Definition
Headphones that convert electrical audio signals into sound using a piezoelectric crystal element, which flexes when voltage is applied and moves a small diaphragm to produce sound. They have very high impedance and require no external power, but produce relatively low audio quality compared to modern dynamic or electret types.
Plain English
An older style of headphone that uses a small crystal to turn an electrical signal into sound. The crystal physically bends when voltage hits it, and that tiny movement makes the sound you hear.
Context Anchor
Seen in older aircraft radio, headset, and avionics maintenance discussions, especially when the type of earphone required for a receiver is being described.
Derivation
Named for the piezoelectric crystal inside. 'Piezoelectric' comes from the Greek piezein, meaning 'to press' — pressing or flexing the crystal generates electricity, and applying electricity makes it flex. That flexing is what produces the sound.
Why Pilots Care
Most pilots will only meet crystal earphones when working with vintage aircraft radios or restoring older equipment. They are not interchangeable with modern low-impedance headsets — plugging one into the wrong system can result in no audio or damaged equipment.
Intuition Check
“Crystal” does not mean decorative glass here. It means a small electrical element that physically moves to produce sound.
Example Sentence 1
The restored 1950s aircraft radio was originally designed to drive crystal earphones, so a modern headset would not work with it directly.
Example Sentence 2
Maintenance crews checked the crystal earphones before reinstalling them in the vintage cockpit headset.