For example , in sound recording , fluctuations in air pressure ( that is to say , sound ) strike the diaphragm of a microphone which causes corresponding fluctuations in a voltage or the current in an electric circuit . For example , in sound recording , fluctuations in air pressure ( that is to say , sound ) strike the diaphragm of a microphone which induces corresponding fluctuations in the current produced by a coil in an electromagnetic microphone , or the voltage produced by a condensor microphone .