Piezo Transducer
From DT Online
When a piece of piezo material (quartz) is squeezed or compressed it produces a small electric current. Conversely, if a small current is passed through a piece of piezo crystal the material will move. This effect is used in piezo transducers.
When used as input devices, vibration creates a small signal which can be used to trigger transistor and logic circuits. As output devices an a.c. signal causes them to vibrate and thus create sound.