From DT Online
Microphones convert sound energy into electrical energy.
Features and Applications
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.
""Ribbon"" - a lightweight foil ribbon is suspended in a strong magnetic field. When sound hits the ribbon it vibrates within the magnetic field and generates a small current.
Capacitor - a lightweight charged diaphragm is positioned close to another charged plate. 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.