Derived from a Greek word that means to press, for the purposes of audio, Piezo microphones are manufactured by coupling a diaphragm to a small layer of crystal. When vibration causes the diaphragm to vibrate and therefore "push" against the crystal, a shift of electrons occurs within the crystal, thus creating a potential difference. Piezo microphones are sensitive to the source of the vibration, but insensitive to the outside world, and are useful as contact microphones or triggers for electronic drums.