The Modulation of Sound Decay Curves

Abstract

The irregularity of decay curves has long been noted as a difficulty in the measurement of the reverberation time of a room. It has been suggested that these irregularities may be an important factor in the evaluation of the subjective acoustic qualities of a room. The departures from an exact exponential decay arise from three causes: (1) Beats are produced by interference among the resonance frequencies of the room. (2) Multiple decay rates result from the shape of the room or the non-uniform distribution of absorption on the walls. (3) The direct waves from the source and its first images are important for large or highly absorptive rooms, and at high frequencies. To investigate these phenomena, objective measurements have been made on nine rooms of widely differing sizes and shapes. Apparatus has been designed which compensates for the exponential slope of the decay, leaving only the remaining variations to be recorded by a high speed level recorder. Experimental data are presented for the nine rooms.