Performance of hemi-anechoic Rooms

Abstract

Because the constructive and destructive interference between a source and its image formed by the ground plane obscures the conformity of the sound field with the inverse square law, we have used a carousel method to evaluate the performance of hemi-anechoic rooms (i.e., free field rooms with acoustically hard floors). Using the carousel method, the spatial variation of the SPL in hemi-anechoic rooms having the overall shape of a rectangular parallelepiped periodically fluctuates around a mean value. The amplitude of these spatial fluctuations is a function of the room wall acoustic properties and decreases as the carousel radius decreases and as the frequency and bandwidth increases. We take the rms value of these spatial fluctuations as a measure of the room performance. We conclude that the performance of hemi-anechoic rooms having thick, fibrous glass blankets on the walls and ceiling compares favorably with anechoic rooms having wedge-lined walls and ceiling. Furthermore, since the walls and ceiling of rooms having fibrous glass blankets can be represented by a partially absorbing plane of known surface impedance, the performance of such rooms can be predicted.