Implementation of a Phased Microphone Array in a Closed-Section Wind Tunnel

Abstract

This paper describes the design and implementation of a phased microphone array system for aeroacoustic measurements in a closed-section wind tunnel at the Department of Engineering at the University of Cambridge. The tunnel has a test-section area of 1.67 x 1.22 m and can generate wind flows up to 60 m/s. The research started by conducting a feasibility study to find out whether an array could be used in the presence of the tunnel background noise. The boundary-layer pressure fluctuations were found to be correlated according to the Corcos model (Corcos, G. M., Journal of Fluid Mechanics, Vol. 18, No. 3, 1964, pp. 353-378.). Their level can be reduced by recessing microphones away from the flow, but the benefits of such an approach in this facility were found to be insufficient for it to be followed. Two different size arrays, for frequency ranges 650 ∼ 6500 Hz and 5 ∼ 50 kHz, were designed and installed. A range of characterization and shakedown experiments are described; in particular, it is shown that averaging can increase signal-to-noise ratio and that the array can detect sources up to 6 dB below the mean cross-spectral level of the background noise.