The condenser microphone, created in 1916 by E. C. Wente at Western Electric, is also known as a capacitor microphone or electrostatic microphone—capacitors were once known as condensers. The diaphragm is used as one of the plates in a capacitor, and the vibrations cause the distance between the plates to alter.
DC-biased microphones and radio frequency (RF) or high frequency (HF) condenser microphones are the two sorts, depending on how the audio signal is extracted from the transducer. The plates of a DC-biased microphone are charged with a constant charge (Q).
According to the capacitance equation (C = QV), where Q = charge in coulombs, C = capacitance in farads, and V = potential difference in volts, the voltage maintained across the capacitor plates fluctuates with the vibrations in the air. In a parallel-plate capacitor, the capacitance of the plates is inversely proportional to the distance between them.
An "element" or "capsule" is a collection of fixed and moveable plates.
On the capacitor, a nearly constant charge is maintained. The charge across the capacitor fluctuates very slightly as the capacitance changes, but it remains quite steady at audible frequencies. The bias resistor's magnitude (100 M to tens of G) and the capsule's capacitance (about 5 to 100 pF) combine to generate a high-pass filter for the audio signal and a low-pass filter for the bias voltage. The product of the resistance and capacitance equals the time constant of an RC circuit.
The charge is nearly constant over the time-frame of the capacitance shift (as much as 50 ms for 20 Hz audio signal), and the voltage across the capacitor changes instantly to reflect the change in capacitance. Above and below the bias voltage, the voltage across the capacitor varies. The series resistor shows the voltage differential between the bias and the capacitor. For performance or recording, the voltage across the resistor is amplified. Because the voltage differential is quite large, up to several volts for high sound levels, the electronics in the microphone contribute little voltage gain in most circumstances.
Because this is a circuit with a very high impedance, only current gain is normally required, while the voltage remains constant.
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