On the diffusion of sound in an auditorium

A condition of perfect diffusion of sound is said to exist in an auditorium if, at any point within it, the reverberant sound travels in all directions with equal probability, and if the level of the reflected sound is everywhere equal. In deriving the reverberation time formula, which predicts how long sound will bounce around an enclosed space after the source has stopped, W.C. Sabine assumed perfect diffusion within it. When this is not the case, his formula may predict inaccurate results. For example, the Sabine equation will not give correct results in an auditorium with poor diffusion, as when there is a large overhanging balcony, or if one of the dimensions of the enclosed space is very much greater than the other dimensions, or if the auditorium is divided into spaces having different acoustical properties. An auditorium with excellent diffusion beneficially affects the uniformity of decay of sound within the space and pleases the listener’s ear. Among techniques that contribute to good diffusion are the surface irregularities found in the elaborate styles of architecture of the past. Illustrations will be presented showing some approaches within the modern architectural idiom that have yielded successful results.