A crystal earpiece is a type of

piezoelectric Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied stress (mechanics), mechanical s ...

earphone, producing

sound In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...

by using a piezoelectric crystal, a material that changes its shape when

electricity Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

is applied to it. It is usually designed to plug into the

ear canal The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the auricle to the eardrum and is about in length and in diameter. S ...

of the user.

Operation

A crystal earpiece typically consists of a piezoelectric crystal with

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

electrodes attached to either side, glued to a conical plastic or metal

foil Foil may refer to: Materials * Foil (metal), a quite thin sheet of metal, usually manufactured with a rolling mill machine * Metal leaf, a very thin sheet of decorative metal * Aluminium foil, a type of wrapping for food * Tin foil, metal foil ma ...

diaphragm, enclosed in a plastic case. The piezoelectric material used in early crystal earphones was Rochelle salt, but modern earphones use barium titanate, or less often

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

. When the

audio signal An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals or a series of binary numbers for Digital signal (signal processing), digital signals. Audio signals have frequencies i ...

is applied to the electrodes, the crystal bends back and forth a little with the signal, vibrating the diaphragm. The diaphragm pushes on the air, creating sound waves. The plastic earpiece casing confines the sound waves and conducts them efficiently into the ear canal, to the

eardrum In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit changes in pres ...

. The diaphragm is generally fixed at its outer edge, relying on bending to operate. The air path in the earpiece is generally a horn shape, with a narrowing column of air which increases the air displacement at the eardrum, increasing the volume.

Application

Crystal earpieces are usually

monaural Monaural sound or monophonic sound (often shortened to mono) is sound intended to be heard as if it were emanating from one position. This contrasts with stereophonic sound or ''stereo'', which uses two separate audio channels to reproduce so ...

devices with very low sound fidelity, but high sensitivity and impedance. Their peak use was probably with 1960s era transistor radios and

hearing aid A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers ...

s. They are not used with modern

portable media player A portable media player (PMP) or digital audio player (DAP) is a portable consumer electronics device capable of storing and playing digital media such as audio, images, and video files. Normally they refer to small, Electric battery, batter ...

s due to unacceptable sound quality. The main causes of poor performance with these earpieces are low diaphragm excursion, nonlinearity, in-band

resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...

and the very short horn shape of the earpiece casing. The resulting sound is very tinny and lacking in bass. Modern headphones use electromagnetic drivers that work similarly to speakers, with moving coils or moving iron cores in a magnetic field. One remaining use for crystal earpieces is in crystal radios. Their very high sensitivity enables them to use the very weak signals produced by crystal radios, and their high impedance (on the order of 20 kilohms) is a good match for the typical crystal radio. They have also been used as

microphone A microphone, colloquially called a mic (), or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publi ...

s, with their high output requiring less amplification. Crystal earpieces can also be used as rudimentary, low voltage, audio circuit troubleshooting tools; it is sufficient to touch the tip of the earpiece's audio connector on a point of interest while simultaneously touching the other (sleeve) connection with one's finger. The high impedance of the earpiece means that any audio-range signal applied to the tip of the connector will be heard in the earpiece. This quick-and-dirty technique can remove the need for setting up and probing with an oscilloscope or connecting an amplifier to the test point in the first instance.

References

Headphones Audio electronics {{Electronics-stub