Effect of Absorbing Surfaces on the Statistics of Diffuse Sound Fields

Abstract

The random-incidence absorption coefficient of a sound absorbing surface, calculated from the difference of decay rates of the sound field in a reverberation room, invariably exceeds the random-incidence absorption coefficient computed from the acoustic admittance. The excess at high frequencies is too large to be explained by diffraction due to the finite size of the absorbing sample. The increase can be related to the anisotropy of the sound field in the immediate vicinity of every surface. When surfaces have different absorption coefficients (and hence at least some differ from the average), the total absorption is always greater than the product of the total surface area times the average absorption coefficient. This theory, to a first approximation, provides a qualitative explanation of the difference in absorption coefficients calculated from reverberation room and impedance tube measurements. It also shows that the coefficients measured for an absorbing patch in a reverberation room should increase slightly either as the size of the patch decreases or as the total surface area in the room increases—in qualitative agreement with previous experimental observations [e.g., Kosten, Acustica 10, 400–411 (1960)].