Steady-state acoustic energy distribution in a reverberation chamber at low frequencies

Abstract

The spatial variation of the mean-square sound pressure in a hard-walled rectilinear reverberation chamber is analyzed by extending the “Waterhouse theory” (based on a free-wave model for an omnidirectional sound field impinging on the corner formed by three orthogonal planes) to apply to a closed chamber at low frequencies. This is done by expressing the mean-square sound pressure in terms of the contributions from the actual pressure microphone and from an infinite array of image microphones due to multiple reflections in the chamber walls, and then transforming this expression into a weighted sum of the normal eigenfunctions for the chamber. Experimental sound-pressure levels, measured along different linear paths in the 425-m3 NBS reverberation chamber, are compared with the predictions of this analysis. It is found that the spatial dependence of sound-pressure level is accurately predictable, at least in the NBS chamber, to significantly lower frequencies than has usually been thought possible. Expressions are given for relating the spatial average (over the volume of the chamber) mean-square sound pressure to that which is measured at a given position in the room. The implications of these findings to sound power determinations are discussed.