The Reverberation Time of a room depends essentially on the volume and on the acoustic characteristics of the sound reflecting/absorbing surfaces of the room.

Sound absorption of a surface is determined by the Sound Absorption Coefficient in percent. A surface with a 'Sound Absorption Coefficient' of e.g. 10% absorbs 10% of the sound energy of an incoming sound wave, 90% will be reflected. The 'Sound Absorption Coefficient' depends on the material of the wall and on the frequency. It normally rises with the frequency.

The average value of the 'Sound Absorption Coefficients' (SAC) of all surfaces considers the size of the respective surface and its specific SAC. This average value is a unique number valid for the current room, and is also frequency dependent (see diagram above).