Concepts for reducing the self-noise of in-flow acoustic sensors and arrays

Several recent studies have demonstrated the feasibility of aeroacoustic measurements in closed-section wind tunnels using m-flow phased microphone arrays. Such applications subject the sensors to boundary-layer noise that can limit the acoustic response of the array. This paper presents a simple estimate of the effects cf boundary-layer self-noise on measurement accuracy, which include effects of sensor count, individual sensor signal-to-noise ratio, and data block count. This estimate compares favorably with the results of a Monte Carlo simulation of three different arrays with 35, 70, and 140 elements in a typical array test configuration. The effect of sensor self-noise on spatial pattern dynamic range was also evaluated. Two methods for directly reducing in-flow sensor self noise, recessed microphone placement, and laminar flow control on the array fairing, were evaluated experimentally and found to be effective in reducing self-noise by 5 to 15 dB, which result in levels. comparable to isolated microphones with quiet forebodies. Further development of these noise-reducing concepts is needed for practical application to large arrays.