The Application of Correlation Techniques to Some Acoustic Measurements

Abstract

This paper discusses the application of an analog correlator which computes the crosscorrelation function between two sound pressures to such measurement problems as the localization of noisesources, the determination of transmission loss, and the reduction of microphonewind noise. The crosscorrelation function φ21 (r) between two nonperiodic signals will have a peak in amplitude if a component of each signal originates from a common source. The value of time delay τ for which this peak occurs equals the difference in time required for the individual components to propagate from the common source to the two points under study. An analog electronic correlator described in a companion paper has been constructed which, by employing this property of the crosscorrelation function, can separate the acoustic signal at a given point into components according to: (1) their points of origin (assuming independent sources), (2) the transit time from source to the point in question, and (3) frequency. Preliminary experimental results show that correlation provides a practical method for determining the amount of sound contributed to the field at a given point by each of several sources. The correlator, by separating the signal transmitted directly through the structure from the flanking signal on the basis of arrival time, is also a useful tool for measuring the transmission loss of walls.