THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

11.7 Decay of Sound 253Following Sabine’s definition, we define the reverberation time T as the timerequired for the sound level in the room to decay by 60 dB:T = 60 D = 55.2V(11.13)AcExpressing volume V in m 3 and area S used to compute A in m 2 , and setting soundpropagation speed c = 343 m/s, Equation (11–13) becomesT = 0.161V(11.14)AEquation (11.14) becomes for English unitsT = 0.049V(11.15)Awhere volume V is rendered in ft 3 and A in ft 2 (or sabins, with 1 sabin equal to1ft 2 of absorption area αS). (One metric sabin is equal to 1 m 2 of absorption area.)It becomes apparent here that the reverberation time for a room can be controlledby selecting materials with the appropriate acoustic absorption coefficients. Theabsorption coefficient of a material can be measured by the introduction of a definitearea of the absorbent material in a specially constructed live room or reverberation(or echo) chamber. A photograph of such a chamber is given in Figure 11.6.Figure 11.6. Photograph of a reverberation chamber. (Courtesy of Eckel Industries, Inc.)