Combining asynchronous microphone array measurements for enhanced acoustic imaging and volumetric source mapping

This paper explores the advantages of combining asynchronous microphone array measurements for acoustic source mapping in two and three–dimensional applications. Four different approaches are considered, three consisting of the combination of the source maps (arithmetic mean, geometric mean, and minimum value), and a fourth one obtained by combining the cross–spectral matrices of all the asynchronous measurements into a larger matrix and applying beamforming with it. Both synthetic and experimental test cases convey enhanced results concerning the single measurement baseline, especially by reducing the spurious sidelobes. Two aeroacoustic experiments are considered, the first features a distributed sound source over a flat plate model tested in a wind tunnel and the second features a hovering drone with multiple sound sources, representing typical challenges for 2D and 3D source localization, respectively. For the 2D source mapping configuration, the sidelobe level was reduced up to 5 dB with respect to the baseline, while maintaining a similar beamwidth of the main lobe. For the 3D test cases, the approaches enable volumetric source mapping capabilities, even when planar microphone arrays are considered, commonly available in typical test facilities. Overall, the minimum value approach presents the best performance for all cases in terms of reduced main lobe beamwidth and sidelobe levels.