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# Electret Microphone

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A MICROPHONE is a TRANSDUCER which converts sound waves into an electrical signal of the same frequency.

## 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)

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.

### Electret Microphone Circuit

The resistor to the positive supply is not well described in the data sheets. (Supply_Voltage x 3) kΩ seems to work so in the circuit below, 33kΩ to 47kΩ should be used.

### Pre-Amplifier Circuit

This can be used to amplify a microphone signal before passing the output to an audio power amplifier or perhaps to the sound-card in a PC.

### Possible Layout

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