The measurement of acoustic reflection coefficients by using cepstral techniques

Abstract

The author has recently developed a new free field technique for the in situ measurement of acoustic reflection coefficients by the use of transient acoustic signals [J. S. Bolton and E. Gold, J. Sound Vib. 110(2), 179–202 (1986)]. The measurement is conducted by using an omni-directional loudspeaker to generate a test signal; a microphone placed between the loudspeaker and the reflecting surface is used to detect both the incident (i.e., the direct) and reflected signals. Cepstral processing was used to eliminate some of the difficulties associated with existing reflection measurement methods. Specifically, in the new technique the direct and reflected signals are measured simultaneously and are allowed to overlap in time, thus relaxing geometrical constraints. Further, if the incident signal is sufficiently broadband, a knowledge of its spectrum is not required nor is it necessary to measure it individually. The technique is based on the direct extraction of the impulse response of the reflecting surface from the power cepstrum of a signal containing both the direct and reflected signals. Full-scale acoustical trials of this technique will be described and measurements of the reflection coefficients of samples of polyurethane foam will be shown.