Reverberation Time in “Dead” Rooms

Abstract

An analysis based on the assumption that image sources may replace the walls of a room in calculating the rate of decay of sound intensity after the sound source is cut off gives the more general reverberation time equation T = .05V−S loga(1−aa), where T is reverberation time, V the volume of the room, S the surface of the room, and aa the average coefficient of absorption of the wall surface. In the past Sabine's formula, T = .05VSaa, has been used to calculate reverberation time, but for rather large values of aa, that is, for “dead” rooms this equation gives too long a time of reverberation, or in some cases, if the time is measured, it gives a value of aa greater than unity. This we found to be the case in the Sound Stage, Sound Picture Laboratory, Bell Telephone Laboratories, Inc. Reverberation time measurements made in this room support the new formula, however. Sabine's formula turns out to be the special case of the more general equation and is to be used when aa is small, that is, for “live” rooms, but not for “dead” rooms. The new formula is of importance in the talking picture industry for it indicates that a “dead” room may be obtained by the use of less absorbing material than that calculated by the old formula.