The property of certain crystals that causes them to produce voltage when a mechanical pressure is applied to them such as sound vibrations. This technique is used to build crystal microphones, phonograph cartridges and strain gauges, all of which turn mechanical movement into voltage.
The piezoelectric effect also works in reverse, causing an oscillation when the material is subjected to electrical pressure (voltage). For example, earphones and inkjet nozzles are made with piezoelectric materials. See
inkjet printer.
Ultra-Thin Speakers
Engineers are working on piezoelectric speakers that are as thin as a sheet of plastic. A feature of such speakers causes sound to reach someone without affecting people nearby. For example, a piezoelectric speaker in a phone means only the person holding the phone would hear it. The downside thus far is that such speakers cannot product low-frequency bass sounds, which means music cannot be played adequately.