A
tweeter is a
loudspeakerA loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
designed to produce high audio frequencies, typically from around 2,000
HzThe hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
to 20,000 Hz (generally considered to be the upper limit of human hearing). Some tweeters can manage response up to 65 kHz. The name is derived from the high pitched sounds made by some
birdBirds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, especially in contrast to the low woofs made by many
dogThe domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
s, after which low-frequency drivers are named (
wooferWoofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof"...
s).
Operation
Nearly all tweeters are electrodynamic drivers, using a
voice coilA voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...
suspended within a fixed magnetic field. These designs operate by applying current from the output of an
amplifierGenerally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
circuit to a coil of wire, called a
voice coilA voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...
. The
voice coilA voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...
produces a varying magnetic field, which works against the fixed magnetic field of a magnet around which the cylindrical voice coil is suspended, forcing the voice coil—and the diaphragm attached to it—to move. This mechanical movement exactly resembles the waveform of the electronic signal supplied from the amplifier's output to the voice coil. Since the coil is attached to a diaphragm, the vibratory motion of the voice coil transmits to the diaphragm; the diaphragm in turn vibrates the air — thus creating air motions or audio waves, which we hear as high sounds.
Modern tweeters are typically different from older tweeters, which were usually small versions of
wooferWoofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof"...
s. As tweeter technology has advanced, different design applications have become popular. Many soft dome tweeter diaphragms are thermoformed from polyester film, or silk or polyester fabric that has been impregnated with a polymer resin. Hard dome tweeters are usually made of aluminium, aluminium-magnesium alloys, or titanium.
Tweeters are intended to convert an electrical signal into mechanical air movement with nothing added or subtracted, but the process is imperfect, and real-world tweeters involve trade-offs. Among the challenges in tweeter design and manufacture are: providing adequate damping, to stop the dome's motion rapidly when the signal ends; ensuring suspension linearity, to allow high output at the low end of its frequency range; ensuring freedom from contact with the magnet assembly, keeping the dome centered as it moves; and providing adequate power handling without adding excessive mass.
Dome materials
All dome materials have advantages and disadvantages. Three properties designers look for in domes are low mass, high stiffness and good damping.
CelestionCelestion is a British maker of loudspeakers.-History:The work of what would become Celestion started in Hampton Wick in 1924. Celestion Radio Company and Celestion Limited were formed in 1927, and two years later the company moved across the Thames to Kingston. The company grew rapidly, but was...
were the first manufacturers to fabricate dome tweeters out of a metal,
copperCopper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
. Nowadays other metals such as
aluminiumAluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
,
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
,
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, and
berylliumBeryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
, as well as various alloys thereof, are used, being both light and stiff but having low damping; their resonant modes occur above 20 kHz. More exotic materials, such as
synthetic diamondSynthetic diamond is diamond produced in a technological process; as opposed to natural diamond, which is created in geological processes. Synthetic diamond is also widely known as HPHT diamond or CVD diamond, denoting the production method, High-Pressure High-Temperature synthesis and Chemical...
, are also being used for their extreme stiffness.
Polyethylene terephthalatePolyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...
film and woven
silkSilk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity...
suffer less ringing, but are not nearly as stiff, which can limit their very high frequency output.
In general, smaller dome tweeters provide wider dispersion of sound at the highest frequencies. However, smaller dome tweeters have less radiating area, which limits their output at the lower end of their range; and they have smaller voice coils, which limit their overall power output.
Ferrofluid
FerrofluidA ferrofluid is a liquid which becomes strongly magnetized in the presence of a magnetic field.Ferrofluids are colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid . Each tiny particle is thoroughly coated with a surfactant to inhibit clumping...
is a suspension of very small (typically 10 nm) iron oxide magnetic particles in a very low
volatilityIn chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
liquid, typically a synthetic oil. A wide range of
viscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
and magnetic density variants allow designers to add damping, cooling, or both. Ferrofluid also aids in centering the voice coil in the magnetic gap, reducing distortion. The fluid is typically injected into the magnetic gap and is held in place by the strong magnetic field. If a tweeter has been subjected to elevated power levels, some thickening of the ferrofluid occurs, as a portion of the carrier liquid evaporates. In extreme cases, this can degrade the sound quality and output level of a tweeter, and the fluid must be removed and new fluid installed.
Professional sound applications
Tweeters designed for sound reinforcement and musical instrument applications are broadly similar to high fidelity tweeters, though they're usually not referred to as tweeters, but as "high frequency drivers". Key design requirement differences are: mountings built for repeated shipping and handling, drivers often mounted to horn structures to provide for higher sound levels and greater control of sound dispersion, and more robust voice coils to withstand the higher power levels typically encountered. High frequency drivers in PA horns are often referred to as "
compression driverA compression driver is a type of loudspeaker driver which uses the technique of "compression" to achieve high efficiencies. In this context compression refers to the fact that the area of the loudspeaker diaphragm is significantly larger than the aperture through which the sound is radiated....
s" from the mode of acoustic coupling between the driver diaphragm and the horn throat.
Various materials are used in the construction of compression driver diaphragms including titanium, aluminium, phenolic impregnated fabric,
polyimidePolyimide is a polymer of imide monomers. The structure of imide is as shown. Polyimides have been in mass production since 1955...
and
PET filmBoPET is a polyester film made from stretched polyethylene terephthalate and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aroma barrier properties and electrical insulation.A variety of companies manufacture boPET and other...
, each having its own characteristics. The diaphragm is glued to a voice coil former, typically made from a different material from the dome, since it must cope with heat without tearing or significant dimensional change. Polyimide film,
NomexNomex is a registered trademark for flame resistant meta-aramid material developed in the early 1960s by DuPont and first marketed in 1967.- Properties:...
, and glassfibre are popular for this application. The suspension may be a continuation of the diaphragm and is glued to a mounting ring, which may fit into a groove, over locating pins, or be fastened with machine screws. The diaphragm is generally shaped like an inverted dome and loads into a series of tapered channels in a central structure called a 'phase plug', which equalizes the path length between various areas of the diaphragm and the horn throat, preventing acoustic cancellations between different points on the diaphragm surface. The phase plug exits into a tapered tube, which forms the start of the horn itself. This slowly expanding throat within the driver is continued in the horn flare. The horn flare controls the coverage pattern, or directivity, and as an acoustic transformer, adds gain. A professional horn and compression driver combination has an output sensitivity of between 105 and 112dB/watt/meter. This is substantially more efficient (and less thermally dangerous to a small voice coil and former) than other tweeter construction.
Cone tweeter
Cone tweeters have the same basic design and form as a
wooferWoofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof"...
with optimizations to operate at higher frequencies. The optimizations usually are:
- a very small and light cone so it can move rapidly;
- cone materials chosen for stiffness (e.g., ceramic cones in one manufacturer's line), or good damping properties (e.g., silk or coated fabric) or both;
- the suspension (or spider) is stiffer than for other drivers—less flexibility is needed for high frequency reproduction;
- small voice coils (3/4 inch is typical) and light (thin) wire, which also helps the tweeter cone move rapidly.
Cone tweeters are relatively cheap, but do not have the dispersion characteristics of domes. Thus they are routinely seen in low cost applications such as factory car speakers,
shelf stereothumb|right|200px|A Magnavox 2.0 shelf stereoThe term shelf stereo refers to any home stereo system that is physically suitable for placement on a shelf or other similar location. Shelf stereo systems are becoming more popular as their capabilities increase while size decreases. The price of shelf...
systems, and boom boxes. Cone tweeters can also be found in older stereo hi-fi system speakers designed and manufactured before the advent of the dome tweeter. They are now a rare sight in modern hi-fi usage.
Dome tweeter
A dome tweeter is constructed by attaching a voice coil to a dome (made of woven fabric, thin metal or other suitable material), which is attached to the magnet or the top plate via a low compliance suspension. These tweeters typically do not have a frame or basket, but a simple front plate attached to the magnet assembly. Dome tweeters are categorized by their voice coil diameter, and range from 19 mm (0.75 in), through 38 mm (1.5 in). The overwhelming majority of dome tweeters presently used in hi-fi speakers are 25 mm (1 in) in diameter.
A variation is the ring radiator in which the 'suspension' of the cone or dome becomes the major radiating element. These tweeters have different directivity characteristics when compared to standard dome tweeters.
Piezo tweeter
A
piezoPiezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
(or piezo-electric) tweeter contains a piezoelectric crystal coupled to a mechanical diaphragm. An audio signal is applied to the crystal, which responds by flexing in proportion to the voltage applied across the crystal's surfaces, thus converting electrical energy into mechanical. The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the heart of the transducer as it converts the electrical energy to acoustic energy, and vice versa. The active element is basically a piece of polarized material (i.e. some parts of the molecule are positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces. When an electric field is applied across the material, the polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. This alignment of molecules will cause the material to change dimensions. This phenomenon is known as
electrostrictionElectrostriction is a property of all electrical non-conductors, or dielectrics, that causes them to change their shape under the application of an electric field. - Explanation :...
. In addition, a permanently-polarized material such as
quartzQuartz 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,...
(SiO2) or
barium titanateBarium titanate is the inorganic compound with the chemical formula BaTiO3. Barium titanate is a white powder and transparent as larger crystals...
(BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect.
Ribbon tweeter
A ribbon tweeter uses a very thin diaphragm (often of aluminum, or perhaps metalized plastic film) which supports a planar coil frequently made by deposition of aluminium vapor, suspended in a powerful magnetic field (typically provided by
neodymiumNeodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
magnets) to reproduce high frequencies. The development of ribbon tweeters has more or less followed the development of
ribbon microphoneA ribbon microphone is a type of dynamic microphone that uses a thin aluminum, duraluminum or nanofilm ribbon placed between the poles of a magnet to generate voltages by electromagnetic induction...
s. The ribbon is of very lightweight material and so capable of very high acceleration and extended high frequency response. Ribbons have traditionally been incapable of high output (large magnet gaps leading to poor magnetic coupling is the main reason). But higher power versions of ribbon tweeters are becoming common in large scale sound reinforcement line array systems, which can serve audiences of thousands. They are attractive in these applications since nearly all ribbon tweeters inherently exhibit useful directional properties, with very wide horizontal dispersion (coverage) and very tight vertical dispersion. These drivers can easily be stacked vertically, building a high frequency line array that produces high sound pressure levels much farther away from the speaker locations than do conventional tweeters.
Planar-magnetic tweeter
Some loudspeaker designers use a planar-magnetic tweeter, sometimes called a quasi-ribbon. Planar magnetic tweeters are generally less expensive than true ribbon tweeters, but are not precisely equivalent as a metal foil ribbon is lighter than the diaphragm in a planar magnetic tweeter and the magnetic structures are different. Usually a thin piece of
PET filmBoPET is a polyester film made from stretched polyethylene terephthalate and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aroma barrier properties and electrical insulation.A variety of companies manufacture boPET and other...
or plastic with a voice coil wire running numerous times vertically on the material is used. The magnet structure is less expensive than for ribbon tweeters.
Electrostatic tweeter
An electrostatic tweeter operates on the same principles as a
full-rangeA full-range loudspeaker drive unit is defined as a driver which reproduces as much of the audible frequency range as possible, within the limitations imposed by the physical constraints of a specific design. Frequency range is of these drives is maximized through the use of a whizzer cone and...
electrostatic speakerAn electrostatic loudspeaker is a loudspeaker design in which sound is generated by the force exerted on a membrane suspended in an electrostatic field.-Design and functionality:...
or a pair of electrostatic headphones. This type of speaker employs a thin diaphragm (generally plastic and typically
PETPolyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...
film), with a thin conductive coating, suspended between two screens or perforated metal sheets, referred to as stators.
The output of the driving amplifier is applied to the primary of a step-up
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
with a center-tapped secondary, and a very high voltage—several hundred to several thousand volts—is applied between the center tap of the transformer and the diaphragm. Electrostatics of this type necessarily include a high voltage power supply to provide the high voltage used. The stators are connected to the remaining terminals of the transformer. When an audio signal is applied to the primary of the transformer, the stators are electrically driven 180 degrees out of phase, alternately attracting and repelling the diaphragm.
An uncommon way of driving an electrostatic speaker without a transformer is to connect the plates of a push-pull vacuum tube amplifier directly to the stators, and the high voltage supply between the diaphragm and ground.
Electrostatics have reduced even-order harmonic distortion because of their push-pull design. They also have minimal phase distortion. The design is quite old (the original patents date to the 1930s), but occupies a very small segment of the market because of high costs, low efficiency, large size for full range designs, and fragility.
AMT tweeter
The
Air Motion TransformerThe Air Motion Transformer is a loudspeaker mechanism, or audio transducer, invented by Dr. Oskar Heil. It operates on a different principle than both electrodynamic and electrostatic speaker drivers...
tweeter works by pushing air out perpendicularly from the pleated diaphragm. Its diaphragm is the folded pleats of film (typically PET film) around aluminium struts held in a strong magnetic field. In past decades, ESS of California produced a series of hybrid loudspeakers using such tweeters, along with conventional woofers, referring to them as Heil transducers after their inventor,
Oskar HeilOskar Heil was a German electrical engineer and inventor. He studied physics, chemistry, mathematics, and music at the Georg-August University of Göttingen and was awarded his PhD in 1933, for his work on molecular spectroscopy.-Personal life:At the Georg-August University in Göttingen, Oskar Heil...
. They are capable of considerable output levels and are rather more sturdy than electrostatics or ribbons, but have similar low-mass moving elements.
Most of the current AMT drivers in use today are similar in efficiency and frequency response to the original Oskar Heil designs of the 1970s.
Horn tweeter
A horn tweeter is any of the above tweeters coupled to a flared or
hornA horn is a tapered sound guide designed to provide an acoustic impedance match between a sound source and free air. This has the effect of maximizing the efficiency with which sound waves from the particular source are transferred to the air...
structure. Horns are used for two purposes — to control dispersion, and to couple the tweeter diaphragm to the air for higher efficiency. The tweeter in either case is usually termed a compression driver and is quite different from more common types of tweeters (see above). Properly used, a horn improves the off-axis response of the tweeter by controlling (i.e., reducing) the directivity of the tweeter. It can also improve the efficiency of the tweeter by coupling the relatively high acoustic impedance of the driver to the lower impedance of the air. The larger the horn, the lower the frequencies at which it can work, since large horns provide coupling to the air at lower frequencies. There are different types of horns, including radial and constant directivity (CD). Horn tweeters may have a somewhat 'different' sonic signature than simple dome tweeters. Poorly designed horns, or improperly crossed-over horns, have predictable problems in the accuracy of their output, and the load that they present to the amplifier. Perhaps concerned about the image of poorly designed horns, some manufacturers use horn loaded tweeters, but avoid using the term. Their euphemisms include "elliptical aperture" "Semi-horn" and "Directivity controlled". These are, nonetheless, a form of horn loading.
Plasma or Ion tweeter
Because ionized gas is electrically charged and so can be manipulated by a variable electrical field, it is possible to use a small sphere of
plasmaIn physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
as a tweeter. Such tweeters are called a "plasma" tweeter or "ion" tweeter. They are more complex than other tweeters (plasma generation is not required in other types), but offer the advantage that the moving 'diaphragm' is optimally low mass, and so very responsive to the signal input. These types of tweeters are not capable of high output, nor of other than very high frequency reproduction, and so are usually used at the throat of a horn structure to manage usable output levels. One disadvantage is that the plasma arc typically produces
ozoneOzone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
, a poison gas, in small quantities as a by-product. Because of this, German-made Magnat "magnasphere" speakers were banned from import to the USA in the 1980s. See also
plasma speakerPlasma speakers are a form of loudspeaker which varies air pressure via a high-energy electrical plasma instead of a solid diaphram. Connected to the output of an audio amplifier, plasma speakers vary the size of a plasma glow discharge, corona discharge or electric arc which then acts as a...
.
In the past, the dominant supplier was DuKane near St Louis in the US, who made the Ionovac; also sold in a UK variant as the Ionophane.
Electro-VoiceElectro-Voice is a manufacturer of audio equipment, including microphones, amplifiers, and loudspeakers. A subdivision of Telex Communications Inc., Electro-Voice markets its products for use in small or large concert venues, broadcasting, houses of worship, and in retail situations.-History:On...
made a model for a short time under license from DuKane. These early models were finicky and required regular replacement of the cell in which the plasma was generated (the DuKane unit used a precision machined quartz cell). As a result, they were expensive units in comparison to other designs. Those who have heard the Ionovacs report that, in a sensibly designed loudspeaker system, the highs were 'airy' and very detailed, though high output wasn't possible.
In the 1980s, the
PlasmatronicsPlasmatronics was a company, founded by former Air Force Weapons Laboratory scientist Dr. Alan E. Hill, which produced a plasma speaker design...
speaker also used a plasma tweeter, though the manufacturer did not stay in business very long and very few of these complex units were sold.