Crystal earpiece
Encyclopedia
A crystal earpiece, more properly called a piezoelectric earphone, is an earphone 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 radio
s and hearing aid
s. They are not used with modern portable media player
s 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) make them sensitive enough to convert very weak signals.
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
, a material that changes its shape when electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
is applied to it. It is usually designed to plug into the ear canal
Ear canal
The ear canal , is a tube running from the outer ear to the middle ear. The human ear canal extends from the pinna to the eardrum and is about 35 mm in length and 5 to 10 mm in diameter....
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
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
or more commonly barium titanate
Barium titanate
Barium titanate is the inorganic compound with the chemical formula BaTiO3. Barium titanate is a white powder and transparent as larger crystals...
. When the audio signal
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
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
Monaural
Monaural or monophonic sound reproduction is single-channel. Typically there is only one microphone, one loudspeaker, or channels are fed from a common signal path...
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 radio
Transistor radio
A transistor radio is a small portable radio receiver using transistor-based circuitry. Following their development in 1954 they became the most popular electronic communication device in history, with billions manufactured during the 1960s and 1970s...
s and hearing aid
Hearing aid
A hearing aid is an electroacoustic device which typically fits in or behind the wearer's ear, and is designed to amplify and modulate sound for the wearer. Earlier devices, known as "ear trumpets" or "ear horns", were passive funnel-like amplification cones designed to gather sound energy and...
s. They are not used with modern portable media player
Portable media player
A portable media player or digital audio player, is a consumer electronics device that is capable of storing and playing digital media such as audio, images, video, documents, etc. the data is typically stored on a hard drive, microdrive, or flash memory. In contrast, analog portable audio...
s 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
Barium titanate
Barium titanate is the inorganic compound with the chemical formula BaTiO3. Barium titanate is a white powder and transparent as larger crystals...
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) make them sensitive enough to convert very weak signals.