A material's sound absorbing properties are expressed by the sound absorption coefficient, α, (alpha), as a function of the frequency. α ranges from 0 (total reflection) to 1.00 (total absorption).  

 

1. Transmitted energy
2. Converted energy
3. Incident energy
4. Reflected energy
* absorption coefficient

When a sound wave strikes one of the surfaces of a room, some of the sound energy is reflected back into the room and some penetrates the surface. Parts of the sound wave energy are absorbed by conversion to heat energy in the material, while the rest is transmitted through. The level of energy converted to heat energy depends on the sound absorbing properties of the material.

The sound absorption coefficient can be measured by two very different methods - the room method and the tube method. The room method is normally used for presenting product information (as in this publication) and as input to calculation models. The measuring method follows an international standard designated EN ISO 354. The corresponding American standard is ASTM C 423 (measurements according to this often show slightly higher figures). The measurements are done in a large room with a diffuse sound field, i.e. the sound has evenly distributed angles of incidence against the test surface.