Crystal earpiece

A Crystal Earpiece (more properly called a piezoelectric earphone, pronounced pee-zo) is made of a material that changes its shape when connected to a source of electricity. Some crystals such as quartz and Rochelle salt are piezoelectric. Some ceramics (such as those made with barium titanate) are also piezoelectric.

Crystal earpieces are more or less always mono devices with very low sound fidelity. They typically use a crumpled conical metal film diaphragm, and have very high impedance and sensitivty. They are popular for use with crystal radios due to this ideal combination of characteristics.

In the past crystal earpieces were also used as microphones, with their high output requiring less amplification, saving on costs. Their poor sound quality makes crystal microphones an unlikely choice today.

There is also another common design of piezoelectric transducer. This is made of a disk of brass that is coated with barium titanate ceramic. When electricity is connected to it, the ceramic bends the brass disk, and we can hear the vibrations this causes in the air. These provide much greater sound output than crystal earpieces, but have insufficient bandwidth for earpiece use. They are usually used as resonant bleepers.

This article uses material from http://en.wikipedia.org/wiki/Crystal_earpiece.