Acoustical defects

The perfect acoustical condition in a room or auditorium are obtained when an average sound rise to a suitable intensity with

a. no distortion of the original sound

b. die (decay) out quickly.

The following acoustical defects have to be remove or minimize in order to achieve a perfect acoustic in an enclosure.

A. Echoes

• Echoes cause disturbance and unpleasant hearing.

• An echo is sound repetition produced when a reflected sound waves from surface of wall, floor, ceiling coming from the same source reaches the ear just when a direct sound is already heard.

• The formation of echo normally happen when

a. the time lay between the 2 sounds is about 1/17 of a second.

b. the reflecting surfaces are situated at a distance greater than 15m.

c. the shape of the reflecting surface is smooth and curved.

• Echo can be overcome by using absorbent material (rough and porous) to disperse energy of the echo.

B. Reverberation

• Reverberation is the continuation of sounds caused by multiple reflections between the surfaces of an enclosure.

• Reverberation is not the same as an echo. The reverberative reflections are heard as an extension of the original sound.

• Hard surfaced rooms will have a longer reverberation time than rooms finished with sound absorbing materials.

• The reverberant sound combines with the direct sound to produce a continuing ‘reverberant field’.

• A room totally without reverberation is termed ‘anechoic’ and is achieved by using special absorption techniques at each surface.

Reverberation Time

• Reverberation time is the time taken for a sound to decay or diminish by 60dB (decibel) from its original level.

• The time taken for this decay in a room depends upon the following factors:

1. Areas of exposed surfaces

2. Sound absorption at the surfaces

3. Distances between the surfaces

4. Frequency of the sound

• Reverberation time is an important index for describing the acoustical quality of an enclosure.

• The correct reverberation time in between this two limit is called ‘optimum time of reverberation’.

• The ideal reverberation time depend on:

1. Size of an enclosure or room

Recommended Optimum Reverberation Time for various building.

Building	Reverberation (seconds)
Cinema / Theatre	1.3 – 1.5
Church	1.8 – 3.0
Conference	1.0 -1.5
Music Concert Hall	1.0 -1.5
Parliament / Assembly Hall	1.5 – 2.0

2. Types of activity

   1. Speech: 0.5 – 1 seconds reverberation time

Short reverberation times are necessary for clarity of speech otherwise the continuing presence of reverberant sound will cause the speech to be blurred.

2. Music: 1 – 2 seconds reverberation time

Longer reverberation times are considered to enhance the quality of music otherwise short reverberation will cause the sound ‘dry’ or ‘dead’.

Sabine Formula

Sabine Formula give predictions of reverberation time for rooms,

Reverberation Time, t = 0.16 V = 0.16 V

A S x ά

Where t = reverberation time (s)

V = volume of the room (m³)

A = total absorption of room surfaces (m² sabin)

= Σ (surface area x absorption coefficient)

= S x ά

S = total room surface area m²

ά = mean absorption coefficient of room surfaces

C. Sound foci

• There is a possibility for reflected sound rays to meet at a point called sound focus when sounds are reflected by a concave shape interior surface or dome ceiling of an enclosure.

• This causes a concentration effect for the reflected echoes and consequently a sound of large intensity at this spot.

• The spot of unusual loudness or intensity is known as sound foci.

• Remedy:

a. Absorbing materials should be used on reflecting surface.

b. Ensure the design throughout the interior hall has no concentration of reflected sound.

D. Dead Spot / Sound

• This is a side effect of the sound foci.

• Dead spot is the deficiency of reflected sound rays where these spot of low sound intensity causing unsatisfactory hearing to the audience.

• Remedy:

a. Use suitable diffuser to enable uniform sound distribution throughout the enclosure.

E. Exterior Noise

• This defect is caused due to poor insulation and partly due to poor planning.

• The exterior noise is carried inside the enclosure through the loose doors, windows and ventilation.

• Remedy:

a. This penetration of outdoor noise inside the enclosure can be corrected by properly planning the building location of the building with respect to the road.

b. Providing and adequate insulation for various components of the enclosure.