A method for reducing the flow-induced self-noise of acoustic array microphones was demonstrated in a wind tunnel. Unsteady boundary-layer flow in the wind tunnel induces large pressure fluctuations on exposed, flush-mounted microphone diaphragms, reducing the signal-to-noise capability of microphone arrays. Two important steps were take to reduce this background noise. First, the microphones were recessed to separate the microphones from the unsteady flow. Second, a porous surface material was placed above the microphones to act as an aerodynamic surface while allowing acoustic signals to pass through to the microphones. Previous attempts at this approach used perforated plates as the surface material, which tended to fatigue in the unsteady flow. The increased acoustic impedance of thicker materials caused reverberation between the surface and the microphone mounting plate. This latest attempt used a stretched sheet of Kevlar® as the surface. The extreme strength and durability of the Kevlar® withstood flow-induced fatigue while providing very low acoustic impedance with little attenuation of sound for most frequencies. Data is presented for two wind tunnel tests that demonstrate the capabilities of the recessed Kevlar® array.
Stephen M. Jaeger, W. Clifton Horne and Christopher S. Allen, “Effect of Surface Treatment on Array Microphone Self-noise,” AIAA 2000-1937, 6th AIAA/CEAS Aeroacoustics Conference, Lahaina, HI, June 2000.
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