Principles of Acoustics

The behaviour of sound plays an effective role in the acoustical design of different types of buildings and in the sound insulation process.

A sound originating from a source, such as music or operation of machine, is transmitted through the medium in all directions. The transmitted sound strikes on some surface, like wall, ceiling, floor or any other barrier. Depending on the type of surface part of it is reflected back, and a part being absorbed by the surface. If the sound is not absorbed by the material, it will be transmitted in part to another side of barrier.

If the ultimately reflected sound is not properly controlled the reflection may result in acoustical defects, viz., echoes and reverberations. This reflected sound is important in the acoustical design of buildings. The part of sound absorbed by the surface is represented by an absorption coefficient. This coefficient is the ratio of the energy absorbed by the area of the surface to the energy striking the area. This coefficient is a function of the frequency of sound. The values of the absorption coefficient for different surfaces for a normal frequency of 500 cps are presented in Table 28.2. It is assumed that the sound wave will completely pass through a window, then it is taken as 1.

Table 28.2 Absorption coefficient for different surfaces

The reduction of intensity of sound of a transmitted sound through a barrier is called as transmission loss. This transmission loss is a measure of the effectiveness of a surface as an insulating material.

Thus transmitted and absorbed sounds have important bearing on the acoustical condition of a building. However, both transmitted and absorbed sounds are inter related and influence the acoustic and sound insulation.

Highly porous materials have the quality to dissipate considerable energy and the absorption will be relatively high. An ideal material for sound absorption should possess the following qualities:

1. It should have a high coefficient of sound absorption of economy.
2. It should be fire resistant.
3. It should have high structural strength.
4. It should be unaffected by decay, attack by insects and vermin.
5. It should be easily workable to fit in any location.