A class of microphone, of which condenser and electret are types, in which air pressure changes cause changes in the capacitance of a condenser. The capacitor is normally biased by a voltage which is supplied from batteries or via phantom power from the signal cable. The electret is an exception, as this requires such a small biasing voltage that it is possible to charge it permanently at the time of manufacture; Sennheiser mics use a proprietary biasing scheme which utilizes RF instead of a DC voltage.