Mapping of contributions from car-exterior aerodynamic sources to an in-cabin reference signal using Clean-SC

Use of an out-of-flow array in a wind tunnel to map car-exterior aerodynamic noise sources will typically show the contributions to the sound pressure at the array. The cross-spectral matrix is first averaged, followed by beamforming and use of a deconvolution technique, such as Clean-SC, to improve spatial resolution and to suppress sidelobes. Clean-SC makes use of the fact that the aerodynamic noise sources across the exterior car surface are highly incoherent, so it builds up a sparse distribution of incoherent point sources on the mapping surface, which together represent the measured cross-spectral matrix to a chosen accuracy. The main idea of the present paper is to produce a Clean-SC map, where each point source is assigned a value equal to that part of a measured in-cabin reference signal, which is coherent with the point source. Effectively, a map is obtained of the car-exterior contributions to the in-cabin reference. The method uses the measured cross spectra between the in-cabin reference and the out-of-flow array microphones, and therefore it relies on the coherence loss across the shear layer being small or properly compensated. A new Diagonal Removal algorithm will be presented and results from practical measurement will be given.