An optical microphone which comprises a source of light connectable to a power source for illuminating an acoustically sensitive membrane; a photo detector having an output for transforming light reflected by the membrane into electrical signals proportional to the intensity of the reflected light; and a circuit for measuring the level of signals at the output of the photodetector for controlling the current fed to the source of light by the power source so that the current is low when the output signals of the photodetector are lower than a defined value, and the current is high when the output signals of the photodetector are higher than the defined value.