Electret Microphone

Electret Microphone

This device has a microphone and pre-amplifier built into a small enclosure. It is important to connect it the right way round because the built-in preamp might be destroyed otherwise. The preamp uses a field effect transistor (FET).

This microphone uses two metal plates as a capacitor. One of the plates moves or vibrates in step with incoming sound waves. This causes the capacitance to change.

A Mathematical Explanation

Q = CV

Q = charge (Coulombs)

C = capacitance (Farads)

V = voltage (Volts)

Since Q stays constant (it does not have time to change) and since C changes, it follows that V will change. Thus incoming sound waves are converted into a proportional alternating voltage. This voltage is amplified by the built-in FET. This is a delicate, easily damaged component.

Moving Coil Microphone

Moving coil microphones are similar in structure to moving coil loudspeakers but much smaller and more delicate.

When sound hits the microphone, a paper cone inside the structure is moved by the sound waves. This in turn moves a coil. The coil is suspended in a strong magnetic field so when it moves, voltages are induced.

These voltages are proportional to the coil movement. This in turn is proportional to the incoming sound wave pressures.