The Optical Microphone - Fact or Fiction

This paper describes various techniques by which the displacement of a microphone diaphragm may be measured optically.The need for'a nonvelectn cal method of measurement came about as a result of previous research into a force-feedback microphone In this microphone the electrodes (one on each side of the diaphragm) were used to carry the feedback signal.The normal technique of DC excitation which generates a change in voltage irom the change In capacitance could not thereiore be employed.In the prototype force-feedback capsule the change in capacitance was used to modulate the frequency of an oscillator and this FM signal was then frequency shifted and demodulated.Unfortunately the feedback signal was found to aiiect the frequency of the oscillator directly.ionning an unwanted high-frequency feedback path that proved difficult to stabilise.In addition to this the demodulator was iound to introduce excessive phase lag at high frequencies which limited the amount of attainable feedback.The closed-loop frequency response was however excellent and so alternative methods of measuring the diaphragm displacement were considered.