In a condenser microphone, also known as a capacitor microphone, the diaphragm acts as one plate of a capacitor, and the vibrations produce changes in the distance between the plates.
If a D.C. voltage is applied to the circuit, the voltage across the capacitor varies as the sound pressure varies.
Condenser microphones can range from cheap karaoke microphones to extremely high quality and expensive recording microphones. Condenser microphones require a power source which is necessary for establishing the capacitor plate voltage, and also for internal amplification of the signal to a useful output level. This can be provided either from microphone inputs as phantom power which can be found on most mixing desks or from a small battery. Condenser microphones are also available with two diaphragms, the signals from which can be electrically connected such as to provide a range of polar patterns, such as cardioid, omnidirectional and figure-eight.