Acoustic intensity measurements in the presence of low Mach number flow

Abstract

Abstract : Acoustic intensity is an important analysis tool since it provides wave propagation directionality along with absolute magnitude, and it can be measured in the near or far field of a source. Acoustic intensity measurements acquired in the presence of a mean flow are susceptible to errors due to the effects of the flow noise on the sensor. To determine if this error could be accurately quantified, intensity measurements were acquired, with the standard two microphone cross spectral technique, in a sound field that contained both mean flow and an independent random broadband noise source. The microphones were flush mounted, at several different separation distances, in the test section of a wind tunnel that provided the desired flow conditions, while a large speaker provided the independent random noise source. The error calculations were based on a technique that had already been derived theoretically and published, but had not been proven experimentally. The experiments performed validate that the error is indeed a bias error and that it can be accurately quantified. In addition, accurate quantification of the error is not limited to one-dimensional sound fields that contain only plane waves, and the method can be easily extended to tow or three directions with complex wave propagation. Acoustic intensity, Low mach number, Flow, Two microphone, Cross spectral, Wind tunnel.