1. A microphone consisting essentially of a flexible metal diaphragm and a rigid metal plate that together form a two-plate air capacitor.

Sound waves set the diaphragm in vibration, producing capacitance variations that are converted into audio-frequency signals by a suitable amplifier circuit.

2. A microphone design where a condenser (the original name for a capacitor) is created by stretching a thin diaphragm in front of a metal disc (the back plate).

By positioning the two surfaces very close together an electrical capacitor is created so the capacitance varies as a function of sound pressure.

Any change in sound pressure causes the diaphragm to move, which changes the distance between the two surfaces.

Electrostatic microphones, unlike other types, require a battery or other voltage source.