Crystal Microphone

A microphone used for aircraft radio transmissions in which sound pressure waves distort a piece of piezoelectric crystal. A diaphragm in the microphone is attached to a crystal of Rochelle salt or other piezoelectric material in such a way that, when sound waves vibrate the diaphragm, it distorts the crystal. When the crystal vibrates, it produces a voltage across two of its opposite faces that changes in exactly the same way the sound pressure changes.