Factors Affecting Acoustics in a Building

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The factors affecting the acoustics in a building are as follows.

  1. Reverberation and Reverberation time.
  2. Loudness.
  3. Focussing due to walls and ceilings.
  4. Echo.
  5. Echelon effect.
  6. Resonance.
  7. Extraneous noises.

1. Reverberation and reverberation time

  • The prolonged reflection of sound from the ceiling, walls, floor, and chairs etc., is called Reverberation.
  • It is defined as the persistence of sound even after the sound source is cut-off.

The reverberation time is the time for which the sound stays inside the hall. The reverberation time is defined as the time taken by the sound intensity to fall below minimum audibility i.e, 10-6 times its initial intensity, after the source is cut-off.

  • When the reverberation time is too large, the successive sounds overlap and the clarity of sound is lost.
  • When the reverberation time is too small the loudness will be inadequate and the sound heard will not be intelligible.
  • If the reverberation time is zero, no sound is heard and the hall becomes a dead hall.
Decay of Energy density
Fig. 1.1 Decay of energy density

Therefore the reverberation time should neither be too large nor too small but it must have a definite optimum value which is satisfactory to both the speaker and the listener. This optimum reverberation time can be found by Sabine’s formula.

Remedies

The reverberation time can be controlled by the following means:

a. By providing windows and ventilators (open or closed).
b. By decorating the walls with maps and paintings.
c. By providing heavy curtains with folds.
d. The walls can be lined with absorbing materials like felt, wood, celotex, fibre boards etc.
e. By covering the floor with carpets.
f. By providing acoustical tiles and slit resonators.
g. By having full capacity of audience.
h. By using cushioned chairs.

2. Loudness

Loudness is the degree of sensation produced on the ear. Loudness depends on the amplitude of sound i.e., loudness is proportional to the square of the amplitude. The loudness must be adequate inside the hall so that the sound is heard by every listener when he is seated in any part of the hall.

Remedies

Loudness can be increased:
a. By reflection of sound.
b. By using public address systems with amplifiers.
c. By providing low ceilings for reflection of sound towards the audience.
d. By keeping large wooden boards behind the speakers.

3. Focussing due to walls and ceilings

The focussing effect is caused by curved structure of walls and ceilings. If the curved surface is concave then the sound will get focused at a particular point and it will not be distributed uniformly throughout the hall. Hence a concave surface (Fig. 1.2) produces an adverse effect. This should be avoided, i.e., the walls and ceilings should not have any concave surface. Also any decorative object placed inside should not possess a concave shape.

Remedies

  1. By avoiding curved surfaces.
  2. By covering such curved surfaces with suitable absorbing materials.
  3. By providing low ceilings.

4. Echoes

When the reflecting surface is far away from the source (more than 17 m ) then the sound is reflected back as a distinct repetition of direct sound, this reflected sound is called echo. An echo is heard when direct and reflected sound waves coming from the same source reach to the listener with a time interval of 1/7th  of a second.

Remedy

a. Echoes can be avoided by covering walls and ceiling with suitable sound absorbing materials.

5. Echelon effect

When a hall contains structures of regular repetitions, like stair cases etc., the sound produced by the source may be scattered. This may create nuisance effect due to echoes. “The echoes are formed when the time interval between direct and reflected waves is about 1/15th  of a second”. The regular repetition of echoes of the original sound to the observer is called as Echelon effect. This should be avoided.

Remedies

a. Avoiding such structures.
b. Covering the staircase with carpets.
Concave Surface
Fig. 1.2 Concave surface
Stair Case
Fig. 1.3 Stair case

6. Resonance

Sometimes the window panes, sections of the wooden portions and walls lacking in rigidity (loosely fitted) are thrown into vibrations and they create other sounds.

For some note of audio frequency, the frequencies of new sounds may be the same thus resulting in resonance. Such vibrations are called resonant vibrations. Due to this the original sound get distorted. Hence, it can lead to unpleasant effect. These resonant vibrations should be suitably damped.

Remedies

  1. Avoid loose fittings in the hall.
  2. Ranging large number of curtains in the hall.

7. Noises

The unwanted sound wave to produce adverse effect to our health is called noise. There are three types of noises. They are:
a. inside noise,
b. air borne and
c. structure borne.

a. Inside noise

Noise produced inside the hall itself or adjacent hall is called as inside noise. These are due to moving people, furnishers working machines etc.

Remedies

To avoid these:

  1. Machines may be placed over the sound absorbing materials.
  2. Covering the walls, ceilings with suitable sound absorbing material.

b. Air borne noise

The noises produced outside by the vehicles such as aircrafts, automobiles, trains etc., and carried by air are called as air borne noise.

Remedies

To avoid air borne noise:

  1. Construct the auditorium well away from the road at a quiet place.
  2. Construct the compound wall as high as possible little away from the auditorium.
  3. Construct a double wall enclosure.
  4. Provide the windows at proper places with curtains.

C. Structure borne noise

Noise which is transmitted through the structure of the building is called structure borne noise.

Eg: Noise due to movement of furniture, machinery operation will produce structural vibrations giving rise to structure borne noise.

Remedies

To avoid the structure borne noise:

  1. Rubber gaskets can be provided in between the pipelines.
  2. Double walls fitted with or packed with sand, which is called sand pugging can be constructed.
  3. A false ceiling can be used.
  4. By providing carpets.
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