Optimization of Microphone Array Wall Mountings in Closed-Section Wind Tunnels

Abstract

The objective of the present paper is to improve the implementation of microphone arrays on the wall of closed-section wind tunnels. A test campaign has been conducted in the ONERA F2 1.4 m × 1.8 m wind tunnel to investigate the performance of thin and smooth metallic clothes to protect the microphones from the hydrodynamic fluctuations in the turbulent boundary-layer flow and, still, to preserve the transmitted acoustic signal. Three microphone mountings have been considered : individually recessed microphones in (i) cylindrical and (ii) conical apertures and (iii) a recessed, screened, large-scale microphone mounting plate (microphones flush with the plate). It was found that by covering the top of the cylindrical and conical microphone apertures with a wire-mesh, the strong self-noise due to the interaction of the boundary-layer flow with the cavities can be suppressed. The most significant noise reduction is obtained when the large-scale microphone mounting plate is recessed behind the metallic wire-mesh. A reduction by up to 20 dB is then obtained with respect to a flush-mounted, unscreened, microphone. This microphone mounting out-performs the cone-nose-protected and Kevlar-protected microphone mountings, two conventional low-noise devices. Thanks to an original calibration technique, the acoustic response of the tested microphone mountings could be evaluated in presence of flow (� 80 m/s). The recessed, screened, microphone mounting plate has an oscillating acoustic response (in function of the frequency) by less than 5 dB deviation. This attenuation can be post-corrected by using a very simple acoustic model.