The ratio of the absorbed sound energy to the incident energy is the sound absorption coefficient. It is also known as sound absorptivity.

Every material has a property that changes acoustic energy into usually heat energy through the process of absorption. Absorption of a given material is frequency-dependent and is affected by the size, shape, location, and mounting method used. Sound absorption coefficient describes the efficiency of the material or the surface to absorb the sound.

Units and Measures

The sound absorption coefficient indicates how much of the sound is absorbed in the actual material. The absorption coefficient can be expressed as the ratio of sound intensity absorbed over incident sound intensity. It is a unitless measure between 0 to 1

The most common way to measure sound absorption coefficient is to lay a piece of the material in a reverberant chamber then measure the reverberation time so the coefficient can be derived from reverberation time of the room without the material.

Example Units

Sound absorption coefficient of some materials measured in 125Hz

  • Carpet - 0.01
  • Brick (natural) - 0.03
  • 12mm Plasterboard - 0.29

Useful References

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