In-Flow Calibration Approach for Improving Beamforming Accuracy

Abstract

The phased microphone array technique is a well established tool in aeroacoustic testing for the localisation and quantification of sound sources. The measurements are often carried out in closed or open wind tunnels using scaled models. In both types of wind tunnels flow effects derogate the accuracy of beamforming results. In the open test-section the sound waves have to pass through the wind tunnel shear layer before reaching the microphone. That results in a spatial coherence loss due to turbulent scattering and refraction and potentially leads to degraded source maps and under-estimated source levels. The present study addresses this matter and proposes a calibration approach for an open wind tunnel using an in-flow calibration source. The source employs a ribbon loudspeaker which can emit sufficiently high sound pressure levels in a broad frequency range and its properties are known. The presented results comprise details of the calibration procedure and their evaluation with respect to the achieved beamforming accuracy.