Microphone

Microphones convert sound energy into electrical energy. There are a number of types :-

Moving coil - similar in construction to a loudspeaker but working in reverse. The sound vibrates the diaphragm which in turn moves the coil and induces an emf into it.

Crystal or piezo - sounds hitting the piezo material causes minute currents to flow.

Capacitance - when sound hits the diaphragm it varies the gap and hence the capacitance. As the microphone needs to be polarised (i.e. have a fixed voltage applied across it), varying the capacitance also varies the voltage and therefore gives a true representation of the sound at the output.