The Electrostatic (Condenser) Microphone

Author: N.H. Crowhurst

If an electrophorus (a simple instrument used in demonstrating the properties of electrical charges) is charged up and connected to an electrostatic voltmeter, a low reading will be obtained with the plates of the electrophorus in contact with each other. When the moveable plate of the electrophorus is lifted from its base, the reading will rise - probably shoot off the scale. When the plate is replaced, the reading will return to the earlier value.

The principle of condenser microphone operation

This shows that when the distance between the charged plates changes, the voltage due to the charge changes. This principle is used in the electrostatic or condenser microphone. (Nowadays, the use of the term "condenser" is discouraged for most purposes - it should be capacitor, but most people still use the older term for the electrostatic microphone.)

In a condenser microphone, one plate is flexible, whereas the other has holes in it that permit air to flow into the space between them, (This permits the flexible plate to move freely due to sound waves from either direction.) The motion of the flexible diaphragm changes the spacing between it and the fixed plate, and produces voltage fluctuations.

Cutaway view of a condenser microphone

To make a condenser microphone work, it must have a steady electric charge upon it (obtained from a source of high voltage) that is isolated from the microphone so that a change in the spacing between the plates due to incident sound waves causes the voltage between them to go up and down. This is done by connecting a high voltage across the plates through a large resistance. The fluctuating voltage due to sound waves is fed through a coupling capacitor to an output resistor.

The capacitance of the coupling capacitor is greater than that between the microphone plates, and both the resistors are so large that the charges on capacitors do not have time to change during the slowest fluctuations due to sound waves. Because the charge on the coupling capacitor does not change, the voltage across it also must be constant. Therefore, the voltage that appears across the output resistor has the same fluctuations put out by the microphone, without, however, the polarizing voltage applied to microphone plates.

Condenser microphone circuit