A novel design of a feedback-controlled optical microphone for aeroacoustics research

This study constitutes a proof of concept of a new feedback-controlled optical microphone for potential use in phased 'beam-forming' arrays utilized in aeroacoustics research. In the new microphone design, a fiber-optic lever sensor is employed as a means for measuring the center displacement of a stretched thin membrane caused by incident acoustic pressure. The membrane is constructed from polyvinylidene-fluoride which exhibits piezoelectric properties allowing actuation of the membrane in a feedback system to nullify the optically detected deflection. The feedback provision was used to actively modify sensor parameters, most notably membrane stiffness, resonant frequency and damping. Testing of a prototype microphone was performed using a plane wave tube calibrator. Using feedback control, the fundamental resonant frequency of the prototype capsule was increased from 3.61 to 5.1 kHz, a 41% increase. Also, feedback was used for dc attenuation, equivalent to 'stiffening' of the microphone membrane. The results demonstrate that feedback control is an effective method for improving the microphone's transient response, as well as for 'self-tuning' and matching of microphone parameters in sensing arrays.