Experimental investigation on a microphone using the change in the total reflection of light by sound

Abstract

A new microphone, which uses the optical total reflection at the boundary surface between glass and air, is investigated theoretically and experimentally. The critical angle for total reflection changes by the refractive index of air, which depends on the air density. The density changes by the sound pressure. Therefore, the sound pressure is measurable by detecting the intensity of the reflected light from the total reflection area, and it is expected that there is no limitation in the frequency range as the mechanical vibration is not used. The sound pressure sensitivity of the microphone and the effect of surrounding conditions are investigated theoretically. Some experiments are carried out to verify the theoretical investigations, employing a laser diode and a sensor made by cutting off a part of cylindrical glass rod. Experimental results show that the microphone can be used for the measurement of the waveform of high frequency sound though the sensitivity of the microphone is low as expected by the theoretical investigation. The remaining problem is to improve the sensitivity.