an optical microphone having enhanced sensitivity only along a specified axis and free from the effect of ambient noises . the optical microphone comprises a diaphragm vibrating with sound pressure , a light source for irradiating the diaphragm with a light beam , a photodetector for receiving a fraction of the light reflected from the diaphragm and outputting a signal corresponding to the vibration of the diaphragm , and a light source driving circuit for driving the light source by supplying a specified current . the optical microphone is additionally provided with a negative feedback circuit for supplying the light source driving circuit with a part of the output signal from the photodetector as a negative feedback signal .