Reverberation time (RT60) is defined as the time required for the reverberant sound to decrease by 60 dB from its original level. It generally varies with frequency.
In a bare room, where all surfaces absorb the same fraction of the sound that reaches them, the theoretical reverberation time is proportional to the ratio of volume to surface area.
The following empirical formula to calculate reverberation time from the dimensions (in meters) of a room was proposed by Wallace Sabine in the late 1890s: RT60 = , where is the volume of the room in cubic meters and is the effective “total absorption.”
The “total absorption” area is calculated as the sum of all surface areas in the room, each multiplied by its respective absorption coefficient for a particular frequency.
Typical frequencies for which absorption coefficients are determined are 125, 250, 500, 1000, 2000, and 4000 Hz.
A low absorption coefficient indicates a more reflective material.
Air contributes a substantial amount to the absorption of high frequency sound and can be accounted for (in a simplistic way) in the formula for reverberation time as RT60 = 0.161 , where is a constant that varies with air temperature, humidity, and frequency.
The optimum reverberation time of a room is dependent on the use for which it is designed (speaking, chamber music, opera, orchestral music, organ, etc.).