Solid-state photo-microphones or pressure sensors by total reflection

Abstract

Solid-state photo-microphones or pressure sensors are proposed. These sensors use a new principle, involving the optical total reflection at the boundary surface between glass and air. The critical angle for total reflection changes by the refractive index of air, which depends on the air density. Sound pressure changes the air density. Therefore, the sound pressure is measurable by detecting the intensity of the reflected light from the total reflection area. The sensitivity of the sensor is investigated theoretically. It is expected that the sensor has sufficient sensitivity for practical use, employing laser light and a curved boundary surface with a large radius of curvature. Some experiments are carried out to verify the theoretical investigations. A He-Ne laser or a laser diode is employed as a light source in the experiments. Experimental results show that the sensor has equivalent sensitivity to that which was theoretically estimated, but that sensitivity is very low. The sensor is useful as a pressure sensor, but it is difficult to realize a microphone for general use at the present. The microphones have no diaphragm and the upper limit in the frequency range is extremely high in principle.