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ACOUSTICAL DEFECTS IN

ENCLOSED SPACE

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Acoustical defects List of acoustical defects

Reverberation.

Formations of echoes.

Sound foci.

Dead spots.

Insufficient loudness.

Exterior noises.

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REVERBERATIONAcoustical defects

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• Reverberation is the persistence of sound in the enclosed space , after the source of sound has stopped.

• Reverberant sound is the reflected sound , as a result of improper absorption.

• Reverberation may results in confusion with the sound created next.

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However some reverberation is essential for improving quality sound.

The time during which the sound persists is called the reverberation time of sound in the hall.

As per Prof. W .C. Sabins reverberation time ‘t’ is given by formula :-

t= 0.16V /A where V=volume of room in cubic meters A= total absorbing power of all the surfaces of room/ hall.

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Reverberation time & quality of sound Reverberation time should remain within

limits as per Indian Standard Code: 2526-1963.

Sr. No.

RECOMMENDED TIME IN SECONDS

ACOUSTICS

1 0.50 to 1.50 Excellent

2 1.50 to 2.00 Good

3 2.00 to 3.00 Fairly good

4 3.00 to 5.00 Bad

5 Above 5.0 seconds Very bad

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Remedies

The reverberation time can be controlled by the suitable choice of building materials and furnishing materials.

Since open windows allow the sound energy to flow out of the hall, there should be a limited number of windows. They may be opened or closed to obtain optimum reverberation time.

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Formation of echoes.Acoustical defects

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Echo's Not all sound that hits matter is

absorbed. Some of it is reflected. That means sound bounces off the solid matter the way a tennis ball bounces off a wall. Sound reflected back to its source is an echo.

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An echo is produced when the reflected sound wave reaches the ear just when the original sound from the same source has been already heard.

Thus there is repetition of sound. The sensation of sound persists for 1/10th

of a second after the source has ceased. Thus an echo must reach after 1/10th

second of the direct sound

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Multiple echoes may be heard when a

sound is reflected from a number of

reflecting surfaces placed suitably.

This defect can be removed by selecting

proper shape of the hall .

And by providing rough and porous interior

surfaces to disperse the energy of echoes.

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Remedies

This defect is avoided by selecting proper shape for the auditorium. Use of splayed side walls instead of parallel walls greatly reduces the problem and enhance the acoustical quality of the hall.

Echoes may be avoided by covering the opposite walls and high ceiling with absorptive material.

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SOUND FOCIAcoustical defects

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Some times shape of the hall makes sound waves to concentrate in some particular areas of hall creating a sound of large quality.

These spots are called sound foci. This defect can be removed by

Geometrical design shapes of the interior faces.Providing highly absorbent materials on critical

areas (curved spaces).

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DEAD SPOTSAcoustical defects

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This defect is the out come of formation of sound foci.

Because of high concentration of reflected sound at sound foci , there is deficiency of related sound at some other points.

These spots are known as dead spots where sound intensity is so low that it is insufficient for hearing.

This defect can be removed by suitably placing diffusers and reflectors.

Right proportions of internal spaces.

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Geometrical shape of roof helps in proper distribution of sound

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EXTERNAL NOISEAcoustical defects

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External noises from vehicles , traffic engines ,

factories , machines etc. may enter the hall

either through the openings or even through

walls and other structural elements having

improper sound insulation.

This defect can be removed by proper planning

of the hall with respect to its surroundings and

by proper sound insulation of external walls.

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Acoustical design of halls The initial sound from the source should be of

adequate intensity so that it can be heard

throughout the hall .

For halls of big sizes suitable sound amplification

system should be installed.

The sound produced should be evenly distributed

so that there is no dead spots and sound foci.

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Acoustical design of halls The boundary surface should be so designed

that there are no echoes or near echoes.

Desired reverberation time should be

achieved by proper placement of absorbents

on wall.

The out side noise should be eliminated.

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Noise Noise is unwanted sound which masks the satisfactory

hearing of speech and music. There are mainly three types of noises that are to be

minimized. They are (i) air-borne noise, (ii) structure-borne noise and (iii) internal noise.

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(i) Air-Borne Noise

The noise that comes into building through air from distant sources is called air-borne noise.

A part of it directly enters the hall through the open windows, doors or other openings while another part enters by transmission through walls and floors.

Remedies The building may be located on quite sites away from

heavy traffic, market places, railway stations, airports etc.

They may be shaded from noise by interposing a buffer zone of trees, gardens etc.

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(ii) Structure-Borne Noise

The noise which comes from impact sources on the structural extents of the building is known- as the structure-borne noise. It is directly transmitted to the building by vibrations in the structure. The common sources of this type of noise are foot-steps, moving of furniture, operating machinery etc.

Remedies The problem due to machinery and domestic appliances

can be overcome by placing vibration isolators between machines and their supports.

Cavity walls, compound walls may be used to increase the noise transmission loss.

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(iii) Internal Noise

Internal noise is the noise produced in the hall or office etc.

They are produced by air conditioners, movement of people etc.

Remedies The walls, floors and ceilings may be provided with

enough sound absorbing materials. The gadgets or machinery should be placed on sound

absorbent material.