Measuring Jet Noise Source Locations with Acoustic Beamforming

Abstract

Numerous methods have been used by the aeroacoustics community to locate noise sources of different frequencies in subsonic jets. These include: acoustic mirrors, microphone arrays, two-microphone methods, causality correlation and coherence techniques, acoustic beamforming, and automated source breakdown. Except for the acoustic beamforming, most of these methods are quite time consuming. No one seems to have ever compared the performance of these techniques. This paper compares the performance of various existing techniques. As the beamforming technique is the least time consuming, emphasis has been given to examining the performance of this technique in measuring source location in a subsonic jet for a range of Reynolds numbers. A 48-microphone acoustic beamformer was used to locate the noise sources for several subsonic Mach numbers from a one-inch and a half-inch nozzle. The effects of tabs on source location are also examined for the one-inch nozzle. Once set up, this method allows for faster and thus more numerous tests. It is shown that jet noise source locations may be a strong function of either Reynolds number or nozzle exit boundary layer. In general, there is a shift of all the noise sources away from the nozzle exit as Reynolds number decreases.