Beamforming In Acoustic Testing

Abstract

Non-intrusive measurements of wind tunnel model noise can be made in non-acoustic, hardwall wind tunnels, as well as free jets and other challenging settings, by using beamforming techniques with sparse, wide-band phased arrays of microphones. The process, which was developed for this application at Boeing in the last few years, begins by measuring a full array cross-spectral matrix for each frequency of interest. A grid of potential source locations in the test section is defined. A complex array steering vector is computed for each grid point with non-uniform flow, microphone imperfections, and installation effects taken into account. Beamforming combines the cross-spectral matrices and the steering vectors to produce maps of the wind tunnel model’s acoustic source distribution. Several beamforming algorithms are described with intuitive motivation and expected performance. Practical details of steering vector construction are presented. Techniques for removing various types of interference are detailed.