Acoustic measurements and background noise separation in wind tunnels

Abstract

An attempt is made at developing an integral methodology for experimental acoustic measurements and separation of background noise in wind tunnels. To this end, an array beamforming technique known as delay-and-sum beamforming is identified and tested. The theories underlying delay-and-sum beamforming and minimum redundancy arrays are discussed briefly. Two linear arrays, a seven microphone uniform linear array and a four microphone minimum redundancy array, are designed. To provide a source of monochromatic sound for calibration purposes, an acoustic driver is designed based on Helmholtz resonator theory. Array beamforming results show that tests conducted with the two linear arrays in the open field provide good resolution of the driver sound source Direction Of Arrival (DOA) peaks from the background noise, and provide a semianechoic reference with which to compare wind tunnel results. The minimum redundancy array proved more accurate than the uniform linear array in terms of resolution because of the minimum redundancy array’s ‘increased effective aperture over the uniform linear array. Beamforming results obtained for the driver placed inside the wind tunnel with no flow and tunnel velocities of 45 and 81 fWsec successfully resolved the DOA peaks of the driver from the background noise of the tunnel. NOMENCLATURE A area of the opening of Hehnholtz resonator