Crystal earpiece

Crystal radio using piezoelectric earpiece

Common piezoelectric earpiece used with portable radios

A crystal earpiece, more properly called a piezoelectric earphone, is a headphone that produces sound by using a piezoelectric crystal, a material that changes its shape when electricity is applied to it. It is usually designed to plug into the ear canal of the user.

It typically consists of a piezoelectric crystal with metal electrodes attached to either side, glued to a conical plastic or metal foil diaphragm. The piezoelectric material used in early crystal earphones was Rochelle salt, but modern earphones use quartz or more commonly barium titanate. When the audio signal is applied to the electrodes, the crystal bends back and forth with the variations in the signal, vibrating the diaphragm. The diaphragm pushes on the air, creating sound waves. The plastic earpiece confines the sound waves and conducts them efficiently into the ear canal, to the eardrum.

Crystal earpieces are usually monaural devices with very low sound fidelity, but high impedance and sensitivity. They are popular for use with crystal radios due to these characteristics. Their peak use was probably with 1960s era transistor radios and hearing aids. They are not used with modern portable media players because of their poor sound quality.

They have also been used as cheap microphones, with their high output requiring less amplification.

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.

They are used in crystal radio projects and experiments. Their very high impedance (on the order of 20 kilohms)^[1] make them sensitive enough to convert very weak signals.

References

1. ^ "Crystal Earphone (Piezoelectric)". shop.miniscience.com