Investigation of Room Acoustics by Steady State Transmission Measurements

Abstract

It should be possible to deduce the necessary acoustical information concerning the bounding surfaces of a room from steady-state measurements of the transmission between a source and a receiver appropriately placed in the room. This problem is being studied by using a small model chamber yielding discrete normal modes of vibration in the range from 200 to 1500 cycles. A small sound source having substantially constant output over this frequency range was developed and comprises a multiple capillary formed by packing a brass tube with parallel round wires, the sound from a dynamic receiver passing down the canals between the wires. Resonant pressure peaks in the chamber corresponding to the first fifty normal modes of vibration have been observed and identified. Analysis and experiment indicate that the total absorption in the chamber for a given mode of vibration can be measured by observing the width of the resonance curve at the half-intensity height. By selection of different modes of vibration, absorption coefficients are obtained as functions of both frequency and angle of incidence, the latter including grazing incidence. The decay rates predicted by these measurements have been checked experimentally. Plans will be discussed for the extension of these results to large scale rooms by an appropriate statistical analysis.