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 mechanical stress. The word '' ...

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'' b ...

by using a

piezoelectric crystal 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 mechanical stress. The word ''p ...

, a material that changes its shape when

electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as describ ...

is applied to it. It is usually designed to plug into the ear canal of the user.

Operation

A crystal earpiece typically consists of a piezoelectric crystal with

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

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 ...

diaphragm, enclosed in a plastic case. The piezoelectric material used in early crystal earphones was

Rochelle salt Potassium sodium tartrate tetrahydrate, also known as Rochelle salt, is a double salt of tartaric acid first prepared (in about 1675) by an apothecary, Pierre Seignette, of La Rochelle, France. Potassium sodium tartrate and monopotassium phospha ...

, but modern earphones use

barium titanate Barium titanate (BTO) is an inorganic compound with chemical formula BaTiO3. Barium titanate appears white as a powder and is transparent when prepared as large crystals. It is a ferroelectric, pyroelectric, and piezoelectric ceramic material ...

, or less often

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

. When the audio signal 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 The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...

, creating sound waves. The plastic earpiece casing confines the sound waves and conducts them efficiently into the ear canal, to the eardrum. 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 or monophonic sound reproduction (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 reproduc ...

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

transistor radio A transistor radio is a small portable radio receiver that uses transistor-based circuitry. Following the invention of the transistor in 1947—which revolutionized the field of consumer electronics by introducing small but powerful, convenient ...

s 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 ...

s. They are not used with modern portable media players due to unacceptable sound quality. The main causes of poor performance with these earpieces are low diaphragm excursion, nonlinearity, in-band

resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied Periodic function, periodic force (or a Fourier analysis, Fourier component of it) is equal or close to a natural frequency of the system ...

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 coil A magnetic cartridge, more commonly called a phonograph cartridge or phono cartridge or (colloquially) a pickup, is an electromechanical transducer that is used to play records on a turntable. The cartridge contains a removable or permanently ...

s 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 publ ...

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