Absorption or Insulation?

When approaching a noise control problem, the difference between sound insulation and sound absorption should be appreciated.

 Sound Absorption

Refers to the attenuation of reverberant noise within the same room or area as the noise source. This normally involves lining all or part of the walls, floor, roof or ceiling of a building with a material which absorbs sound.

A material’s ability to absorb noise efficiently can be gauged from its sound absorption coefficient, which is defined as the ratio of the sound energy absorbed to the sound energy incident. A material which absorbs 100% of the sound energy striking it, has a sound absorption coefficient of unity i.e. 1. (Whilst test results can indicate a sound absorption coefficient greater than unity, this is simply a feature of the particular test method.)

 Sound Insulation

Sound insulation is concerned with preventing noise being transmitted from one area into another, for example by erecting a partition or wall.

The ability of a partition or wall to resist the flow of sound energy through it is largely determined by its mass. However, here again sound absorbing materials can have an important role to play.

It can be shown that doubling the mass of a partition/wall will normally reduce the sound transmission through it by one quarter, or in decibel terms 6dB. (In practice it is more realistic to claim a 5dB improvement for each doubling of mass.) This rule of thumb is known as the “mass law” and is useful in providing a guide to the improvement that can be achieved by increasing mass.

However, in many instances significant improvement can be made by the addition of efficient Acoustic Insulation products to the construction.