This study is an extension of a previous study conducted at MSU on a feedback-controlled optical microphone for potential use in "beam-forming" arrays employing anywhere from thirty to one hundred microphones in aeroacoustics research. The membrane of the microphone is made from PVDF (polyvinylidene-fluoride), a material with piezoelectric properties, giving a means for opposing the sound-produced deflection of the membrane using a feedback loop. The feedback capability of the microphone allows it to self-calibrate and gives it response-matching capabilities in array applications---a unique advantage over current microphone technology, as manual, individual calibration of each microphone in an array can be time-consuming.The present research goal is to extend the operational range of the microphone. Two provisions were proposed to accomplish this goal: the addition of a microphone back-plate with small holes to create a means to damp the membrane motion; also, by employing an optical sensor that measures membrane displacement averaged over an area rather than at a point, as used in the previous study, to attenuate the high-order resonance modes which cause instability in the control. The improved feedback microphone will ultimately enhance the capability of "beam-forming" arrays used for research. The results show that individually, each of the damping and area-averaged sensing was able to improve the operational range of the microphone, and together, they combined for an even greater improvement in stability, leading to tripling of the bandwidth of the baseline open-loop microphone.
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