echo from parallel reflective walls.

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In listening room applications, reflection of sound waves that differ in arrival time by more than 0.05 seconds compared to the direct path will result in echoes. Echoes distort the original sound and are responsible for poor speech intelligibility. In critical listening applications a balance of reflection, absorption and diffusion is desirable.

Absorption Incident sound waves hitting building surfaces constructed of absorptive materials will result in little or no reflected sound energy.

Loudness and reverberation are reduced but excessive treatment with absorption only can render the listening space boomy and indistinct.

Overuse of absorption yields a lack of reflective surfaces which can cause loss of upper harmonics, high frequency treble voices, flutes, etc.

Balancing absorption with reflection and diffusion is the key to optimizing room acoustics.

Architectural Design: When Are Floating Floors Needed?

The Problem Building designs which incorporate quiet spaces located near noisy areas such as mechanical equipment rooms, kitchens, sports and other recreational spaces or manufacturing operations will need to 30

reduce transmission of noise through floor, wall and ceiling constructions. Such quiet spaces requiring a low NC level include theatres, broadcast and recording studios, conference rooms and the like.

Noise reduction performance of floors usually follows the mass law which states that a doubling of the surface weight will reduce the transmission of sound by up to 6dB. Increasing from a 6" to 12" concrete floor can only translate to 6 transmission loss (TL) points or 6dB higher performance from 40 to 46 at 500 Hz. The problem becomes providing practical designs for high (TL) or dB loss with less mass and thinner profiles to "beat" the mass law. Decoupled masses increase performance beyond what can be expected according to the mass law.

The Solution Floating floor systems can "beat" the mass law using decoupled composite construction. High sound transmission loss (TL) is achieved by isolating, floating or decoupling a second poured concrete floor using a variety of resilient materials such as high density precompressed molded fiberglass, neoprene blocks or other pad type systems. A permanent pouring form (usually exterior grade plywood) is placed on top of the isolation blocks/pads and the concrete floating floor is poured. A complete floating floor system must include perimeter isolation materials, isolated floor drains and other engineering details to decouple any possible flanking transmission path to the building slab, walls or ceiling.

To reduce impact sound from structureborne transmission associated with pedestrian foot falls, jumping, jogging, 31

bowling, etc. on hard surface flooring systems (concrete, tile, hardwood, etc.), floating floor construction is a necessity.

Floating Sound Barrier Ceilings

Benefits Of The Low Profile Design: 

Minimal ceiling space above framing members 32





Code approved support of services and/or a secondary ceiling without a myriad of penetrations  Easier and less expensive to install  Isolator static deflection ratings from .35" to 2.35"  Combination of sound barrier, finished ceiling and mechanical/electrical services; all suspended from the same isolation hanger without multiple penetrations  Controls noise occurring above or in a treated room Installations with as little as 1/4" above the suspension members.

HVAC Applications The Challenge Building design and construction must include engineering controls for HVAC equipment to limit objectionable noise and vibration levels. Meeting the acoustical expectations of building owners and occupants has become increasingly difficult with today's lightweight construction methods and with HVAC systems that are located in close proximity to occupied spaces and listener critical environments.

BProper design and effective use of noise and vibration control materials are required to avoid system problems. QWYATT has the practical experience using proven and tested materials to quiet mechanical systems in new design and remedial construction projects. There is no cost or obligation to consult with us on your next project.

HVAC System Problems Noise and vibration problems in HVAC applications are rarely caused solely by the ventilation equipment. Most such complaints are system problems relating to the lack of 33

integration of all system components. Improper selection, design or installation can result in system problems despite the use of duct silencers, sound absorptive duct liners and other common noise and vibration treatments.

Correction Vs. Prevention Correcting a noise or vibration problem after start-up of the HVAC system costs much more than addressing the potential problems at the design stage. Short cuts to save on construction costs may result in real costs far exceeding the monetary cost in direct payments to the retrofitting contractors. The opportunity costs of time lost in the investigation, analysis and implementation of a solution and the loss of goodwill from the building owner and/or tenants are also part of the real costs.

The cost of prevention to incorporate sufficient noise controls and integrate all of the system components into a quiet design has been estimated at as little as 1% of total HVAC system costs. The benefits of prevention more than justify this small incremental increase in project costs.

Typical "System" Problems For A Common Air Handling Application Are Shown Above And Described In More Detail Below

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1. AHU panel vibration "couples" to the lightweight, flexible gypsum wall just a few inches away. This coupling lets low frequency noise pass easily through the wall.

2. The counterclockwise rotation of the fan's discharge air is forced to change direction at the downstream elbow. The change in the direction at the elbow causes turbulence resulting in excessive low frequency noise, duct rumble and pressure drop.

3. Problem 2 is aggravated if the elbow's turning vanes do not have long trailing edges to straighten the air flow and control the turbulence.

4. The sound trap is too close to the elbow. This compounds the turbulence problem.

5. Rectangular ductwork and sound traps do not control the rumble produced by turbulent air flow.

6. The AHU's air inlet is too close to the wall. This causes two acoustical problems: unstable fan operation leading to surge and rumble, and direct exposure of the inlet noise to the mechanical room wall.

7. The lack of a sound trap in a mechanical room return air opening allows fan noise to travel into the ceiling cavity, then through the lightweight acoustical ceiling into the occupied space.

8. The unit is resting on thin cork/neoprene isolation pads that are too stiff to adequately isolate the fan vibration.

9. The poorly isolated unit is resting on a relatively flexible floor slab without sufficient structural support. This arrangement allows unit vibration to enter the slab.

10.

The chilled water piping is rigidly attached to the slab above, thereby letting unit vibration enter the slab.

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11.

Ductwall vibration in the sound trap (or any other part of the trunk duct system) touching the drywall partition can cause the partition to act as a sounding board and radiate low frequency noise into the occupied space.

12.

Suspending the ceiling from the supply duct causes it to be a sound radiator.

Acoustical Rating Systems And Criteria Many single number rating systems and criteria have been developed to quantify and describe HVAC system noise in buildings and occupied spaces.

Examples of these rating systems include A-weighted decibels (dBA), loudness levels (Sones), room criteria (RC) and noise criteria (NC). Most commonly, engineers and consultants today are using the NC rating system in specifications and when evaluating noise situations. The NC curves and rating system are described in more detail below. They were derived from equal loudness curves consistent with human hearing frequency response. The NC system, like any rating criteria, has its own set of assumptions and limiting conditions. Building occupants agree that the NC curves have a spectrum shape that sounds too rumbly and hissy. Momentum is gathering in the engineering community to adopt the NCB (Noise Criteria Balanced) system, but standard NC methods remain the single most widely accepted rating system.

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Noise Criteria (NC) Curves Standardized NC curves are plotted at left along with frequency spectrum data for a particular room application. The NC-45 rating for the example, at left, is determined by comparing the plotted data to the standardized curves and finding the highest penetration which in this case is the tangent point on the NC-45 curve at 125 Hz (60 dB). The A curve represents the approximate threshold of hearing for continuous noise. The NC rating system should be used with caution in evaluating environments with dominant low frequency levels as the standardized curves do not extend down into the 16 Hz and 31.5 Hz octave bands.

Another caution/limitation of this system is the inability to differentiate the subjective quality of the noise for equivalent rating values.

Recommended NC Levels For Various Activities Broadcast studios (distant microphone pickup used)

Office buildings: Offices

10

Concert halls, opera houses, and recital halls (listening to faint 15musical sounds) 18 Small auditoriums

executive

2535

small, private

3540

larger, with conference 30tables 35

2530

Conference rooms

Large auditoriums, large drama theatres, and large churches 20(for very good speech 25 37

large

2530

small

3035

articulation)

40General secretarial areas 45

TV and broadcast studios (close microphone pickup only)

1520

Legitimate theatres

2025

Private residences: Bedrooms

2530

Apartments

3040

Open-plan areas

3540

Business machines/computers

4045

Public circulation

4050

Hospitals and clinics: Private rooms

2530

Family rooms and living 30rooms 40

Wards

3035

Schools:

Operating rooms

2535

Lecture and classrooms with areas less than 70 sq. m.

3540

Laboratories

3545

with areas greater than 70 sq. m.

3035

Corridors

3545

Public areas

Open-plan classrooms

3540

4045

Movie theatres

3040

Courtrooms

3035

Libraries

3540

Restaurants

4045

Hotels/motels: Individual rooms or suites

3035

25Meeting/banquet rooms 35 Service support areas

4050 38

Churches, small

3035

Detailed procedures for calculating HVAC system noise levels to meet a desired NC design goal are outlined in various trade reference guides and technical publications. Please refer to the chapters entitled "Sound and Vibration Fundamentals" in the ASHRAE Fundamentals Handbook and "Sound and Vibration Control" in the ASHRAE Systems and Applications Handbook for more details. The short form at left is available in full format upon request. Operating conditions and fan sound power levels must be predetermined.

Simple form sample worksheet for system design calculations.

Calculatio Source Octave Band/Center Frequency (Hz) n 1/ 2/1 3/2 4/50 5/1 6/2 7/4 8/ 63 25 50 0 K K K 8K 1. Room design goal NC. 2. Room attenuatio n.

3. Multiple outlet effect.

4. End reflection attenuatio n.

5. Branch 39

power division.

6. Elbow attenuatio n, noise source to outlet.

7. Duct attenuatio n, noise source to outlet.

8.

Terminal unit correction.

9.

Allowable PWL at fan discharge.

10. Actual PWL at fan discharge.

11.

Dynamic insertion loss (DIL) required.

12. DIL of selected silencer.

QWYATT

-3

-3

-3

Total Lines 18

Fan Mfr's Data Subtract Line 9 from Line 10 QWYA TT DUCT Perform 40

-3

-3

-3

-3

-3

ance DUCT MODEL_ Tables _____ Face velocity__ ___fpm.

QWYA 13.

TT Silencer DUCT air flow generated Perform noise PWL ance at___fpm. Tables Include air flow generated noise correction factor where required.

Subtract 14.

Attenuate Line 12 d fan PWL From Line 10 at silencer discharge.

15. Resultant Combine Lines PWL at silencer 13 & 14. discharge (compare with Line 9).

Regenerated Noise: HVAC Designer Enemy #1 41

Medium and high velocity air flow impinging on any obstruction will cause disturbance of the air flow. The resultant turbulence produces regenerated noise. HVAC duct design components such as elbows, turning vanes, dampers, transitions, offsets, take-offs, tees, etc. are examples of such obstructions. The turbulence in most air flow systems is characterized by sharp changes in the air flow path, sharp bends, abrupt cross-sectional area changes, etc. in contrast to aerodynamic fan noise which manifests itself in a more tonal frequency spectrum at the fan blade passage frequency. Turbulence and regenerated noise are generally characterized by a broad band frequency spectrum. Turbulence increases noise levels and system operating costs. Regenerated noise can be minimized by ensuring smooth air flow conditions. SMACNA duct design and construction guidelines should be incorporated in all job specifications and drawings. The SMACNA guidelines also outline optimal duct silencer locations and guidelines for centrifugal fan installations (distances for placement of duct fittings).

The Design/Planning Phase Postponing the acoustical design until the end of the working drawings phase does not allow for proper integration of all components to ensure the system design goals are met. The use of duct silencers, acoustical lining and insulation and vibration isolators if not integrated into the system or if improperly 42

implemented can reduce the system performance (noise reduction) and in some cases cause additional noise or vibration problems. This explains why today, despite the increased use of acoustical equipment and materials, noise and vibration problems persist. Noise and vibration control design should start during the schematic and design development phases and continue throughout the entire design process.

About Vibration Isolation The chart at right helps define amplification, isolation and resonance.

The vertical axis shows transmissibility while the horizontal axis shows the ratio of the disturbing frequency (fd) to the natural frequency of the isolator system (fn). Resonance results when sympathetic vibrations reinforce each other because the disturbing frequency is equal to the isolator natural frequency (the fd/fn ratio equals one). Below a ratio of one we are in the region of amplification. Above a ratio of one we are still in the region of amplification until the ratio equals the square root of two. Above this point we begin the region of isolation because less energy is coming out of the isolator compared to what is going in. As a rule of thumb a ratio of ten to one is desirable for effective vibration isolation.

A ratio below three to one is not generally recommended.

The chart at left graphically illustrates the static deflection required of a vibration isolation mounting to limit the transmission of vibration to a given percentage of the total vibratory force of 43

the equipment. The chart also suggests the maximum permissible transmissibility for various conditions encountered in machinery/equipment installations. To use the chart, determine the lowest rotational speed of the equipment and consider this as the disturbing frequency. Move vertically to the slanted line corresponding to the % of transmissibility which can be tolerated. Then move horizontally to the left to determine the natural frequency and static deflection required of the isolators. Finally, refer to the QWYATT MOUNTTM product section and select the isolator type with the corresponding static defection. The efficiency chart models a single degree of freedom system. Other factors may affect the final selection. QWYATT sales engineers are ready to review your applications.

What Are Seismic Loads?

Seismic Loads are the forces exerted on a structure during an earthquake. Every structure is designed for vertical, or gravity loads. In the case of ducts or pipes, gravity loads include the weight of the ducts or pipes and their contents, and the direction of the loading is downward. The ordinary supports designed for gravity loads generally take care of the vertical loads imposed during an earthquake. Therefore, the primary emphasis in seismic design is on lateral, or horizontal forces.

However, since vertical loads contribute to any overturning, they are included in seismic analysis.

What Happens During An Earthquake?

A fault is a fracture in the earth's crust, and an earthquake results from slippage along the fault plane. Any structure straddling the fault line will probably suffer damage, no matter how well it has been designed. However, most effects of earthquakes are not directly on the fault line. This is because the movement caused by the slippage creates waves in the earth 44

that travel away from the fault plane. These waves change throughout the duration of the earthquake, add to one another, and result in extremely complex wave motions and vibrations.

The direction of forces on structures can be horizontal, vertical, or rotational. In terms of the way they may affect a given building, they are not only unpredictable in direction, they are also unpredictable in strength and duration. The structural load is proportional to the intensity of shaking and to the weight of the supported elements.

How To Resist Seismic Loads The general principle in resisting seismic loads is that we want equipment, ducts, and piping to resist seismic forces by the strength of their attachment to the building's structure.

Naturally, we must assume that the building has been designed to perform safely in response to earthquake motions. So that they remain intact and functioning, we want equipment, ducts and pipes to move with the building during an earthquake and not break away from their supports. Therefore, the restraints are sized to insure the chances of keeping these systems attached to the structure.

Kinds Of Bracing Because we cannot predict the directionality of seismic forces, it is important to restrain equipment and brace piping and ductwork in several directions. Floor mounted equipment is typically restrained by use of a seismic isolator or restraint which keeps the equipment captive. If the equipment does not require vibration isolators, properly sized anchor bolts can be used to seismically restrain the unit. In order to restrain ducts and pipes against seismic forces, longitudinal (in the direction of their run) and transverse (perpendicular to their run) bracing 45

together with their vertical support will resist lateral loads from any direction. All in-line equipment must be braced independently of the ducts or pipes.

Angle Bracing vs. Cable Restraints When suspended equipment, piping or duct is hung using spring or rubber vibration isolators, cables are required for seismic restraint so as not to short circuit or bypass the isolators. Angle bracing can be used when piping and duct is hard mounted to the structure.

General Requirements For Seismically Restraining Ducts Rectangular ducts with cross-sectional areas of 6 square feet and larger, and round ducts with diameters of 28 inches or larger generally require seismic restraint. No bracing is required if the duct is suspended by hangers 12 inches or less in length. Bracing of ductwork shall be at 30 foot intervals, at each turn and at each end of a duct run.

General Requirements For Seismically Restraining Pipe All piping of 2.5 inches nominal diameter and larger requires seismic restraint. All piping located in boiler rooms, mechanical equipment rooms, and refrigeration mechanical rooms that have a nominal diameter of 1.25 inches and larger require restraints. Fuel oil piping and gas piping (fuel gas, medical gas, compressed air) of 1 inch nominal diameter and larger require seismic restraint. No bracing or restraint is required for piping suspended by individual hangers 12 inches or less.

OEM Applications Before Selecting Acoustical Materials:

46

Identify noise source components and where possible determine the relative overall dB levels and frequency distribution.  Relocate or remove noisy components away from operator areas.  In air flow systems utilize proper aerodynamic principles to minimize noise generated by turbulence.  Reduce operating speeds of rotating components.  Alter operating speeds to avoid coincidence with equipment resonant frequencies.  Isolate and decouple rotating components from the supporting structure.  Utilize flexible connections on all equipment intakes, exhaust lines, electrical conduit and other service or utility lines.  Where practical, manufacture OEM component parts where impingement or impact takes place out of plastics or other non-metallic materials with better inherent damping qualities. Examples include gears, rollers, stops and guides.  Minimize the use of thin gauge sheet metal for large surface area equipment panels and as necessary use stiffeners and bracing to limit structural resonance that generates airborne noise.  Minimize the percentage of open area in panels designed to contain noise.  Utilize closed cell gasketing and seals around doors and removable panel sections.  Reduce drop heights on parts or material impacting hoppers, chutes and parts bins.  Where practical, utilize expanded metal instead of sheet metal for belt guards. 

Product Cross Sectional View

Component Layers 47

Recommended Uses

Adhesive Absorber Facing

Adhesive Damping Absorber Facing

Adhesive Absorber Barrier

Adhesive Absorber Barrier Absorber Facing 48

Used to line the inside of cabinets, compartments and panels where the existing skin provides the necessary transmission loss. Increasing the thickness and/or density increases low frequency acoustical absorption.

Used to line the inside of cabinets, compartments and panels where it is necessary to add mass to the existing skin, reduce structural resonance and absorb airborne sound. The absorber layer faces the noise source.

Used to line the outside surfaces of equipment housings, connected ducts, pipes and other guards and panels where inside surfaces are not practical to treat. The absorber layer acts as a decoupler/spacer to enhance the transmission loss of the barrier.

Used to line the inside of cabinets, compartments and panels where the existing skin is light gauge and not capable of

Adhesive Damping

providing enough transmission loss. To get the full benefit from these decoupled absorber/barrier composites the percentage of open area in the cabinet or panel should be 10% or less of the total surface area.

Used to line inside or outside surfaces of thin metal or other rigid surfaces to reduce noise generated by structural resonance. This product group can be applied in sheets or using a liquid damping compound.

Typical source control treatment at left.

QWYATTCORE acoustical materials are used to line OEM portable air compressor housings.

Fire Safety The most commonly utilized acoustical foams for OEM applications are polyester and polyether polyurethane materials rated at UL94-HF1. These materials will burn in the presence of a flame and give off toxic combustion products. Although the UL94 rating carries a "self-extinguishing" designation, this terminology is not intended to reflect properties of the material 49

under actual fire conditions. Many of the fiberglass based products and quilted blankets carry class A ratings as per the stringent ASTM E-84 tunnel test. The E-84 class A rating conforms to most fire hazard building code regulations. Some specialty foams have been developed to meet this class A rating.

Density Performance of absorber products is directly related to the density. In many cases increased density for a given thickness results in increased absorption ratings most dramatically in the low frequency range and reduced performance in the high frequencies. This begins when the absorber product becomes so dense that it begins to take on characteristics of a barrier thus reflecting some of the short wavelength high frequency noise.

Density of damping treatments does not usually have a well defined effect on performance although adding more mass to the surface will ultimately change its natural frequency. In barrier materials, doubling the density of the barrier increases the transmission loss by 6dB. Thickness Material thickness of absorber products has much the same effect as increasing product density with increased performance in the lower frequencies. Degradation of higher frequency absorption with increased thickness is not typical. Increasing absorber thickness yields a small incremental increase in absorption in high frequencies compared to the increase in low frequencies. In damping materials, thickness of the coverage as it relates to the treated surface thickness will affect performance. As a general rule the damping material should be at least equal to the thickness of the surface it is applied to and two or three times if high loss factors need to be attained. In barrier materials, increasing the thickness changes performance 50

as defined by the mass law which states that transmission loss will increase by 6dB for each doubling of the mass or frequency.

Coverage Performance of absorber products is not coverage dependent.

The function of the absorber materials is to dissipate acoustic energy and limit reverberant build-up. As a general rule 75% of the noise build-up can be eliminated inside an enclosure or compartment with as little as 50% coverage of inside reflective surfaces. Likewise, damping treatments and coatings are not coverage dependent. Attacking surface areas where vibrational motion is most prevalent is more important than 100% coverage. A40% to 60% coverage is usually sufficient. To the contrary barrier materials rely on complete coverage as close to 100% as possible to realize their full acoustic performance.

Potential practical limitations for various coverages are as follows: maximum 10dB reduction for 90% coverage, 15dB reduction for 98% coverage and 20dB reduction for 99% coverage.

Facings Usually it is necessary to incorporate some type of thin membrane facing to cover the absorber layer exposed to the noise source inside a cabinet or enclosure. This protects the product from contamination and provides a surface that can be wiped down. As long as the film facing is in the 1 to 4 mil thickness range there will be a minimal effect on acoustic performance. Many times there is only a frequency shift with slightly lower absorption in higher frequencies and slightly higher absorption in lower frequencies. Damping and barrier materials are many times part of composite products not exposed to the environment and are not covered with facings.

Some typical facings are Tedlar, mylar and urethane.

51

Installation Most products are available in standard rolls, sheets or die cut to meet OEM specifications. The products can usually be hand cut using a utility knife, scissors, band saw or other common cutting tools. Attachment is recommended with solvent based contact adhesives for the urethane foam absorbers and composites. Adhesive recommendations should be reviewed in detail at the time of application as special considerations must be made depending on the surface shape and preparation, whether the surface is oriented horizontal or vertical and what working time is required. Pressure sensitive adhesive systems (PSA) are available for most products and are highly recommended. High tack acrylic based PSA backings are preferred to assure the best bond. Stick clips, insulation pins and other mechanical fasteners may be necessary in addition to the adhesives.

Sizing Machinery Mufflers Many OEM process systems utilize rotarypositive blowers that will require mufflers or silencers on both the intake and discharge.

Without such treatment noise levels could easily be in the 110 to 115dBA range. Blower sizes are described in inches (for example 12x25) where the first number represents the timing gear diameter and the second number represents the length of the rotor. The product of the gear circumference in feet and the blower RPM is the peripheral velocity of the timing gear. Noise and pulsation produced by rotary-positive blowers inherently reaches a critical level at gear pitch-line velocities of about 3300 feet per minute (FPM) for intakes and 2700 FPM for discharges. These critical gear pitch-line velocities are commonly referred to as 52

the blower transition speeds. Muffler selections shown in the QWYATT FLOW muffler product section (page 154) make reference to the blower transition speed. Consult QWYATT Sales Engineers on muffler applications for process systems and for other equipment such as compressors, engines, generators, turbines, etc...

Designing Sound Blankets Custom fit removable thermal/acoustic blankets such as the one shown at right are designed based on the equipment or component field measurements and/or housing/casing drawings. For proper construction and fit it is necessary to determine the equipment casing surface temperature. The inner and outer jacketing should be completely water resistant and suitable for both caustic and acidic environments. For durability, all blanket construction should be double sewn lock stitch with a minimum of 7 stitches per inch. All mating match seam blankets should have an overlapping flap cover which is also an extension of the loaded vinyl flexible sound barrier layer (internal to the blanket). This is essential for minimizing noise leaks. Stainless steel quilting pins should be utilized no greater than 18" apart to prevent shifting of the insulation. Hog ring construction should be avoided wherever possible in order to assure the highest blanket design quality possible.

53

Selecting Ventilation Silencers Fan and HVAC silencers are commonly used on low pressure air handling systems and equipment. Acoustic louvers are utilized in conjunction with enclosure or mechanical room ventilation where noise isolation is important. The fundamental tone or frequency that a fan produces is a function of each blade on the fan wheel passing the cutoff sheet on the fan housing. This is commonly referred to as the blade passage frequency (BPF). The first and second multiples of the BPF (harmonics) are prominent but less critical. Calculating the BPF is done by multiplying the fan RPM times the number of blades on the fan wheel, divided by 60 (converts to cycles per second or Hertz). Silencer performance can be approximated by finding the rated insertion loss of the silencer in the octave band where the BPF falls.

Designing Acoustical Enclosures Noisy equipment and systems may require path control modular acoustical enclosures where source treatments are not practical or effective. Sound enclosures can be freestanding structures, partially integrated or fully integrated with the equipment. Key design considerations for OEM acoustical enclosures include durability, aesthetic appeal, accessibility and visibility, acoustic performance, cost of installation and material manufacturing costs. Depending on the type of equipment, system or process; lighting and ventilation are also important considerations. The goal of the product engineer is to 54

meet the acoustic requirements with a design that incorporates all the necessary features.

Enclosure Design Rule #1: Air Leaks The chart at right shows how actual sound transmission loss relates to the enclosure wall transmission loss potential (from lab test data) and the percent open area. As shown in the example, as little as 2% open area over the entire surface area of an enclosure reduces the 38dB potential transmission loss to only 18dB actual transmission loss. Air leaks occur around windows and doors, at panel joints, around cutouts to accommodate pipes, ducts, utility lines and other obstructions and where the enclosure does not seal against the floor. Where enclosure ventilation is required, openings that are not acoustically baffled will further reduce the actual dB transmission loss.

Effective Enclosure Design Will Assure the Lowest $ per dB Cost Enclosure Design Rule #2: Ventilation

55

Heat build-up inside acoustical enclosures can be dissipated using forced ventilation fans and blowers in a draw through (see right) or blow through design. The intake and discharge of forced ventilation systems must be acoustically treated with louvers, lined baffles, hoods or duct silencers. The owner or designer of the enclosed equipment needs to determine what is sufficient air flow to dissipate heat. The formula at left is a good guideline. Proper placement and location of intake and discharge openings should ideally bring in air low on one side or end of the enclosure and draw air over the equipment before exiting high on the opposite end or side.

Enclosure Design Rule #3: Access Removable panel sections (bottom left) can allow for infrequent maintenance access in a limited area.

Hinged doors (at right) provide for more frequent access but require clearance for the door swing outside the enclosure. Sliding doors (see left) also provide easy access without protruding into work area space outside 56

the enclosure. Acoustical performance of sliding doors is less than that of hinged doors. Double doors provide access to a larger area. Sliding or removable roof panels and sections are best for major repairs requiring a crane. Small enclosure designs can be a knock down design where all panels are latched together for quick and easy disassembly.

Near Field

The near field is the region close to a sound source usually defined as 1/4 of the longest wavelength of the source. Near field noise levels are characterized by drastic fluctuations in levels as much as 10dBA for small changes in distance from the source. Near field references can pertain to both indoor and outdoor environments.

Far Field

The far field describes a sound field beyond the near field limits described above where the sound pressure level (SPL) drops off at the theoretical rate of 6dB for every doubling of distance from the source. This rule of thumb is called the Inverse Square Law. Please note that if the far field does not meet the criteria for a free field as described below, then less than the theoretical drop rate will pertain. In such case doubling the distance from the source may yield a drop rate of 3-4dB.

Free Field

To be considered free field there can be no obstructing surfaces in the sound path of spherical wave propagation. Free field conditions are characterized by SPL loss rates following the Inverse Square Law. Free field references pertain to large open outdoor spaces or in rooms where 57

walls and other surfaces are almost completely absorptive. Anechoic (without echoes) acoustical test chambers simulate free field conditions where omnidirectional sound wave propagation exists.

Direct Field

The direct sound field is also used to describe far field conditions that follow the Inverse Square Law SPL loss rate of 6dB for every doubling of the distance. The actual formula used to make calculations at various distances in the far/direct field is as follows: SPL1 [20x log (d2/d1)] = SPL2 where SPL1 is the noise level at the location closer to the source at a distance of d1 from the source and SPL2 is the noise level at a location farther from the source at a distance of d2.

Diffuse Field

In a diffuse field there are so many reflections contributing to the total sound field that sound levels measured virtually anywhere in the sound field are the same. Diffuse fields usually pertain to indoor environments. Rooms that are categorized as "live" have larger diffuse fields than free fields.

"Dead" rooms have much larger free fields than diffuse fields.

The reverberant field is essentially the same as the diffuse field. For indoor sound field discussions it is used to contrast direct fields. Reverberation test Reverberant chambers have all room surfaces almost Field completely reflective so that total sound energy remains constant throughout the environment and sound levels can be measured independent of location and distance.

58

Please refer to the figure below which shows the relationship between sound fields.

Sound Fields Relative To Distances From A Source

Environmental Applications Community Reaction To Noise Listed below are some of the key factors which can reduce the community tolerance level for noise in environmental applications. 





Where there are exceptionally low background ambient noise levels.

A noticeable fluctuation in sound level which would call attention to the source.

Pure tones or discrete frequency sounds regardless of the overall intensity.  Elevated noise sources such as vents, stacks, outdoor cooling towers and other clearly visible noise sources.  Any noises that disturb or interfere with sleep, communication or recreation.

59

ntermittent, impulsive or startling noises.  Low frequency sound which causes vibrations in windows, walls and other parts of building structures.

Distracting noise sources, such as breaking of glass at a bottling plant.  Any changes in noise patterns. 



Predicting Acoustical Barrier Wall Performance The nomogram at right can be used to describe acoustical barrier sound attenuation. Transmission loss or sound blocking through a freestanding partition or barrier wall will be determined in part by the acoustical properties of the barrier. The second factor affecting barrier wall performance is spillover noise following the diffracted path as illustrated in the figure at right. Sound waves will have a tendency to bend or diffract over the top and around the sides of a barrier wall especially in the lower frequencies. In the higher frequencies sound waves diffract less and are much more directional in nature. The shielding effect of the acoustical barrier and resultant noise shadow area beyond it are determined by the geometric relationship between the source, the receiver and the barrier height.

60

How To Use The Nomogram In the figure at right, distances A, B and D should be determined as follows. Distance A is from the point noise source (not the height) of the equipment to the top of the acoustical barrier. Distance B is from the top of the barrier to the receiver position (figure ear/head level). Distance D is from the source to the receiver (straight line). In the example at right the path length difference (A+B-D) equals 2 ft. Plotting a straight line from the path length difference through the frequency of noise in question on line F (1000 Hz) intersects the dB line at 16 in the example. Thus the estimated attenuation for this application would be 16 dB. Please note that the nomogram does not take into consideration the contribution from reflective surfaces. To be conservative in applications where reflective surfaces are present it is recommended that the final dB figure be discounted 20% to 25%. As the angle (â‚Ź) between the direct and diffracted paths increases, so does the noise reduction.

61

Predicting Community Reaction To Noise 1. Plot octave band sound pressure levels on Figure 3 at each frequency 63Hz to 8000Hz. 2. Determine the value of N where the plotted data intersects the highest curve.

3. Determine the sum total of all correction factors that apply as outlined in Figure 1. The sum equals value CF. These factors will influence the composite noise rating N1.

4. Calculate the composite noise rating N1 from the formula N1 = N - CF. 5. Refer to Figure 2 for predicted community response based on the calculated composite rating N1.

6. When dealing with sensitive community noise issues it may be necessary to contract the services of an acoustical consultant.

62

Sound Propagation Outdoors

Sound propagation is affected by changes in atmospheric conditions. Temperature variations will influence sound wave propagation in the direction of cooler air. Above left shows the shadow zone created as sound waves bend toward cooler air at higher altitudes. When this occurs, a noise source may be visible at a distance but quieter than expected. The other extreme shown above right occurs when air is cooler closer to the ground such as at night or over calm ground. If the ground surface is reflective, sound waves will continue to bounce and hop, traveling much farther than otherwise expected.

Wind directions and currents also affect sound propagation outdoors. Noise sources emitting sound in the direction of wind travel (downwind) will tend to propagate farther than expected as shown above right. Conversely, sound emitting in the direction against the wind (upwind) will travel less than expected because of the shadow zone created as illustrated above left. This phenomenon when combined with temperature fluctuations can explain the common occurrence of aircraft noise fading in and out of hearing range while the plane is moving toward the listener.

Treating Pure Tones And Fundamental Harmonics 63

The above example plotted for an induced fan air system shows a frequency spectrum with spikes at the fan fundamental or blade passage frequency and decreasing spikes at each harmonic or whole number multiple. Most types of rotating equipment such as compressors, engines, blowers and fans generate these pure tone spikes that are elevated above the other frequencies. The tones and harmonics are related to the rotational speed of the equipment and the number of blades, lobes or other driving components. In the example above, the fan tone is a function of the RPM divided by 60 times the number of blades on the fan wheel. For applications such as cogeneration (boiler induced draft), dust collectors, scrubber systems, incinerators, etc. the ventilation fan generates its fundamental tone in the 100 to 300 Hz frequency range. This low frequency noise warrants special treatment with tuned silencer designs. Standard packed silencers provide overall A scale reductions but can miss the offending fan tone which is usually the source of neighborhood complaints in the first place.

64

Acoustical design is burdened by many time-honored misconceptions.

Acoustics can be a mysterious science sometimes. Logarithmic addition just doesn't come naturally to most of us, and the concepts of sound absorption vs. sound transmission, reflections vs. room modes, and reverberation vs. resonance aren't always intuitive.

It is little wonder, then, that applied acoustics - especially when the application is studio design - is full of myths, fallacies and misconceptions. Sometimes it's a misunderstanding of the principles. Sometimes, it's taking a grain of truth and using it incorrectly in a different situation. Sometimes it's solving one problem but creating a bigger one in the process. Whatever the cause, a second look at traditional design concepts and construction techniques reveals that some acoustical "truths" are false.

Yet some of these misconceptions have managed to become such standard practice that acoustically speaking, they can be downright dangerous if you aren't aware of them. This article takes some prevalent acoustical myths, each of which is encountered frequently in broadcast facility designs, and shows that there may be a better way to get the acoustical performance you need.

Myth No. 1: Absorption improves transmission loss Absorption means reducing the sound, right? So putting some fuzzy material on the wall will keep the neighbors happy, right?

Unfortunately no. It is true that when sound strikes a surface, some of the energy is absorbed and some is reflected from the surface. It's also true that some materials absorb more sound 65

than others. But in most cases, although this may do a lot for the sound within the room, it doesn't help much when the problem is sound transmitted through the walls or ceiling of the room.

It is tempting to believe that soaking up all the sound will keep it from going somewhere else. Other things held equal, increasing a room's absorption will indeed reduce sound pressure levels in the room. But the rooms we live and work in generally have moderate absorption to begin with, so in a practical sense it is rarely possible to use "normal" finishes to make order-of-magnitude differences in the overall room absorption. As a result, it is difficult to affect steady-state sound pressure levels in the space by more than a few decibels with absorption alone. That doesn't mean that you can't make a room more pleasant to work in or a better monitoring environment, only that you can't make a noisy space significantly quieter by changing the finishes. The harshness of a highly reverberant space doesn't stem from loudness as much as from factors, such as poor intelligibility and the direction and frequency content of the reflected sound. Even in a completely absorptive (anechoic) environment, the sound pressure level at a wall surface still has a direct sound component, which is dependent only on the sound energy that the source is producing and the wall's distance from it. No amount of absorption can further reduce the level.

Remember, too, that it is much more difficult to keep lowfrequency sound from going through a wall than a highfrequency sound. It is equally difficult to obtain effective lowfrequency absorption over a wide bandwidth (e.g., a full octave or two). So the effect of absorption on sound isolation is at least where you need it the most.

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Sound absorption can be one effective component of a larger noise control solution for problems involving mechanical equipment. In those cases, the sound power of the noise source is fixed. When dealing with voices or reproduced sound, however, an acoustically "dead" environment sometimes encourages you to speak louder or increase the volume to compensate. This may offset any reduction in the overall room levels, or may actually make them worse.

Myth No. 2: The 3-panel partition

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How many times have you seen magazine articles on studio design in which "highperformance" partitions are detailed? Often these are touted as "triple walls" or described as a seemingly endless stack of different sheet goods with air spaces interspersed among them. (We used wallboard plus fiberboard plus wallboard then a 1-inch gap plus wallboard plus rubber plus plywood then a 2-inch gap plus...") By serendipity these walls may be sufficient for the needs of an individual studio, but they're Figure 1. Plan view of a not always a cost-effective use simple double stud partition of materials or available space. (a). Adding dry wall will actually lower its sound Take the example of a simple isolation if it creates a triple double stud partition. Starting (b) or quadruple (c) wall. A with a single layer of gypsum mass-airspace-mass board on the outside faces and arrangement offers the best cavity insulation (Figure 1a), use of materials and space. this wall has a sound Additional dry wall at the transmission class (STC) rating outer faces (d) increases of STC-56. If an attempt is attenuation dramatically. made to "improve" the wall by putting two additional layers of gypsum board on the inner face of one stud (Figure 1b), the STC rating actually decreases, to STC-53. Following this "more is better" mindset, if two or more layers of gypsum board are added to the inner face of the other stud (Figure 1c), the STC rating is still lower, at STC-48.

(Never mind the difficulty in actually building this version.)

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This seems grossly counter-intuitive-more barriers should improve attenuation, not reduce it. Remember that in a cavity wall, transmission loss depends on the mass (and stiffness) of the surfaces and on the thickness (and absorption) of the airspace between them. In this example, putting gypsum board on the inner faces of the studs - creating a 3-panel or 4-panel wall - divides the airspace into smaller segments, and the lowfrequency sound transmission loss (which in this case dominates the STC rating) is reduced.

If only one layer of gypsum board was added to each outer face of the original wall (Figure 1d), an STC rating of STC-63 is achieved. This uses less material and less space than the 4panel wall (Figure 1c) but gives significantly better performance. To optimize acoustical performance, how the materials are put together is often more important than what materials are selected.

Myth No. 3: Angled glass In traditional studio designs, interior windows - between a control room and a booth, for example- often have two panes of glass, with one or both tilted a few degrees from vertical.

(Sometimes it's three panes - see myth No. 2.) Several reasons are given for this design technique.

Many people contend that taking the two panes out of parallel eliminates resonance (standing waves) in the air cavity between them, which would otherwise limit the transmission loss at the resonant frequencies. In theory, this is a valid concern. In actual construction, however, there is always a practical limit on the overall thickness of the wall into which the window is built.

Achieving the tilt by spreading the two panes of glass wider apart of their top edges would put each pane's center of gravity further out from the wall, and the structural support provided by the wall could be questionable. So, the usual "solution" 69

achieves the tilt by moving the glass in at the bottom of the window, thus putting the two panes close together.

The result is an average airspace between the panes that is sometimes little more than half of what it could be if both panes were vertical. (See Figure 2.) Because sound transmission loss through the assembly is highly dependent on the width of the airspace, the acoustical benefit of angling the glass is often negated by the reduced separation between the panes. For a given overall wall thickness, maximizing the overall airspace between the panes minimizes sound transmission through a window.

A second reason for tilting the glass is to redirect reflections of sound from the window. Because of sight line requirements, studio windows are almost always at a height where significant reflections into microphones can occur.

Usually the angle necessary to eliminate this problem is more than what the window frame's depth can accommodate. The detrimental reflection just occurs from a different point on the glass, as Figure 2 also illustrates.

Figure 2. Angling the glass in a studio window reduces the average airspace between the two panes, thereby increasing sound transmission through it. In addition, angling panes to eliminate sound reflections is generally ineffective. Reflections are not eliminated but simply moved.

There are valid reasons to angle glass in double pane windows, but they have nothing to do with improving the sound 70

transmission loss through the window. One reason is to alleviate flutter echo between the window and an acoustically hard surface on a parallel wall. Another is to reduce the multiple visual reflections that can occur between parallel glass surfaces. But the optimal solutions allow the glass to be kept vertical, relying on good room geometry and finishes to fix the first problem and proper lighting to solve the second.

In any event, the acoustical characteristics of the glass itself, the mounting details, and the interior perimeter absorption (on the boundary surfaces of the space between panes) all have a much greater effect on the sound isolation of the window than the angle of the glass.

Myth No.4: Acoustically "transparent" materials The sound-absorbing properties of standard building materials are often given as a noise-reduction coefficient (NRC) rating.

Unfortunately, this standard measurement takes into account only speech frequencies and ignores the extremes of the audio spectrum. More important, it measures the absorption of a material or assembly in a test chamber with random incidence of sound on a relatively small sample.

In practice, absorptive materials are often place on walls where the sound is almost always at "grazing" incidence or nearly parallel to the surface. When you drop a rock into the water it sinks, but when you throw it parallel to the water, it will sometimes skip along the surface. Sound behaves in much the same way: many materials that appear "transparent" based on NRC ratings or porosity are actually highly reflective to sound at grazing incidence.

One example is perforated metal, which frequently is incorporated into prefabricated modular acoustical enclosures to provide an "absorbent" interior surface. If a modular room is 71

shaped to provide a reflection-free zone (RFZ) for a specific listening area or if loudspeakers are mounted near the perforated metal surfaces, sound will strike the surface at grazing incidence and the absorptive properties will be rendered much less effective than intended.

Myth No.5: The field-fabricated door Doors are almost always the weak link in the sound isolation of an acoustically critical room. Moving parts cannot be built as solidly and airtight as fixed components, and real life products don't seal completely or stay in perfect alignment.

To make matters worse, some manufacturers promote "acoustical doors" with ratings based on tests in which a nonoperable door panel is fixed into an opening. Seeing this, many people (including some studio designers) have made valiant but futile attempts to improve a door's sound-isolation performance by making the door panel better. Years ago it was common to see two solid core wood doors bolted together with a layer of "machine rubber" sandwiched in between. Hey, it may not work, but it sure is bulky and unattractive.

What is usually overlooked, however, is that the door panel itself is rarely the limiting factor. The acoustical leaks are almost always worse at the seals around the perimeter of the door. Even the best field-applied door seal can quickly go out of adjustment and lose optimum contact and closure between the door and its frame.

If we consider a 3' x 7' door with a gap around its perimeter of only 1/64 inches, the gap represents only 0.1% of the total door area. This is enough, however, to effectively reduce an STC-36 72

door to an STC-29 rating. More important, if the door panel is beefed up to stop an additional 10dB of sound, the composite transmission loss increases only 1dB. In other works, improving the door panel barely affects the overall performance, because the perimeter seals aren't improving in a proportional manner.

Sound-rated doors - in which the door, frame and seals are manufactured as an integral unit - are the only reliable means of getting acoustical performance that is significantly better than a relatively simple door panel and field-applied seals.

Alternatively, using multiple doors in a vestibule arrangement or keeping the door opening separated from the noise sources will help obtain appropriate sound isolation.

Myth No. 6: Mostly right is good enough Failures in studio construction happen more frequently from lack of attention to detail than from an error in the overall design. One typical example is in building a drywall partition.

Assume that such a partition is carefully erected with isolated stud framing, filled with acoustical insulation, and finished with multiple layers of drywall carried from the floor slab all the way up to the metal deck above. Later the electrician uses a claw hammer to fun some conduit through the wall, and the plumber puts in a sprinkler pipe or two. You note that there are some gaps around these penetrations and that the drywall doesn't fit into the corrugations at the deck, so you issue instructions that all gaps are to be stuffed with insulation. That seems harmless enough, but you've probably just wasted half of the effort and materials that went into the wall.

The insulation provides sound absorption, but it isn't a barrier to sound transmission through and around the wall. Even though a 3/4-inch gap along the top of a 15-foot length of wall 73

represents only one square foot of opening, stuffing it with insulation instead of sealing the gap can limit the wall's overall performance by more than 10dB. Actual field tests of a drywall partition of these dimensions confirm this. Initially the gap had been stuffed with insulation, but later a barrier designed to conform to the gap was installed and sealed airtight into place.

This single modification improved the sound isolation from STC-31 to STC-44.

What is important in facility design and construction is balance. There is no point in putting a great door into an inferior wall or vice versa. And the best, most expensive partition is only as good as its leakiest electrical box. As the sound-isolation requirements of a room increase, the effect of an acoustical weak link becomes more and more devastating.

Each of the components must meet the required performance or they will fail collectively.

Myth No. 7: Reverberation time in the control room Articles that discuss the acoustical design of a facility often refer to measurements of "reverberation times" (T60) in small spaces, such as broadcast control rooms. Some designers have even gone so far as to specify optimum T60 values in the range of 0.5 seconds or less for small rooms.

The definition of reverberation time involves the statistical decay of sound in the reverberant field of an enclosed space. In a small room, particularly one with the type of absorptive finishes generally found in control rooms, there is no location in the room that is said to be in the reverberant field. Nor do the reflections of sound within the space develop any statistical decay. Certainly the amplitude and time-of-arrival patterns of these reflections are of paramount importance in defining the acoustical environment. However, reverberation time is not an appropriate metric to use in quantifying that information.

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Often, the measurements cited for reverberation times in small rooms are questionable. Much of the test equipment used to analyze decay characteristics over full-octave or third-octave bands has a filter slope near the values of the "T60s" themselves. The measurements may have nothing to do with the room; they may be measuring the capabilities of the test gear.

Myth No. 8: You can't hear heat From the standpoint of audio fidelity, it is desirable to minimize the length of the cables that connect a loudspeaker to its amplifier. What better place, then, for the amplifier but directly beneath the speaker? Unfortunately, if you fall into this trap, saving a few feet of speaker wire may cost you dearly in attendant acoustical problems.

Temperature gradients and air movement between a speaker and listener can drastically affect the sound field, much like heat rising from hot pavement can distort an optical image.

This is most commonly noticed at windy outdoor concerts, where the frequency response of a distant PA speaker stack seems to be changing. The cause of this is not the wind "blowing the sound around" and changing its direction by pulling it along with the moving air, as is commonly thought. It is the result of the sound waves passing through air temperature gradients introduced by the moving air currents. The frequency-dependent refraction (bending) of some sound waves and not others results in the changing frequency response. The actual propagation direction of the sound remains relatively unaffected.

In the control room, this same phenomenon can cause perceptible effects most frequently noticed in shifting of the 75

acoustical stereo image. Putting amplifiers directly beneath the monitor speakers allows them to vent heat directly in front of the speakers, and the thermal turbulence creates audible distortion. Similarly, the heat generated by some mixing consoles (coupled with poor ventilation design) ironically renders them unsuitable for use where accurate monitoring is required.

This same phenomenon is often observed where air diffusers for the heating, ventilating and air-conditioning (HVAC) systems have been located incorrectly in a room. In any critical monitoring environment, even seemingly "non-acoustical" heat sources and airflow must be carefully controlled to maintain a sonically neutral sound field.

Beware the acoustical myth Many more fallacies and misconceptions in acoustics than what we have related here exist, but you get the idea. Individually, the examples in this article may help you avoid specific pitfalls in studio design and construction. Collectively, they serve to illustrate the dangers in believing everything you read in a magazine or see at a world famous studio. The "it's-alwaysdone-this-way" approach may not be based on sound acoustical principles, let alone be the best means to achieve desired results.

Any time an acoustical myth can be identified and replaced with a little common sense or objective proof, acoustics as a science becomes less mysterious, and one less acoustical "truth" will be preached as gospel.

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Noise Control for Teleconferencing, Distance Classrooms and Webcasting 

The teleconference room, an important part of the audio circuit, demands adequate acoustical treatment of all room surfaces.

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The slightest echo can undermine speech intelligibility.

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Reverberation reduction may require significant sound absorption material.

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Most offices and conference rooms are too noisy and too reveberant to serve as teleconferencing rooms.

The teleconferencing room should be designed as a package with the aid of a professional acoustical consultant, especially when an upscale interior design is required.

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Teleconferencing rooms, webcasting rooms and distance classrooms all require the same considerations for noise as traditional broadcasting rooms. Acoustically there are two main areas for consideration. First is noise coming in the room from the outside. Second is to keep the reverberation of the sound inside the room down to a level where the microphone doesn't "hear" reflections causing a hollow or echoic sound that limits speech intelligibility.

Noise Criteria of 20 decibels (NC-20) should exist in the broadcasting room. This means when the room is empty and quiet the sound pressure level in the room is 20 decibels or less.

The existing conditions in most cases make achieving this level a challenge. Common noises that create problems are heating & 77

air conditioning systems, telephones ringing or people talking in the next room, office equipment, and traffic noise coming from the street.

Speech Intelligibility in broadcasting requires a Reverberation Time of .75 to 1 second. Reverberation Time (RT60) is the time it takes for noise to reduce in volume by 60 decibels.

Untreated rooms have a variety of reverberation times depending on the size and shape along with the absorptive qualities of the surfaces in the room. Generally, you can expect your room to require additional absorptive materials on the walls, ceilings, floors and/or other reflective surfaces, such as tabletops and file cabinets. Existing reverberation times can be calculated. The amount of absorptive material needed to reduce that time to the recommended RT60 can also be calculated.

Once this criteria is established acoustical materials and location can then be specified.

If you are considering installation of a teleconferencing room, webcasting room or distance learning classroom, you should consult with a qualified acoustical designer, as early as possible, to assure your room selection is the optimal location to avoid existing problems. If the location of your room has already been established, your acoustical designer can assist in determining the layout and orientation of monitors, speakers and microphones, as well as other acoustical considerations that will assure the best possible design.

In most applications, Acoustical Treatments are more effective and less expensive when a qualified acoustical designer is consulted early in the process of developing your plan.

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Controlling HVAC Noise When people who work in existing broadcast and production facilities are surveyed about workplace comfort, their most prevalent complaints involve the heating, ventilating and airconditioning (HVAC) systems. The problem they cite most frequently, aside from temperature control, have to do with excessive noise and vibration.

How can this happen so often? First, the requirements for an HVAC system in a technical facility are quite different from those in typical office spaces. Technical spaces have extensive heat loads and other unique characteristics associated with a high concentration of electronic equipment or production lighting. As a result, adequate cooling, airflow, humidity control and filtration are vital issues. Second, a majority of these technical spaces have acoustical requirements that are much more stringent than those for run-of-the-mill buildings.

Combine these two features and it's no surprise that without a great deal of care, the results are often disastrous.

It would be nice if there were qualified engineers and acoustical consultants involved in the design of every construction project (we all have our little fantasies), so that a TV facility's management and staff would not have to understand the ways in which noise and vibration problems can arise from a poor HVAC design. Unfortunately, the reality is that architectural acoustics is one aspect of broadcast and production that is commonly misunderstood, misrepresented or misapplied - by broadcasters and by may design professionals.

This article discusses some problems with HVAC system noise and vibration that are commonly encountered in broadcast and production facilities. Sometimes just knowing the potential pitfalls is enough to avoid falling headlong into them.

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Estimating Heat Loads When designing an HVAC system, the first step is to accurately identify the heat loads generated by the occupants, equipment, lighting and surrounding environment. Although this isn't really an acoustical issue, it's an area that suffers from the "garbage in, garbage out" syndrome. If a project's mechanical and electrical engineers are given bad information about equipment loads by the broadcaster's engineers, consultants or systems integrators, the HVAC systems they design may never be able to provide an appropriate environment for equipment or operators. When HVAC systems don't work right, it can often be traced to a mismatch between capacity and actual load.

Unfortunately, it's difficult to obtain accurate information about the power requirements of broadcast equipment, much less the heat dissipation. Manufactures sometimes list peak power consumption, sometimes power consumption at idle and sometimes just the fuse rating or recommended circuiting requirements. Invariably, if they exist at all, the manufacturer's specs represent some sort of worst case, which for many equipment items (audio amplifiers or any machine with a tape transport motor, for example) bears little resemblance to their actual power consumption over time.

For someone trying to tabulate power consumption of dozens or hundreds of equipment items, it's tempting to guess high when the data you're collecting is ambiguous or undefined.

This isn't much of a problem for the electrical systems, where the downside of excess capacity is only the cost of oversized circuits or transformers. For HVAC systems, however, it's sometimes worse to have too much capacity than to have too little.

Besides the increased noise and vibration that comes with oversized equipment, some HVAC system types - particularly 80

less-expensive varieties - don't respond well to conditions outside their design range. If oversized, many systems will short-cycle, rapidly dropping the temperature in an occupied space over the span of a few minutes, then shutting off for a longer period while the humidity climbs and the air stagnates.

Compounding the problem is the diversity of room types that exist in a typical TV facility. Production control rooms and edit suites may have technical equipment that operates 24 hours a day, 365 days a year, but they undergo fairly drastic swings in their lighting loads and number of occupants over the course of a typical day. Technical operations centers or tape rooms, on the other hand, will have relatively constant equipment loads with few occupants. Furthermore, neither of these types of rooms are subject to the seasonal changes in HVAC system operation that are typical in the office areas. Technical spaces often require cooling year-round.

Studios and stages are yet another category. They are subject to enormous heat loads from production lighting instruments when they're in operation, but have little heat-generating equipment the rest of the time. In addition, the changing requirements of newer fluorescent-type production lighting and the next generation of cameras may combine for an order-ofmagnitude difference in heat loads compared to today's totals.

TIP: Do your homework. When you're asked to estimate power consumption for your technical electronic equipment, the time spent getting accurate information can mean the difference between a successful facility and one that just never works right.

HVAC Equipment Location There is no substitute for keeping equipment that generates noise and vibration as far away as practical from acoustically 81

sensitive spaces. If an air handler is too close to a studio, obtaining adequate sound attenuation through the duct system can quickly become a losing battle. And the risk of excessive noise or vibration via every other potential path is greatly magnified.

Perhaps due to their superficial resemblance to warehouse space, TV studios may appear to inexperienced mechanical engineers to be perfect candidates for rooftop units located directly overhead. This is almost always a big mistake, because exposing a studio to noise and vibration by poking a hole in its roof and placing rotating machinery there makes it virtually impossible to achieve industry-standard background noise levels. Standard building mechanical systems are almost always inadequate to handle the specialized needs of technical facilities. In evaluating an existing building to house technical spaces, it is essential to consider the existing mechanical systems, as well as the space needed for supplementary systems.

TIP: Think ahead. Whether in a new facility or a renovation, knowing where HVAC equipment will be located relative to the acoustically sensitive spaces can keep you from having to face intractable problems later on.

Duct Silencers You would think that any device called a sound attenuator, silencer or sound trap would be invariably beneficial to HVAC noise control. Sound attenuators are an effective means of reducing broadband noise as it travels down a duct system, and have the advantage of predictable performance. Like any other tool, however, these products work properly only when they're used correctly.

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Duct silencers operate by restricting the airflow through a system of baffles exposing as much of the air stream as possible to sound-absorptive filler materials and/or resonant cavities. As a result, they have "self-noise" characteristics, meaning that they generate noise themselves due to turbulence in the airflow through their internal baffles. Silencers should be located far enough upstream of any acoustically sensitive space to ensure that the noise they generate is adequately attenuated before it reaches the occupied room.

Another frequent misuse of sound attenuators involves their placement in the duct system. If a silencer is located within a mechanical room, noise may enter the system through the sheet metal duct after the silencer. If a silencer is located away from the mechanical room walls, noise may escape the system through the sheet metal duct before it is attenuated by the silencer. Ideally, a sound attenuator should be located within or immediately adjacent to the mechanical room's walls.

TIP: If you're using sound attenuators, make sure they're located where they will perform the job that's intended.

Duct-Borne Noise The duct work that connects a fan or air handler to a room is a contained system that will also connect the equipment noise and vibration to the room unless adequate precautions are taken to attenuate the noise before it gets there. Without internal sound-absorptive duct liner or prefabricated sound attenuators, noise travels effectively down the duct system right along with the conditioned air.

Acoustical crosstalk is a similar problem that occurs when two spaces are connected by a common duct system with inadequate internal sound attenuation. Noise from one space enters the duct through the supply-air diffusers or return-air 83

grilles and travels through the duct to a similar opening in another room.

Although noise control issues through the supply air duct system are routinely considered in HVAC design, inexperienced mechanical engineers and contractors often forget that the return-air path is an equally important contributor to noise problems. In fact, because return-air systems sometimes employ common plenums above corridor ceilings, there may be less duct work in the return-air path to attenuate the noise, and the transfer of return air from one space to another may be a significant breach of the sound isolation between them.

TIP: Make sure that all duct systems are laid out to prevent crosstalk and to attenuate the fan noise.

Velocity Noise As conditioned air travels from a fan to an occupied room, it is subjected to acceleration, deceleration, changes in direction, division and a variety of surfaces and obstacles. Each of these effects disturbs the uniformity of the airflow and causes turbulence, which in turn creates noise.

It is essential to limit the velocity of the airflow through all duct work systems in order to keep it from generating excessive noise. This is particularly true at the final branch ducts and the neck of the supply diffusers and return grilles where this regenerated noise is exposed directly into the occupied spaces.

TIP: Keep airflow velocities low throughout the duct systems serving acoustically sensitive spaces.

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Volume Dampers An important feature in the proper design of a duct work system is the ability to control the amount of air that flows through each segment of the duct to ensure that the volume of air supplied to each space is tailored to its conditioning needs and that each supply diffuser in a given room is balanced with the others. To accomplish this, volume dampers are needed to limit the amount of air that is allowed down the duct path.

Unfortunately, dampers accomplish their volume control by pinching down the air stream, increasing the pressure and consequently the noise wherever they occur.

For office spaces, ceiling supply diffusers routinely are installed with face dampers, which are volume control dampers located right at the inlet to the diffuser. The airflow noise created by the face dampers is essentially exposed directly into the room. In acoustically sensitive spaces, even if face dampers are left wide open they can generate audible noise.

TIP: Don't use face dampers on the diffusers to adjust the air volume. Adjust the volume upstream using opposed-blade-type dampers.

Vibration Isolation Airborne noise that radiates directly from HVAC equipment is only one part of the story. Rotating or motor-driven machinery also generates vibration energy that can travel through a building's structure and radiate from the walls, floors and ceilings in the form of airborne noise. It is essential to control vibration at its source, because once it's allowed to transmit into the building structure, vibration from HVAC equipment is widespread and extremely difficult to contain.

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Vibration is best controlled by decoupling the vibrating equipment from the surrounding structure. This can involve spring mounts, elastomeric mounts, inertial bases, floating floors and/or structural isolation joints.

Vibration isolators must be matched to the load they carry. It's fairly intuitive that a spring that is fully compressed doesn't offer any isolation from the supporting structure. What may not be so obvious is that an uncompressed spring is just as ineffective. If the weight of the equipment doesn't deflect the spring, it means that the spring is stiff enough that the vibration can transmit directly through its coils. Oversizing a spring's capacity is just as detrimental as leaving it out entirely.

TIP: Use properly sized isolation mounts to keep vibration at the HVAC equipment from being transmitted into thebuilding structure.

Flanking Paths When isolating HVAC equipment or piping or duct work from the building structure, it's important to verify that there are no flanking paths or other means for their vibration to be transmitted into the surrounding construction. For example, it is futile to mount a fan unit on springs unless there are flexible canvas or neoprene connectors at the supply and return ducts that attach to the fan. Otherwise, the vibration will travel through the duct to the first place it attaches to the building.

Similarly, any piping and conduit that are connected to vibrating equipment must also be isolated with flexible connections.

TIP: Remember that noise and vibration don't always follow the most obvious path in getting from the source to the place you don't want it to go. Don't allow rigid connections to defeat the operation of vibration isolators.

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Penetrations One of the most commonplace problems caused by HVAC systems has nothing to do with noise and vibration generated by the equipment or duct work. As it is distributed throughout the technical spaces, duct work inevitably penetrates the wall, ceilings and floors that are responsible for a room's sound isolation capabilities. If these penetrations are not adequately treated, they allow sound leaks that can render these acoustical barriers completely ineffectual.

First, the penetrating duct work should be supported independently from the partition. If a duct rests on the wall it penetrates, any vibration within the duct can be transmitted into the wall itself, which can then provide a large radiating surface to turn the vibration into airborne noise. Second, the duct should be resiliently isolated from the surrounding construction as it passes through the partition, to avoid any contact that might transmit the vibration into the wall. Third, the area surrounding the penetration should be sealed airtight, using materials that won't allow noise from an adjacent space to leak through the gap.

TIP: Make sure all penetrations through sound isolation walls and ceilings are sealed resiliently and airtight.

The Best Defense When it comes to HVAC noise and vibration control, even with the best of intentions, there are hundreds of ways to make acoustical blunders that can render a technical space virtually unusable. There is no substitute for getting qualified help for the mechanical and acoustical design of HVAC systems in a technical facility.

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If you're cognizant of the general mechanisms behind typical acoustical problems, however, it's much more likely that you'll be able to avoid at least the most common ones.

Room Acoustics Your decision is finally made, after weeks of reading reviews and driving from store to store, you finally decide on the speaker system that's going to make your home theater room come alive. It's a little more money than you wanted to spend, but your local dealer has offered you a deal that's hard to refuse. The next thing you know, you're back home pulling speakers out of cardboard boxes, and draping speaker cables all over the place. Finally, when all the connections are made, you plop yourself in the from row center seat, hit the play button and ...hey, wait, what's going on? Where's the center-channel detail you heard in the store? And where's that thundering bass?

Welcome to the mysterious world of room acoustics. The reason your speakers sounded different in the dealer's demo room most likely had to do with the difference between the acoustic characteristics of his room versus yours. Audio experts have known for years that the physical attributes of listening rooms are every bit as important as the audio equipment involved. As a matter of fact, some would argue that room acoustics are far more important than popular equipment features such as gold connectors, polypropylene capacitors, and the like. Now don't get us wrong. It's not that these details are unimportant, it's just that room acoustics are far more important-and often overlooked.

One of the first scientists to investigate the role of room acoustics in high-quality sound reproduction was Roy Allison.

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Allison's seminal research in this area was revolutionary and led to a new way of looking at speaker and equipment design.

The basic premise is that the acoustic makeup of a listening room can play a significant role in the total system of sound reproduction. Allison explains: "The acoustic parameters of a room have a tremendous effect on the quality of sound you hear. The problem is that acoustics as a science is poorly understood by many audio enthusiasts. However, it should be known that one does not need a Ph.D. in the field to properly treat their own listening room; a few modest room treatments can go a long way. The only trick is knowing which ones are best to use."

Back To School Before we can dig deeper into the subject of listening room acoustics, a basic review of the physics of sound is prudent. By understanding the way that soundwaves travel in listening environments, we can more fully understand how to keep them out of trouble.

Though the sound field in Although the exact acoustic any room can be quite soundfield that occurs in an complex, this illustration enclosed space can be quite shows the three basic ways complex, there are three sound travels in a home components that predominate. The theater. first is the direct sound from the speakers themselves. These are the soundwaves that travel in a straight line directly from the speaker drivers to the listener's ears. These direct sounds are considered the most significant component of sound reproduction because of their relatively 89

large amplitude and transmission characteristics. But they're easy to deal with: All you need do is make sure you have an unencumbered line of sight from the speakers to your listening seat.

Next are the first reflections. These are the sound waves that bounce off surfaces flanking the speakers and the listeners. As the diagram at left illustrates, these sounds typically bounce off nearby walls, the ceiling, and the floor. Acousticians feel that these early reflections are important to the perception of the soundstage. If you don't do something to attenuate these reflections, the soundstaging and imaging produced by your speakers will become less accurate.

The last component is reverberation. Reverberation consists of the countless random reflections that bounce off other surfaces in the room and eventually arrive at the listener's ears. These sound reflections reinforce the feeling of room size and ambience. When you are in a large room with hard surfaces, such as a Gothic stone church, the reverberatory echoes bouncing off the stone walls are the components that give you that cavernous sonic experience.

Listening Room Physics So how do these components affect your listening room? When you fire up your new home theater speaker system, all the sounds bouncing around the room combine to form a unique acoustic room signature. This signature effectively becomes superimposed on the primary recording and modifies it. It's almost as if there are two separate sound systems in the room playing at the same time.

Acousticians agree that the best way to design a listening environment is to strive for sonic balance of these "systems."

For example, you don't want an overemphasis on reverberated 90

sound, or you'll feel like you're listening in a tiled bathroom.

On the other hand, you don't want a room void of reflections, in which case the room will sound dead and lifeless. The ideal listening room allows some of the first reflections and some of the reverberatory components to arrive at the listener's ears, just enough of each to balance the resulting sound and make it sound natural.

There are two ways to control the first reflections and room reverberations to achieve sonic balance: absorption and diffusion.

Absorptive surfaces consist of materials that dampen sound energy so that only a fraction of the energy is reflected. The portion that isn't reflected is actually Surfaces interact with soundwaves by reflecting them, absorbing them, or converted into a tiny diffusing them. amount of thermal energy that dissipates into the air. Acousticians use absorption coefficients to indicate how well a material absorbs sound. The scale ranges in value from 1 to zero. A material with an absorption coefficient of 1 absorbs sound energy completely. A material with a zero coefficient reflects it entirely. As you might imagine, real-world materials lie somewhere in between.

The other technique used to control sound is diffusion. The principle here is to take the soundwaves directly at the diffusive surface and break them into many small components. The resulting soundfield is then scattered around the room at greatly reduced magnitude. This technique can also be augmented in a listening room via natural room components like bookcases and furniture, or with specially engineered diffusion panels.

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How to Do It There are dozens of methods and hundreds of products available to whip your listening room into shape. Rather than catalog all the products and techniques available, we will take a more pragmatic approach- we will consider each room surface individually and suggest appropriate treatments.

First, let's look at the floor and ceiling surfaces. These can be the worst offenders in a listening room because they're often constructed of extremely reflective materials. The absorption coefficient, for example, of your standard plaster/gypsumboard ceiling is approximately 0.5 at 1 kHz. In other words, sounds bounce right off this stuff. And hardwood and tile floors aren't much better, with absorption coefficients at 1 kHz of .01 and .07, respectively. Fortunately, though, there is a standard household building material that soaks up soundwaves: carpeting. Both wall-to-wall carpeting and area rugs do a wonderful job of absorbing sound energy. And fortunately, carpeting goes quite well with most listening-room decors.

Ceilings, however, can be more problematic. One could glue carpeting to the ceiling, but we doubt your interior decorator (read: spouse) would approve. The next best thing is to install acoustic ceiling tiles. The least expensive option is to use the standard 1 - by 1- foot, fiber-based, tongue-and-groove tiles sold by building supply stores. Typically, these are installed 12 inches on center, on 1-by 3-inch strapping that is firmly screwed into the ceiling joists. Properly installed, these fiberbased tiles actually do a pretty good job of absorbing soundwaves, particularly in the higher frequencies.

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Most of the walls in your home probably 93

look like A, which means you can hear Even better sound absorption can be someone speaking loudly in the next obtained via drop room. However, by adding a sheet of ceilings, and most drywall to the existing wall, as building-supply illustrated in B and C, you can stores offer a wide dramatically quiet things down. And with the two independent walls shown in variety of acoustic tiles designed for D, you shouldn't hear a thing coming this purpose. The from any adjoining room. standard "wormhole" pattern tile is common in commercial applications, because it provides a good mix of absorptive and diffusing properties. One thing to note: If you plan to listen at very high sound levels, drop ceiling can rattle. This could be quite annoying, but several companies offer small rubber hangers to isolate the drop-ceiling structure and thus reduce this problem.

You can also consider the professional acoustic ceiling materials. Various companies sell materials that are decorfriendly and specifically designed for ceiling use. IIlbruck, for example, offers their famous Sonex panels, made from a new, Melamine absorptive foam. These panels are available in a number of different surface textures and a variety of colors. For those who are opposed to the commercial look of Sonex, there are plenty of flat-faced products available. These are typically constructed of fiberglass or foam materials. Acoustical Solutions, for example, offers a number of panels that can be applied to the ceiling. Their Alphasorb panels are constructed of fiberglass sheets wrapped with woven decorator fabrics and are offered in 65 different colors. Several companies also offer special ceiling diffusing panels. These panels come in a multitude of shapes and sizes, all designed with intricate diffusing surfaces.

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Listening-room walls often promote sonic misbehavior because they are typically constructed of hard substances such as gypsum board or wood. Since the wall surfaces located near the listener are directly involved in first reflection, it is especially important that they be addressed. For the economical approach, ordinary drapes can go a long way. You can also place furniture, plants, or other items in the way to diffuse the sound.

For example, it's hard to beat an upholstered couch or chair for sound absorption. If you place an object like this right at the spot on the wall where first reflections occur, a remarkable improvement in sound can result.

If these conventional wall-taming approaches are inconvenient, you can consider professional products. RPG Diffuser Systems, for example, offers the Acoustic Tools For Home Theater line.

This line contains several panels that can be used to attenuate wall reflections. Armstrong also offers a line of decorator styled acoustic wall panels designed for easy installation. Their SoundSoak panels are available in many colors of woven fabric.

Many audiophiles use a product called RoomTunes to tame their sidewall reflections. Michael Green Designs manufactures these tall, free-standing monolithic panels, which use a fiber filling with a Mylar diaphragm stretched across the front that reflects very high frequencies. They're designed to provide control of room acoustics without creating a "dead" sound. The company also makes a variety of other products designed for use in corners and other spots in the room.

Even if you've toned down the reflections in the room by using ceiling panels, a carpet, and a couple of acoustic panels on your side walls, you may still need to do some work on the rest of the room, which will contribute to the reverberatory ambience.

Amateur acousticians often test the reverberatory tendencies of 95

a room by standing near the listener's position and clapping their hands together loudly. The resulting echo can tell you a lot about the sound signatures of the room. If the clap produces a distinct echo, you probably have some surfaces that need to be toned down. Generally, the main offender is the rear wall. If it is flat and open, you should consider some treatments. As in the other surfaces, this does not always necessitate professional products. Bookcases, pictures, drapes, and other common household furnishings can absorb and diffuse the sound energy quite well.

The last issue to consider is standing waves. Standing waves occur when lower-frequency sounds bounce off opposing walls and combine to produce unnatural sounding peaks or dips in the bass response. First of all, unless your room has large areas of opposing surfaces, this may not be a problem. However, if your room is stark, standing waves could occur. Absorptive is the technique of choice here.

One of the best ways to soak up standing waves is with furniture; plushly upholstered seating does a great job. If you want to get fancier, several manufacturers offer high-tech professional approaches. One of the best known is Acoustic Sciences Corporation's Tube Traps, which are designed to reduce standing waves. These devices are tall, free-standing cylinders that are designed to absorb specific sound frequencies. Half sections are available to hang on the wall; these might be the best option where large expanses of hard surfaces exist. These tubes are available in a wide variety of shapes and sizes.

Feel Free To Experiment Now that you have an understanding of the techniques involved, tuning your home theater room for spectacular sound be straightforward, right? Well maybe not. We asked Jay 96

Trieber, the president of Home Theater Concepts, a home theater design firm, based in Norwood, Massachusetts, for his thoughts.

"We use all the standard acoustic treatments," Trieber explains, "but the problem is that home theater rooms can be very complex acoustically. Each room is unique, and techniques that work well in one room may not work in others. What we recommend is that you work on known problem areas, such as hardwood floors, first. If further treatment is necessary, take it a step at a time and experiment. The proof is what you hear - let your ears be the final judge."

Basic Principles Of Room Acoustics Room acoustics as a discipline involves the study and analysis of direct and reflected sound. Appropriate room acoustics are essential in all spaces where sound is to be transmitted to a listener; this includes both speech and music. Room acoustics design criteria are determined according to the room's intended use. Acoustic, unamplified music, for example, is best appreciated in spaces that are "warm" and reverberant. Speech, by contrast, is more intelligible in rooms that are less reverberant and more absorptive. It is possible to create suitable acoustics for both speech and music in the same space, although this is rarely accomplished without some degree of compromise.

The term "room acoustics" typically brings to mind spaces where music is performed and recorded: concert halls, recording studios and scoring stages, for example. While acoustics are especially important to the success of these spaces, a much wider variety of facilities benefits from welldesigned acoustics. Lecture and convention halls, classrooms, 97

board rooms, council chambers, courtrooms, places of worship, theaters, cinemas and broadcast studios all depend on their acoustical quality. Speech intelligibility is essential in all of these spaces. Different acoustic design criteria are required for rooms where music is to be played, where "natural" acoustics help support unamplified musical instruments.

Design Criteria The design for any room should be based on its estimated percentage of use for a particular function. This is particularly important for multipurpose spaces that may need to serve, for example, both as a lecture facility as well as for music recitals.

Often, such different requirements pose a design conflict that is difficult to resolve, especially if the room is large. As a general rule, speech is intelligible in rooms having a reverberation time of one second or less. Conversely, music is composed of a wide variety of repertoires and genres, each of which has its own desirable range of reverberation or "liveness" provided by the room.

In addition to criteria for reverberation time, spaces used for critical listening should be designed with concern for the audio signal's imaging and echoes. Imaging includes the apparent size and location of sounds that are part of audio reproduction.

Ongoing psychoacoustic research has attempted to define the early sound field thresholds for perception of reflections, changes to the audio image, and echoes based on the sound level of reflections and their delay after the direct sound.

If a critical listening space is designed to be "neutral," that is, without added "coloration," then the early reflection levels should lie at or below the threshold for image shift. The threshold for image shift is the level at which a sonic image appears to move from its actual location. Achieving these relatively low reflection levels in a studio control room requires 98

treatment of all surfaces involved in providing first-order sound reflections to the listener. One surface that cannot be treated by the studio design consultant is the upper surface of the mixing console. Future mixing console design should consider using control surfaces made of porous material, such as sintered aluminum.

Another consideration in the design of critical listening spaces is eliminating "rattles and resonances" often associated with metal fixtures, such as lighting, ducts, diffusers, and furniture.

Difficulties are often resolved by applying visco-elastic damping material. Damping is normally available as sheet material with a self-adhesive backing or in liquid form. The sound intensity produced by a vibrating surface is normally proportional to the velocity of the panel vibration. Damping reduces the panel velocity and, hence, the sound.

Basic Principles of Room Acoustics The main difference between indoor and outdoor sound propagation is in the level of reflected sound. Indoor environments naturally create more reflected sound than do outdoor environments. Reflected sound can be divided into three distinct categories: early and middle-reflected sound, reverberation (late-reflected sound) and standing waves.

Early reflections contribute more to the subjective perception of reverberance, or "liveness" of a space. Early and middle reflections occur within the first quarter of a second after arrival of the direct sound. Early sound is considered to be 40 ms after arrival of the direct sound for speech while for music 80 ms is more appropriate. Once sound reflections have built up to a point where they are not discernible as discrete events, the late reverberation process takes over. In most well-designed spaces, reverberation is a statistical phenomenon, no longer relying on specific room shape and sound propagation paths.

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For this reason, the statistical study of room acoustics, which ignores the path of specific reflections but considers reflected sound as an aggregate probability, is employed with respect to reverberation. Statistical analysis methods are applicable to rooms with relatively uniform sound absorbing material distribution and reasonable aspect ratios.

In spaces having a diffuse sound field where sound is uniformly distributed throughout the space, reverberation decays logarithmically, although the decay sounds even and consistent to the human listener. The reverberation time is defined as the time for reflected sound to decay 60 dB. Generally, it is necessary to avoid assessing early sound reflections as part of reverberation since the reflections contribute to sound build-up, rather than to sound decay. The first 5 to 10 dB of decaying sound reflections are generally not used to determine the reverberation time, which is determined from the remaining decay.

Specular Reflection The manner in which sound reflects depends on the shape, texture and material of the room boundary. Specular reflections, those reflections conforming to Lambert's law of reflection, where the angle of incidence equals the angle of reflection, typically occur at smooth and relatively flat surfaces.

For a surface to be a good reflector of sound, its dimensions should be at least one wavelength or larger than the lowest frequency being reflected. For instance, the wavelength of the musical note middle C (256 Hz) is approximately 1.35 meters (4.5 feet) long. Two octaves higher, a little above 1 kHz, the wavelength measures just over 0.345 meters (13 inches). In order to adequately reflect low-frequency sounds which have larger wavelengths, the reflectors must be relatively large.

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Diffusion Sound can also reflect in a diffuse manner. The reflection is fragmented into many reflections having less intensity, which are scattered over a wide angle, creating a uniform sound field.

Diffusion can be created in a variety of ways, most often by introducing surfaces having irregularities in the form of angled planes or convex surfaces sized at least as large as the wavelength being diffused. Three-dimensional surfaces such as ornamentations, columns and statuary serve as diffusing elements and were integral to the acoustics of 17th, 18th, and 19th century performance spaces. The depth of the diffusing undulations must be at least one-tenth the wavelength being diffused. However, it is possible, if attempting to create a relatively low-frequency diffuser (for example, the octave below middle C, which has a wavelength of 2.7 meters [9 feet] ), to have specular reflections at higher frequencies. For this reason, in some concert halls, there are macro as well as micro diffusive elements to accomodate diffusion in different frequency (and therefore wavelength) ranges. Most common diffusers work well between 800 Hz and 4 kHz. Echoes Echoes are reflections that can be heard distinctly and separately from the early reflected and reverberant sound. For most general purposes involving speech communication, echoes are normally heard due to intense reflections arriving 40 ms and later after the direct sound signal has reached the listener. In other words, the difference in path length between the direct sound and the reflected sound is at least 13.8 meters (46 feet) corresponding to a propagation time of 40 ms or greater. Ironically, echoes are most commonly detected in the front rows of an auditorium and onstage. This results from the front row being farthest from the rear wall, thus generating the 101

largest path length difference between the direct sound and the sound radiating directly from the rear wall or the combination of the ceiling and the rear wall. Sometimes, only a performer or lecturer is able to perceive an echo! Typically, using sound absorbing or diffusing materials. Even surfaces as small as 10m_ (100 feet_) can require treatment to suppress an echo.

Generally, very absorptive rooms must be designed with extreme care in regard to the placement of reflective materials.

Flutter Echoes A flutter echo results when sound travels back and forth between two parallel surfaces and is attenuated much more slowly than reflections from other surfaces. Flutter echoes, which are usually perceivable at frequencies of 250 Hz and greater, largely rely on parallel room boundaries to be sustained. Angling room boundaries, therefore, can help eliminate high-frequency flutter echoes.

Focusing The cardinal rule in the design of rooms is to avoid sound reflectors that focus in the plane of listening. A focusing surface concentrates sound energy, which may then be intense enough to be perceived as an echo. Surfaces such as domes, barrel-vaulted ceilings and concave rear walls can cause sound focusing and are notorious for generating strong echoes. Such architectural elements should be designed with extreme care to avoid acoustical defects.

Reverberation Reverberation is directly proportional to room volume, inversely proportional to the surface area and inversely proportional to the amount of sound absorbing material. It is 102

possible to reduce reverberation by the following means: adding sound absorbing material, reducing room volume or increasing surface area.

Reverberation time is the measure used to quantify reverberation and is the time required for sound reflections to decay 60 dB, one-millionth of their original amplitude. The Sabine reverberation formula, named for the physicist who first recognized this relationship, applies to rooms that have a relatively diffuse (uniform) sound field: T=0.05V/S_ where T is the reverberation time; V is the room volume in ft_; S is the room surface area (ft_); and _ is the average absorption coefficient.

While there are other reverberation time equations, such as those described by Norris-Eyring and Fitzroy, for example, the Sabine equation was the first developed, and it remains valid in most cases. In order to determine the reverberation time in a diffuse room, it is necessary to sum up all of the room's sound absorption due to each material's contribution. This can be accomplished in each frequency range by multiplying the surface area by the sound absorption coefficient for a particular frequency range for all materials located within the space. Just as reflections are not entirely specular or diffuse, no material is entirely sound absorbing or reflecting.

As a general guide, it is not advisable to concentrate large amounts of sound absorbing material on one surface only, particularly where that surface is distant from a group of listeners. In order for a diffuse sound field to exist, sound absorbing material needs to be distributed over both the wall and ceiling surfaces. In a rectangular space, for example, it is not good design practice to concentrate sound absorbing on two parallel surfaces or on two pairs of parallel surfaces. This simply reduces reflections coming from the absorptive surfaces 103

and may result in an echo by enhancing the audibility of the reflected sound from the remaining pair (or pairs) of room surfaces. The reflections from the absorptive surfaces are decreased in amplitude, resulting in a relative increase in the amplitude of the remaining reflections.

Standing Waves Standing waves are also known as room modes. Room modes are most easily perceived when listening to low-frequency tones in small rooms having hard surfaces. Standing waves usually occur between hard parallel wall surfaces and are of particular concern in relatively small rooms, such as music practice rooms, voice recording booths, small audio control rooms and other spaces used for recording or for monitoring recordings. In an ideal case, it can be assumed that walls are infinitely rigid and stiff, so that minimum sound absorption occurs and there is little phase difference between the incident sound and the reflected sound at the point of reflection. Rooms in which two or more major dimensions (for example, length, width and height) are equivalent to multiples of halfwavelengths are notorious for causing additive standing waves and undesirable resonances. The frequency of resonance is higher in small rooms due to the smaller dimensions and shorter wavelengths. For this reason, standing waves are a much more important consideration in small rooms where the frequency of interest lies within the normal speech range of 100 Hz to 5 kHz. It is noteworthy that standards require acoustical laboratories to have the lowest useful 1/3-octave frequency band contain at least ten modes (standing waves) to assure reasonably accurate measurements. This requirement results in a smoother frequency response (i.e., less amplification of a single frequency), due to overlapping modes. The lower limiting frequency is usually 100 Hz. For this reason, laboratories do not usually measure below this frequency, in 104

spite of the fact that there is a growing need for data below 100 Hz. In studios used for the production or reproduction of audio material, sufficient low-frequency absorption is important. The sound absorption in this case acts as damping, reducing the amplitude and broadening the frequency range of the resonance.

Sound Absorbing Materials All materials have some sound absorbing properties. Incident sound energy which is not absorbed must be reflected, transmitted or dissipated. A material's sound absorbing properties can be described as a sound absorption coefficient in a particular frequency range. The coefficient can be viewed as a percentage of sound being absorbed, where 1.00 is complete absorption (100%) and 0.01 is minimal (1%).

Incident sound striking a room surface yields sound energy comprising reflected sound, absorbed sound and transmitted sound. Most good sound reflectors prevent sound transmission by forming a solid, impervious barrier. Conversely, most good sound absorbers readily transmit sound. Sound reflectors tend to be impervious and massive, while sound absorbers are generally porous, lightweight material. It is for this reason that sound transmitted between rooms is little affected by adding sound absorption to the wall surface.

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 absorptivity of each type of sound absorber is dramatically (in some cases) influenced by the mounting method employed.

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1. Porous absorbers: Common porous absorbers include carpet, draperies, spray-applied cellulose, aerated plaster, fibrous mineral wool and glass fiber, open-cell foam, and felted or cast porous ceiling tile. Generally, all of these materials allow air to flow into a cellular structure where sound energy is converted to heat. Porous absorbers are the most commonly used sound absorbing materials.

Thickness plays an important role in sound absorption by porous materials. 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. Thicker materials generally provide more bass sound absorption or damping.

2. Panel Absorbers: Typically, panel absorbers are non-rigid, non-porous materials which are placed over an airspace that vibrates in a flexural mode in response to sound pressure exerted by adjacent air molecules. Common panel (membrane) absorbers include thin wood paneling over framing, lightweight impervious ceilings and floors, glazing and other large surfaces capable of resonating in response to sound. Panel absorbers are usually most efficient at absorbing low frequencies. This fact has been learned repeatedly on orchestra platforms where thin wood paneling traps most of the bass sound, robbing the room of "warmth."

3. Resonators: Resonators typically act to absorb sound in a narrow frequency range. Resonators include some perforated materials and materials that have openings (holes and slots). The classic example of a resonator is the Helmholtz resonator, which has the shape of a bottle. The resonant frequency is governed by the size of the opening, the length of the neck and the volume of air trapped in the chamber. Typically, perforated materials only absorb the 106

mid-frequency range unless special care is taken in designing the facing to be as acoustically transparent as possible. Slots usually have a similar acoustic response.

Long narrow slots can be used to absorb low frequencies.

For this reason, long narrow air distribution slots in rooms for acoustic music production should be viewed with suspicion since the slots may absorb valuable lowfrequency energy.

Is room acoustics an art or a science? Recent technology has refined the acoustician's ability to predict a room's acoustical requirements. It is now possible, for example, to provide active acoustical enhancement by introducing synthesized sound reflections through an array of loudspeakers, thus improving the quality of the transmitted sound dramatically. More specific design criteria are also evolving to suit different uses.

Acknowledging the uniqueness of the design criteria required for each space is vital to the success of the facility, especially if it is multipurpose.

Art implies intuition and mastery. Science can aid in the development of both. But what role does luck play? Were the grand masters simply lucky? Is it luck or skill that allows an artist to appeal to a broad audience? It is in fact a combination of both. Today's room acoustics, like many arts, is an opiniondominated field, one that is influenced as much by history as it is by technology.

DUBBING STAGE: SKYWALKER RANCH, NICASIO, CALIFORNIA The dubbing stage, along with the various recording, mixing and editing rooms in this facility, was engineered to meet the owner's acoustic criteria for sound isolation, room acoustics and background noise level. Floating floors, double-glazed windows and masonry walls combined with furred drywall 107

construction achieved the sound isolation requirements. The background noise level in the dubbing room was controlled to a maximum of NC 15 using in-duct silencers, plenums, oversized ventilation ducts and a plaque air diffuser supply system. The reverberation time was controlled to 0.4 seconds. A portion of the Technical Building was constructed over a parking garage.

Acoustical tests were conducted and construction designed so as to control the noise intrusion of car engines.

It was desired that arches be part of the room's design. Cost studies conducted during the value engineering phase of the project dictated that the arches be constructed of glass fiber reinforced gypsum rather than plaster. A 1-to-10 scale model was built as both an aesthetic study model as well as an acoustical testing model. A 3mm (1/8-inch)-diameter microphone was used to receive the test signal in the model, and the sound reflection patterns in the model were displayed on an oscilloscope screen. The test indicated that the arches as designed would diffuse the sound, not create echoes. These test results were confirmed after the room was built.

During the construction phase, onsite field visits were conducted every two weeks to review the various sound-rated constructions and the installations of the ventilation system.

Post-construction measurements of background noise were made in all noise-critical spaces to verify that the design criteria had been met.

The acoustical design of this building received an Honor Award from the American Consulting Engineers Council in 1988, in part because some of the recording spaces in the complex are among the quietest in the world.

SCREENING ROOM: DOLBY LABORATORIES, SAN FRANCISCO, CALIFORNIA 108

This room is used for film screening, presentation, audio recording and for training. New products for improving motion picture sound are demonstrated, evaluated and developed here.

It is located on the third floor of a building originally constructed in 1910. The size and shape of the room were optimized for motion picture presentations. The coffered ceiling creates a desirable aesthetic and helps to diffuse sound evenly throughout the room. To achieve Dolby's reverberation time criterion of approximately 0.3 seconds in the midfrequencies, about 70% of the wall and ceiling areas were made sound absorptive, using 25 mm (1 inch)-thick sound absorbing material over deep air spaces. The acoustical quality in the room can be varied using retracting sound absorbing quilts in the side walls.

To develop structure-borne vibration control design standards for the 35mm and 70 mm projectors in the projection room, vibration measurements were made on similar projectors at a nearby theater. The screening room projectors were mounted on a floating concrete slab, isolated from the surrounding floating floor.

Double stud walls, a sound-isolating gypsum board ceiling and 75 mm (3-inch)-thick acoustically gasketed doors control noise intrusion from the outside as well as from the theater to adjoining areas. Double glazing with a 200mm (8-inch) air space was used to control projector room noise transfer into the screening room. The background noise in the room varies between NC 15 and NC 20 depending on the ventilation fan speed and thermal load. The office space.

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References: http://www.acousticalsolutions.com/hvac-noisecontrol-products

http://www.mfmca.com/acoustical_products.html

http://www.acousticsbydesign.com/noise/hvacnoise.htm

http://www.pdhonline.org/courses/m206/m206.htm

http://continuingeducation.construction.com/crs.php?L =320&C=1150

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