A crystal earpiece is a type of piezoelectric earphone, producing sound using a piezoelectric crystal, a material that changes shape when electricity is applied to it. It is usually designed to plug into the user’s ear canal.
A crystal earpiece typically consists of a piezoelectric crystal with metal electrodes attached to each side, glued to a conical plastic diaphragm or sheet of metal, enclosed in a plastic housing. The piezoelectric material used in the earliest crystal earphones was Rochelle salt, but modern headphones use barium titanate, or less often quartz. When the audio signal is applied to the electrodes, the crystal leans a little with the signal, causing the diaphragm to vibrate. The diaphragm pushes on the air, creating sound waves. The plastic earpiece envelope confines sound waves and conducts them efficiently through the ear canal to the eardrum. The diaphragm is usually attached to its outer edge, relying on bending to function. The path of air in the earpiece is usually horn-shaped, with a narrowed air column that increases the air movement around the eardrum, increasing the volume.
Crystal earphones are generally monaural devices with very low audio fidelity, but high sensitivity and impedance. Their peak use was probably with 1960s transistors and hearing aids. They are not used with modern portable media players due to unacceptable sound quality. The main causes of poor performance of these earpieces are the small excursion of the diaphragm, the non-linearity, the resonance in the band and the very short form of the earpiece of the earpiece. The resulting sound is very metallic and lacks bass. Modern helmets use electromagnetic drivers that function in the same way as loudspeakers, with moving coils or iron cores moving in a magnetic field. One remaining use for crystal earphones is in crystal radios. Their very high sensitivity enables them to use the very weak signals produced by the crystalline radios, and their high impedance (of the order of 20 kilohms) is a good match for the typical crystal radio. They have also been used as microphones, with their high efficiency requiring less amplification.