The other crucial elements for audio broadcasts are of course a microphone to convert sound waves into voltage variations in the transmitter circuit, and earphones or speakers to transform the received electric signals back into sound. In fact microphones and earphones are more or less the same device. Both contain a diaphragm that is free to vibrate or either create or respond to sound waves, fixed to a coil of wire that moves over a magnet, and so both have the same reversible electromagnetic effects as motors and generators.