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

246 11. Acoustics of Enclosed Spaces: Architectural AcousticsFigure 11.3. Reception of direct and indirect sound.The phases and the amplitudes of the reflected waves are randomly distributedto the degree that cancellation from destructive interference is fairly negligible. If asound source is operated continuously the acoustic intensity builds up in time until amaximum is reached. If the room is totally absorbent so that there are no reflections,the room operates as an anechoic chamber, which simulates a free field condition.With partial reflection, however, the source continues to add acoustic energy to theroom, that is partially absorbed by the enclosing surfaces (i.e., the walls, ceiling,floor and furnishings) and deflected back into the room. For a source operating ina reverberation chamber the gain in intensity can be considerable—as much as tentimes the initial level. The gain in intensity is approximately proportional to thereverberation time; thus it can be desirable to have a long reverberation time torender a weak sound more audible.11.3 Reverberation EffectsConsider a sound source that operates continuously until the maximum acousticintensity in the enclosed space is reached. The source suddenly shuts off. Thereception of sound from the direct ray path ceases after a time interval r/c, wherer represents the distance between the source and the reception point and c the soundpropagation velocity. But owing to the longer distances traveled, reflected wavescontinue to be heard as a reverberation which exists as a succession of randomlyscattered waves of gradually decreasing intensity.The presence of reverberation tends to mask the immediate perception of newlyarrived direct sound unless the reverberation drops 5–10 dB below its initial levelin a sufficiently short time. Reverberation time T , the time in seconds required forintensity to drop 60 dB, offers a direct measure of the persistence of the reverberation.A short reverberation time is obviously necessary to minimize the maskingeffects of echoes so that speech can be readily understood. However, an extremelyshort reverberation time tends to make music sound harsher—or less “musical”—while excessive values of reverberation time T can blur the distinction between