An Investigation of Microphone Array Installation Effects

Abstract

The ability to perform aeroacoustic measurements in hard-walled, closed section wind tunnels is desirable for the development of quiet aircraft, to supplement dedicated acoustic testing in open-jet anechoic facilities. One of the restrictions of such testing is the poor signal-to-noise ratio (SNR) when using arrays of microphones mounted on the wind tunnel wall. This can limit the ability to discern acoustic sources which are near, or below, the background noise level of the facility. Increasing the number of sensors can help to improve SNR. In this paper we investigate how sensor mounting details can help to improve SNR. A systematic study of microphone mounting strategies is presented. The performance of individual microphone recessing, using straight and countersunk holes, is investigated and compared with flush-mounted microphone performance. The effects of placing the microphones out of the flow by recessing the whole array behind a cloth are described. A parametric study of the effect of recess depth and cloth type has been carried out, along with the effect of adding an acoustic foam liner within the recess enclosure. Results show that recessing individual microphones by the depth of the microphone diameter (d) up to 2d can provide up to 3dB improvement. Increasing the recess depth beyond 2d provided up to 10dB improvement, with recessing to 10d depth providing up to 20dB improvement. The greatest improvements occur below 25kHz, although there is improvement across the 0 to 48kHz range. The effect of countersunk recessing was either no improvement, or an increase in the background noise level of up to 20dB, possibly due to cavity mode oscillations within the recess aperture. Significant differences in SNR were observed between Kevlar cloths of different densities, and with a silk covering. A reduction in background noise level of 5 to 10dB was observed when acoustic foam lining was added to the floor of the recessed array. Overall this study concludes that the use of recessed arrays with acoustic foam lining may significantly improve microphone array SNR in hard-walled wind tunnel testing