The working principle of an electret condenser microphone is that the diaphragm acts as one plate of a capacitor. Vibrations produce changes in the distance between the diaphragm and the back plate. The voltage maintained across the diaphragm and the back plate changes with the vibrations in the air, according to the capacitance equation (C = Q / V), where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts. This change in voltage is amplified by the FET and the audio signal appears at the output, after a DC-blocking capacitor.