In classical statistical acoustics (see Ref. 9 and Ref. 16) the reverberation time T60 (RT) is the time taken for the sound pressure level (created by an impulse source) to decay 60 dB. The reverberation time can be calculated using the Sabine formula

where V is the room volume, c is the speed of sound, and A is the total absorption of all surfaces. The factor 55.3 is a roundoff of 24·ln(10). The total absorption is calculated by

where Si and αi denote the area and absorption of the i’th area. S is the total surface area and is the average absorption of the room. For large rooms this equation is corrected by the absorption of air

where ma is the air absorption constant (see Table 3.1 in Ref. 9). Other versions of the T60 reverberation time exist one is the Eyring–Norris given by

The models for the reverberation time given above are statistical models for diffuse sound field (large number of reflections). At low frequencies the modal behavior of the room becomes important. The lower bound for the application of these statistical models is the Schroeder frequency (see Ref. 10 and Ref. 16) given by