Acoustic Lucky Imaging: Retrieving spatial resolution in beamforming lost by reduced signal coherence

In aeroacoustic wind tunnel measurements beamforming algorithms are frequently employed to detect and quantify aerodynamic noise sources. In open test section wind tunnels the sound originating from a noise source travels from inside the potential core into the quiescent flow region where the microphone array is located, on its way passing through a turbulent shear layer. The velocity fluctuations in the shear layer stochastically alter the propagation time between source and observer. Since this process is random and dependent on observer position the coherence between two microphone pairs decreases. This leads to beamforming maps with reduced peak Sound Pressure Level. This paper explores the applicability of the Lucky Imaging method, originally developed for use in astronomy to correct for image distortion due to the earth’s atmosphere, to improve results of acoustic wind tunnel measurements in order to mitigate the effect of coherence loss. The Lucky Imaging method uses a short-exposure time to freeze the effect of the turbulence. Pictures which are distorted are then discarded and only the pictures with low distortion are further processed. The method has been adapted towards use in beamforming applications. The Acoustic Lucky Imaging uses beamforming maps generated with data acquired in a short-interval. Subsequently the distorted maps are discarded and the remaining maps are further processed using a shift-and-add method in order to spatially align the sound sources. Numerical and experimental results show that this process is able to recover part of the lost resolution and restore peak Sound Pressure Level up to 6 dB.