We use microphones to convert sound in air into and electrical signal.

These electrical signals can be amplified (increasing the voltage and current) so that it can driver a loudspeaker.

Microphones come in two types : electro-mehcanical and piezo-electric.

Electro-mechanical microphones use a small diaphram attached to a coil of wires suspended in a magnetc field. When the vibrations in the air cause cause the diaphram to vibrate, the movement of the coil of wire in the magnetic field generates a small electric current.

Piezo-electric microphones harness the physical properties of certain types of crystal which generate a very small electrical signal when placed under stress. In this case, the stress is placed on the crystal by the vibration of air molecules.