Measurement of acoustic intensity in a duct system with flow

Abstract

A transfer-function method is described of measuring the acoustic intensity of plane waves in a duct with flow. In this method, the transfer function between two locations in the duct is used to determine the reflection coefficient. Acoustic intensity in a duct with flow is then determined from the reflection coefficient using an expression developed by Morfey. In the case of no flow, the transfer-function formulation reduces to a cross-spectral relation similar to the relation used to measure acoustic intensity in three dimensions. In contrast to the relation in three dimensions, however, the cross-spectral formulation for a duct is not limited in accuracy by the microphone spacing. Laboratory tests have been conducted in which the transfer-function method was used inside the pipe to evaluate the net sound power transmitted toward the open end of the pipe. The three-dimensional cross-spectral method of measuring acoustic intensity was used outside the pipe to evaluate the total radiated sound power. The acoustic power-loss during the radiation process was then determined by comparing the two results.