12.5 How does a microphone work?

A microphone works due to electromagnetic induction

A microphone works in the opposite way to a loudspeaker.

The diagram above illustrate the principle of a microphone e.g. for a vocalist or a telephone mouthpiece.

A microphone converts the energy of the pressure variation of sound waves into an electrical energy signal in an ac current.

The oscillation of the sound waves vibrates the diaphragm which induces an oscillation in the electrical circuit - the electrical signal.

The electrical signal could in turn be used to re-generate sound in a loudspeaker.

A microphone behaves like a loudspeaker in reverse -

The right-hand side of the diagram was borrowed from my loudspeaker diagram!

The coil of wire surrounds one pole of a permanent magnet, which is itself surrounded by the other pole of the magnet - look for the N and S on the diagram above.

The coil, in which the current is induced, is connected to the flexible diaphragm cone cover made of thin plastic or metal.

If the coil moves in the magnetic field, a p.d. is induced in the coil.

This is no different in principle to the working of a generator - a coil moves through a magnetic field,

but there is no rotation, it is a 'to and fro' vibration effect.

The initial movement comes from the sound waves hitting the flexible diaphragm.

The sound wave vibrations cause the diaphragm to vibrate and move in resonance with the sound wave.

In turn, the coil automatically moves at the same frequencies as the sound wave inducing a varying current in the coil.

Also, the louder the sound, the bigger displacement of the coil.

Therefore the movement of the coil generates (induces) an electrical current signal that can be used to reproduce both the frequencies and relative volumes (loudness) of the sound waves in a loudspeaker system or a recording device like a tape recorder.

See also Sound waves nature and properties explained