Condenser microphone is a sound pick-up transducer. The mechanism is based on the characteristic of a capacitor. A capacitor is constructed by two conductor plates separated by a dielectric material (isolator). Air is a common dielectric used in condenser microphone. One plate will be stationary and the other plate will be vibrated by the received sound wave. Because a capacitor is always keep its charge (Q=C.V) to be constant constant, then the vibration will variate the capacitance (C) since it depends on the distance between the two plates. As the result, the voltage (V) will vary according to the vibration pattern to keep the charge (Q) constant.

Before a condenser microphone can be used, it should be charged by applying a voltage on its electrodes. Theoretically, after the plates are charged, the microphone will operate although the voltage supply has been removed, but practically it’s not. First because the microphone has a current leakage (between two plates), and the second, is that the load connected to the microphone output (the pre-amplifier) draw current from it. You need a bias current via a resistor an a voltage source to keep the microphone working.

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