Building Science II Project 1: Auditorium Acoustic Design

INTRODUCTION Damansara Utama Methodist Church is one of the prominent Methodist Church located in the Klang Valley.

It's located at Section 13 within an industrial district.

The building consists of two floors with three mezzanine levels.

DUMC houses an auditorium and three halls.

The auditorium has a capacity of 2301 seats and a volume of 18655m^3 and is used for ochestra, speech, sunday activities & etc.

Site Location

BUILDING HISTORICAL BACKGROUND It was first a shoplot premises in 1988, DUMC moved to Taman Mayang to accomodarte up to 500 people.

1996 - Chinese church of DUMC was then established resulted in 1000 members.

1997 - DUMC moved to the former Ruby Cinema in SEAPARK. 2001 - DUMC then expanded again with the establishment of Bahasa Malaysia ministry.

2007 - the Dream Centre became the latest premise for DUMC. Exterior of DUMC

Shape of Auditorium The fan-shaped auditorium creates a more intimate space that brings the audiences closer to the speaker.

Since the distance between the audiences and the speaker is closer, the sound of the speaker is louder hence improve the listening condition of the audiences.

The angle of the auditorium is 150 degree which exceeds the maximum limit of 130 degree for a wide fan arrangement.

A central focus will be formed to create a more intimate relationship between the speaker and the audiences.

However, the audiences which seated beyond the maximum limit will have poor listening condition. There should be no seats beyond the maximum limit of fan shaped arrangement as the sound level is significantly lower than the rest of the audience.

Thus to overcome this issue, mechanical audio devices such as three line arrayed speakers are installed at centre, left and right wings.

The doublestories high auditorium is to accommodate large audiences.

The seating which arranged on level terraces is to provide proper sightlines from the audience to the stage.

The staggered ceiling design forms a concave curvature to reflect the incident sound wave back to the audiences.

Sound Path EFFECTIVE TRANSMISSION AND REINFORCEMENT OF SOUND AIDED BY STAGGERED CEILINGS AND RAISED STAGE.

INEFFECTIVE SOUND PATH DISPERSION OF SOUND

Sound shadow area A sound shadow area is an area which sound wave fail to propagate.

In this case, a sound shadow area is formed in the auditorium under the first floor.

The reflected sound waves fail to reach the people who stay under the sound shadow area.

Due to the design approach, the sound shadow area is considered shallow in this auditorium whereby the design of the second terrace is pushed further outwards which cast minimal shadow area towards the lower seating area working together with the staggered ceiling which produces a useful reflection of the sound wave.

Hence, there are no periphery audio devices required at the back seating area.

Type and location of speaker The speakers employed to transmit sound for the audience are all suspended from the ceiling with a height of approximately 7 meters from ground, to prevent direct reflection from the flat ground floor, which might produce uneven sound concentration.

Line-arrayed speakers are suspended from the ceiling facing the centre, right and left wings to transmit amplified sound in tier-arrayed pattern to transmit the sound evenly to the audiences in every level seat to ensure every audience receive the same amplitude of the sound.

The subwoofers are suspended in between the line-arrayed speakers to boost the low frequency sound range by amplifying the base which will resonate in the auditorium.

To overcome the shortcoming of suspended subwoofers which the energy of direct vibration might attenuate in the air, a parallel hard concrete wall is designed facing the output source of the subwoofers.

Monitor speakers are positioned in an array manner facing the stage to ensure the performers receive the sound simultaneously with the audiences to prevent off-beat of the performance.

The audiences in the front row are situated in the blind spot of line-arrayed speakers, thus small speakers are installed facing them.

Seats

The seats are divided into two categories which are fixed and movable seats to cater different occasion use.

One seat represents the similar sound absorption which a person posses to ensure the acoustic experience that is not directly affected by the occupancy of the hall.

The dimension of the cushion seat is 550 with a leg room of 530 which is comfortable and provide a marginal clearance between a seated person’s knees and the back of the chair in the next forward row.

Fixed seats arelocated at level terrace on the ground floor and first floor.

Movable seats are located at the ground floor pit.

Acoustic Defect Time Delay

Echo Echoes are the distinct repetition of the original sound due to the reflection of sound waves from an obstructing surface.

There will be an occurrence of echo when R1 + R2 – D >=14m for lecture hall and when R1 + R2 – D >=34m for music hall.

Delay = (R1 + R2 – D) / 0.34 Ground Floor Fixed Seats Area R1a + R2a – Da = (11.8 + 17.3 – 18.6) / 0.34 = 30.88msec First Floor Fixed Seats Area R1b + R2b – Db = (18.8 + 14.7 – 30) / 0.34 = 10.29msec Ground Floor Movable Seats Area R1c + R2c – Dc = (10 + 14.5 – 12) / 0.34 = 36.8msec According to the calculations, all the time delays are within 40msec which is acceptable for speech but is not suitable for music.

A music hall need a relatively long time delay to allow music to flow and enhance the music quality.

However, the time delay shouldn’t exceed 100msec as it will cause echo for a music hall.

R1 + R2 – D = 13.7 + 13.6 - 21.5 = 17.06m After the calculation, the result is between 14m and 34m which are acceptable for a mixed acoustic used hall as it is between the occurrences of echo for lecture hall and music hall.

Flutter Echo A flutter echo consists of a rapid succession of noticeable small echoes. The occurrence of flutter echo is due to the sound wave are trapped between two surfaces and get a series of reflections in a short time.

However, when we clapped our hands on the longer side of the first floor seating area, echo can be heard. The time between the first and second time the sound register in our ear is big enough for our ear to tell the difference. It is probably due to the reflection of the sound wave by the Perspex railing. When the sound wave hit the hard and flat surface, it reflects back. The distance between the railings is long enough for us to notice the reflected sound wave.

Flutter echo is noticeable in this auditorium. A short burst of sound is produced when we clapped our hand on the first floor of auditorium. The present of flutter echo probably is due to the parallel hard sound-reflective surfaces which are the ceramic floor tile of mezzanine control area and the ceiling above.

Noise

The main sources of noise can be classified into two groups which may be transmitted through air-borne or structure-borne:

Outdoor Noise

Indoor Noise

The noise coming from the neighbouring room such as washroom, performers’ changing room, mothers & infants room and other room that people will gather and talking.

Indoor noise produces by people talking, impact noise from people’s footsteps and equipment noise from the machinery such as sound system, lighting system and air conditioning system.

Impact noise from the hollow timber were being reduce by covering a layer of carpet on it but the noise still can be heard clearly.

For the utility area, thick stage curtain was being installed to reduce the transmission of sound from the backstage.

At both ground floor and mezzanine floor, the noise from sound system were not being treated well.

R.T. CALCULATION

The reverberation time of large multipurpose auditorium (>7500 cubic meter) is normally 1.0 – 2.0 sec. The calculation above shows that the reverberation time for this auditorium is 1.61sec, which is suitable to be a multipurpose hall. Thus, this auditorium could be a mixed acoustical use auditorium which compromises good speech intelligibility & fullness of music.

Materials

Interior Wall

Plastered walls are only finished with a coat of paint. Dry walls are installed on the side of the auditorium and balcony. Hard plastered drywall have very low sound absorption, thus will reflect the sound waves, forming echoes. To prevent fluttered echoes, parallelism is avoided on the side walls.

Rear walls of this auditorium are mainly dry walls covered with Rockwool. This is to absorb the sound from the stage to avoid echo due to the parallelism ofthe rear walls and the wall behind the stage. Other than that, the low wall of the second floor acting as the balcony is also installed with Rockwool membrane. Diffuser is installed on the lower wall, thus Rockwool membrane functions as sound absorber to reduce the noise produced by the HVAC. On the ground floor, there is a combined use of various materials on the wall, which is fabric wrapped Rockwool on the upper part of the wall and wooden strips claddings on the lower part of the wall. By installing porous membrane such as Rockwool with wrapped fabric on its surface, this could absorb the mid or high frequencies to reduce the reverberation time, achieving speech intelligibility. Coarse weaved fabric is used for the covering of the Rockwool to allow sound waves to dissipate through the porous membrane before being trapped and dampened. On the other hand, the wooden strips act as a medium which absorbs the lower frequency that could not be absorbed by the porous membrane. It also increases aesthetic value of the auditorium, giving a sense of warmth to this worship place.

Curtain

Thick and highly porous acoustical curtain is installed at the backstage. Behind of the curtain are acoustic panels, followed by concrete wall with an air gap in between the curtain and the wall. The air gap is to absorb the low frequencies while the drapery is used to absorb the sound, reducing reverberation time and avoid echo.

Angled Rockwool acoustic panels finished with soft coarse fabric are mounted on the wall with an air space gap. The mechanism principle is similar as cavity absorber, works well with low frequency range, which is appropriate for the hall as bass reliant music are mostly played. When the incident sound energy which is already dampened by the first absorption of the panel enter the air gap, it is reflected back and trapped, subsequently being attenuated, thus reducing the reverberation time and avoid fluttered echo. This increases sound absorption, adds aesthetic purposes while providing indirect lighting as lights are installed behind.

Movable heavy curtains are also installed at the entrance of the auditorium and openings as curtain is an absorptive material. It will not reflect the sound back to the audience, inhibiting speech intelligibility.

Ceiling

Majority of the ceiling is made of plaster while some part of the ceiling is mineral fiber board. Plaster is a kind of material which absorbs low frequencies and is good in reflecting all other frequencies. Proper tilted plaster ceiling increasing the length of useful ceiling reflection,contributes more useful sound reflection. Audience sitting far away from the stage is able to hear the sound from the stage.The concave shape created converges the sound wave. Mineral fiber board is installed where the square diffusers are. This is because gypsum board is strong in low frequency absorption.

Floor

Sloped floor is designed in this large mixed used auditorium.

This could improve the sight line of the audience as to avoid obstructed view for the audience, also known for improving the fidelity from the seating area. While sitting on the sloping floor , audience could also receive more direct sound waves compared to flat flooring.

Exterior Wall

For the exterior wall, another layer of brick masonry wall is built.

Massive thick masonry wall is able to reflect low frequencies, insulating the sound from the exterior entering the auditorium.

Staggered masonry wall is also able to absorb the mid and high frequencies, leaving the frequencies in the air gap in between the overlapping walls.

Stage floor finishing is wooden plank while most of the flooring is in carpet.

The carpet is made up of thin and tight loop piles of synthetic fibers.

This creates a good sound absorbent mateial, dampening the unwanted noises created such as footsteps, creating a quieter environment.

Part of the exterior wall is cladded with cork. Cork can help to absorb and dissipate sounds to reduce disruptions. This could avoid unwanted noise entering the auditorium.

Doors, Openings & Windows

Cons: Thin glass is a poor soundproof insulator due to its hard and light dense properties. When incident sound waves hit its surface, it vibrates and resonates along, producing unwanted noise.

Suggestion: Install a curtain layer to increase absorption.

Cons: - Wood is a bad sound absorbance material some door are installed without curtain - door still have gap which allows sound to escape through.

Pros: - Using curtains as sound deflector & absorber to avoid the sound inside being transfer to outside.

Suggestion: installing door seal to further soundproofing the hall.

Air Diffuser There are several types of air diffusers in the auditorium which are louvered bladed diffuser, jet diffuser and linear slot diffuser. Both ground floor and first floor consist of supply and return air diffusers.

Issue: Conical cone jet diffusers are used at the ground floor for supply air in the area where the ceiling is high. However, they produce larger noise because large air velocities are needed to reach the occupied space.

Suggestion: The use of rock wool insulation lining along the internal duct can effectively eliminate noise along ducts as the insulation has good sound absorption properties to absorb noise attenuated within the ducts.

Lighting Several types of lighting fixtures were installed all over the auditorium.

The buzzing of lighting fixtures when switched on also became one of the noise sources that affect the experience of the users.

THE END