Design and In-situ Calibration of a Beamforming Array for High-frequency Noise Measurements in a Hybrid-Anechoic Wind Tunnel

The design and commissioning of a 120-element beamforming array that is tailored for the measurement of high-frequency noise sources (10 kHz - 70 kHz) within a semi-anechoic Kevlar-walled, or hybrid-anechoic wind tunnel (HAWT) configuration are presented. The components of the array and the experimental approach proposed are individually assessed. Namely, the design considerations of an array layout tailored for high-frequency noise sources are introduced. The impact of microphone mounts is also considered and each microphone is calibrated using a high-frequency noise source generated via laser-induced plasma (LIP). It was found that the microphone mounts are suitable for the proposed use, they do not significantly impact the performance of the microphones, however, using the in-situ LIP-based microphone calibration data, cleaner, and more accurate beamforming maps were obtained. In terms of semi-anechoic HAWT performance, no reverberations were observed in the maps that could originate from the semi-anechoic configuration or from the presence of a large, flat, two-dimensional NACA0012 airfoil, however, mirror image sources were found in the calculated maps that were present due to the acoustically reflective sections of the floor and ceiling of the test section.