Spatial averaging in sound power measurements

Abstract To measure sound power in a reverberation room, we need to estimate the space-time average of squared sound pressure. While the time average usually presents no special problem, the space average may cause difficulties, particularly for acoustic signals having pure-tone or extremely narrow-band spectra. The space average is subject to a random error due to finite sample size. Nomographs and formulas are presented from which an experimenter may determine the confidence level of his estimate in terms of signal bandwidth reverberation time, and number of fixed microphones or the path length for a moving microphone. Finally, the remarkable performance of rotating diffusers is described in terms of their improvement in the statistical accuracy of the measured mean. A way of computing their figure of merit is described. It is shown that the rotating diffuser is effectively a parametric modulator which introduces new frequencies in the room.