Thursday, December 16, 2010

ARCHITECTURAL ACOUSTICS

1. INTRODUCTION

Architectural acoustics is the science of controlling sound within a building. The aim is to have good hearing condition for the listeners all around the corner would not be sufficient. For good speech intelligibility, the direct signal from the loudspeaker to the listener’s ears must be louder than any competing sound and be free from distortion.

The architect designs a great looking and comfortable structure. The people attending the grand opening are impressed with what they see, but they are gathered for more than dazzling display of architecture, lighting, glasses, surface texture, carpets and paint. The outer beauty is recognized by the way it looks, but inner more lasting beauty of the structure is truly know by how it sounds.

2. TYPES OF ACOUSTICS

  • Passive acoustics: Amplifying sound without the assistance of electro acoustics. This is achieved by directing sound through reflectors.
  • Electro acoustics: Amplifying sound through the use of sophisticates equipment.
  • Variable acoustics: Varying the reverberation time a space by changing the amount of absorptive surface in space for specific use, in form of banners, curtains and panels.

3. TERMS CONNECTING ACOUSTICS

Sound is a vibration in an elastic medium. Sound is a relatively simple form of energy, causing variations in pressure and alternations in direction of molecular movement within media. (Usually produces hearing in humans).

Acoustic energy is the total energy of a given part of a transmitting medium minus the energy, which would exist in the same part of the medium with no sound present.

Vibration refers to the oscillating motion of media. (Typically the non-audible acoustic phenomena that can be felt).

Elastic Medium: usually is matter which stress is proportional to strain. Molecules in all substance are constantly moving very quickly (temperature & pressure) and rebounding off of each other. Sound motion is added to the already existing motion.

Frequency is the number of times the cycle of compression and rarefaction occurs in a given unit of time. (Usually cycles per second or cps = Hz)

Wavelength is the distance between regions of identical rarefaction & compression.

Amplitude of motion is the maximum displacement of an element beyond its normal position which is dependent upon the sound pressure (Force) of the sound energy involved.

Sound Intensity Level is the ratio relative to a base. dB

4. ACOUSTIC DESIGN PROBLEMS

4.1 Reverberation time (T60)

The reverberant sound in an auditorium dies away with time as the sound energy is absorbed by multiple interactions with the surfaces of the room. In a more reflective room, it will take longer for the sound to die away and the room is said to be 'live'. In a very absorbent room, the sound will die away quickly and the room will be described as acoustically 'dead'. But the time for reverberation to completely die away will depend upon how loud the sound was to begin with, and will also depend upon the acuity of the hearing of the observer. Reverberation time is defined as the time for the sound to die away to a level 60 decibels or (1/1,000,000) below its original level.

4.2 Echoes

An echo is a reflection of sound, arriving at listener some time after the direct sound. If so many reflections arrive at a listener that they are unable to distinguish between them, the proper term to this phenomenon is called reverberation. Echoes get reflected from walls or hard surfaces like mountains.

4.3 Acoustic resonance

Acoustic resonance is the tendency of an acoustic system to absorb more energy when it is forced or driven at a frequency that matches one of its own frequencies (natural) of vibrations that it does with other frequency.

4.4 Dead spots

This defect is an outcome as a side effect of sound foci. Due to high concentration of sound rays at some points

5. ARCHITECTURAL ACOUSTICS

These different reflecting sounds create standing waves that produce an annoying sound. There are three ways to improve workplace acoustics and solve workplace sound problems – the ABC’s.

A = Absorb (usually via ceiling tile)

B = Block (via workstation panels, wall placement and workspace layout)

C = Cover-up (via electronic sound masking)

5.1 Room Volume

The smaller the volume per seat, greater the sound energy available to each listener. Reverberation time is directly proportional to room volume. As per room volume increases, the need to use sound absorption on room surface increases to obtain the require reverberation time. A gross area of 6-7.5sq.ft is recommended per seat.

5.2 Shape and proportion

As the purpose changes, the shape of theatre changes. Mainly rectangular, fan shaped or reverse fan shaped are used with reflective surface behind and throughout the walls of the stage .it may be of semi circular cross section. According to the modal design theory, the worst possible room shape is a cube. The next worst is a room where all dimensions are multiples of the height. A pretty horrible example is a room 8-ft high, 16-ft wide and 16 ft long

5.3 Audience layout

  • Intimacy: this is the most important requirement of the theatre. It is essential for the audience to see gesture and facial expression of the performer. The distance from the stage and the farthest should be so calculated that every person can enjoy the play. When larger audience is to be accommodated, balconies or fan shaped floor pan are utilized.
  • Balcony depth: the depth should not exceed twice the height of the opening. Shallow side balconies are useful diffusion and intimacy.
  • Special treatment

5.4 Qualifying acoustic diffusion

Process by which sound energy is reduced and scattered evenly over a larger area due to the surface irregulations of the reflecting surface. According to their shape they are divided into spline, bicubic, monoradial, waveform.

5.5 Sound absorbing materials

On striking any surface, sound is either absorbed or reflected. The sound energy absorbed by an absorbing layer is partially converted into heat but mostly transmitted to other side,unless such transmission is restrained by a backing of an impervious,heavy ,barrier. In other words, good sound absorber is an efficient sound transmitter and consequently an inefficient sound insulator.

5.5.1 Porous materials

The basic acoustic characteristic of all porous materials such as fibreboards,soft plasters,mineral wools is an cellular network of interlocking pores. Incident sound energy is converted into heat energy within these pores, while the remainder, reduced energy is reflected from the surface of the material. They are subdivided into

  • Acoustical plasters and sprayed on materials: These acoustical finishes are used mostly for noise reduction purposes and sometimes in auditoriums where any other acoustical treatment would be impractical because of curved or irregular shape of the surface. These are applied in semiplastic consistency by spray gun
  • Acoustical blanket: Acoustical blanket are manufactured from rock wool, glass fibers, wood fibers etc. generally installed on a wood or metal framing system, these blankets are used for acoustical purpose for varying thickness between 1 to 5 inch. Their absorption increases with thickness at lower frequency
  • Carpet and fabrics: These absorb airbone sounds and noises within the room, carpets are lied at portion were absorption is needed. Fabrics are added to the wall to reduce the sound. They give a clean and tidy look for the auditorium

5.5.2 Panel absorbers

Any impervious material installed on a solid backing but separated from it by an air space will act as a panel absorber and will vibrate when struck with sound waves. The flexural vibration of the panel will then absorb certain amount of incident sound energy by converting it into heat energy. Among auditorium finishes and construction the following panel absorbers contribute to low frequency absorption: wood and hardboard panels, gypsum boards, rigid plastic boards

5.5.3 Cavity resonators

This consists of an enclosed body of air confined within rigid walls and connected by a narrow opening to the surrounding space, in which the sound waves travel. Cavity resonators are

· Individual cavity resonator: These made of clay vessels of different sizes, were used in medieval Scandinavian churches. Standard concrete blocks using regular concrete mix but with slotted cavities called sound blox units, constitutes a contemporary version of sound resonators.

5.5.4 Space absorbers

When the regular boundary encloses of a auditorium do not provide suitable or adequate area for conventional treatment, sound absorbing agents, can be suspended as individual units from the ceiling. These are mad of perforated sheets in the shape of panel; prisms, cubes spheres etc are generally filled or lined with sound absorbing materials. Their acoustical efficiency depends on their spacing. In order to achieve a reasonable amount of room absorption, it is essential that large amount of space absorbers be used within a space.

6. ELECTRONIC SOUND SYSTEMS

The basic types of sound-reinforcing systems include:

1. Central: Cluster of loudspeakers located above the actual source of sound.

2. Distributed: Array of loudspeakers located over the listeners.

3. Seat-integrated: Loudspeakers located in the backs of seats or pews.

4. Combination. Combinations of several systems.

6.1 Loudspeaker

Convert electric energy into airborne sound. They should be positioned so their direct sound will be evenly distributed at the proper sound level to all listeners in the room. For rooms with a reverberation time of less than 2 s, the maximum loudspeaker to listener distance d can be found.

6.1.1 Placing of loud speakers

Central Loudspeaker System:

Usually locates the loudspeaker, or cluster of loudspeakers, 20 to 40 ft. above and slightly in front of the actual source of sound. The cluster can be exposed or hidden behind a sound-transparent grille cloth. This system can provide maximum realism because the listener will hear the amplified sound from the direction of the live source location. This is because human ears differentiate sounds better in the horizontal plane (where the ears are located) than in the vertical plane. Therefore, amplified sound from a properly designed central system should not be noticeable. The cluster should be aimed at the audience, which absorbs sound. In this way, reinforcement systems can provide high intelligibility by increasing the level of sound energy from the location of the talker more than they increase the reverberant sound energy sound rating systems.

7. SOUND TRANSMISSION CLASS

Is a single-number rating system which compares the Sound Transmission Loss (the ratio of sound energy incident upon a panel to the sound energy radiated from the opposite side) of a test specimen with a standard contour. The energy actually lost is partially reflected and heat (within the partition). It is measured by the difference in Sound Pressure Level between two spaces which are separated by a barrier.

7.1 FSTC

In practice, the STC of the laboratory sample represents the optimum condition, and is rarely achieved in actual construction. The difference between the actual or Field STC (FSTC) and the laboratory STC is a result of leaks and flanking paths, in other words, sound entering a wall in a common assembly is also entering the floor, traveling through the floor and breaking out in the adjoining space, by-passing the wall.

7.2 CAC

Ceiling Attenuation Class is a single number system used to measure room-to –room sound attenuation with the source room sound passing through the ceiling, along the common ceiling plenum and then down through the ceiling again into the receiving room.

7.3 NRC

Noise Reduction Coefficient is a wideband general indicator of sound absorption. It is an average of absorption coefficients 250 Hz through 2 kHz, with a range between 0 (0%) and 1 (100 %). the values are so obtained from table below

Frequency (Hz)

Concrete/brick

125

250

500

1000

2000

4000

Glass

.01

.01

.02

.02

.02

.03

Plasterboard

.19

.08

.06

.04

.03

.02

Plywood

.2

.15

.10

.08

.04

.02

Carpet

.10

.20

.30

.35

.50

.60

Curtains

.05

.12

.25

.35

.40

.45

8. SYDNEY OPERA HOUSE- A CASE STUDY

Sydney Opera House was inscribed in the World Heritage List in June 2007: “Sydney Opera House is a great architectural work of the 20th century, figure (1). It represents multiple strands of creativity, both in architectural form and structural design, a great urban sculpture carefully set in a remarkable waterscape and a world famous iconic building.” UNESCO

Fig1: Sydney Opera House

8.1 Interiors

The rich and extensive timber interiors. An exemplary example of the use of plywood and laminated hardwood. Timber is used as the primary material for the interiors, with the warmth, colour and tactility of timber providing a contrast with the heavy, monochrome, load bearing concrete of the podium and sails. Each beam was to be made up of two plywood box beams bolted together, with acoustic insulation in the cavity inside each beam Spanning horizontally between the box beams was to be panels of plywood reinforced with hot bonded aluminium. These horizontal elements were attached to the top of one beam and the bottom of the next creating a stepped form to the ceiling. On the top of these panels was to be bonded 2mm of lead for low frequency sound insulation. The ceiling of the performance halls give a feeling of a floating cloud

8.2 Built scheme

As shown in figure (2) Throughout the interiors, prefabricated panels of laminated Brush Box were used for flooring, stair treads and risers and wall panels. An extremely hard and dense timber, Brush Box was chosen for its warm, rich colour and grain, acoustic performance and high durability.

8.3 A Strategy for Design with Timber

Timber and Acoustics Timber has historically been used for acoustic applications for a number of reasons. A timber surface does not just reflect sound, but resonates slightly, giving it a particular quality and colour. A hard surface such as concrete reflects sound with a hard and sharp quality. In the Sydney Opera House, the musicians were happy to have timber used as it was a material they understood, many of their instruments being made from timber. They were accustomed to the quality of sound timber spaces created. This surface treatment with paneling breaks the sound, reducing echoes.

Fig2: Use of acoustic material

9. CONCLUSION

As the purpose of theatre changes from one format to another, acoustic design is also changed. By using materials which can act as good absorbent or good reflectors, design can be varied

Placement of loudspeakers should be such that person at every corner cam hear the voice clearly

Optimum reverberation time is a compromise between clarity (requiring short reverberation time), sound intensity (requiring a high reverberant level), and liveness (requiring a long reverberation time)

Echoes, flutter echoes, sound focusing, sound shadows, and background noise should be avoided in an auditorium design.

The greater the early decay time (up to two seconds), the greater the preference for the concert hall. Above two seconds, the trend it reversed.

10. REFERENCES

1.Moore.N and John Edwin, Design for good acoustics and noise control, 2nd edition, Best press,2004

2. Berry W R, “acoustic design for buildings”, Emmott & Company Limited, London, 1999.

3. Bates R C & Clark W G, “theatre design”, 1st edition,west hill publications,2005.

4. Abraham, Dulcy M., Halpin and Daniel W. (2008). “Sound propogation.”, American acoustic society Journal of Civil Engineering”.

5. ww.sydneyarchitecture.com/roc/qua01.htm

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