Tweeter
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A tweeter is a loudspeaker designed to produce high frequencies, typically from around 2,000 Hz to 20,000 Hz (generally considered to be the upper limit of human hearing). Some tweeters can manage response up to 45 kHz. The name is derived from the high pitched sounds made by some birds, especially in contrast to the low woofs made by many dogs, after which low-frequency drivers are named (woofers).
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[edit] Operation
Nearly all tweeters are electrodynamic drivers, using a voice coil suspended within a fixed magnetic field. These designs operate by applying current from an amplifier to a coil. The electrified voice coil produces a varying magnetic field which works against the fixed magnetic field, forcing the voice coil—and the diaphragm attached to it—to move. Since the coil is attached to a diaphragm, its motions become those of the diaphragm creating air motions which we hear as high sounds.
Modern tweeters are typically different from older tweeters, which were usually small versions of woofers. As tweeter technology has advanced, different design applications have popularized. Many soft dome tweeter diaphragms are thermoformed from polyester film, or silk or polyester fabric which have been impregnated with a polymer resin. Hard dome tweeters commonly employ 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.
[edit] Dome materials
All dome materials have advantages and disadvantages. Three properties designers look for in domes are low mass, high stiffness and good damping. Celestion were the first manufacturers to fabricate dome tweeters out of a metal, copper. Nowadays other metals such as aluminium, titanium, magnesium, and beryllium, 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 diamond, are also being used for their extreme stiffness. Polyethylene terephthalate film and woven silk 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.
[edit] Ferrofluid
Ferrofluid is a suspension of very small (typically 10 nm) iron oxide magnetic particles in a very low volatility liquid, typically a synthetic oil. A wide range of 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.
[edit] 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 drivers" 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, polyimide and PET film, each having its own characteristics. The diaphragm is glued to a voice coil former, typically made from a different material than the dome, since it must cope with heat without tearing or significant dimensional change. Polyimide film, Nomex, 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.
[edit] Types of tweeters
[edit] Cone tweeter
Cone tweeters have the same basic design and form as a woofer 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 stereo 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.
[edit] 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.
[edit] Piezo tweeter
A piezo (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 electrostriction. In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate (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.
[edit] 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 aluminum vapor, suspended in a powerful magnetic field (typically provided by neodymium magnets) to reproduce high frequencies. The development of ribbon tweeters has more or less followed the development of ribbon microphones. 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 further away from the speaker locations than do conventional tweeters.
Early ribbons electromagnetically pushed outward with the musical waveform, and used their natural elasticity as a restorative or return force. They produced high levels of distortion (up to 30%) as a result. Later designs utilized iron ferrite, and later neodymium magnets on both sides of the diaphragm resulting in a push-pull design. Push-pull ribbons are typically far more accurate than single-ended or "push" ribbons; they usually have higher power handling capacities as well, since diaphragm motion was far more tightly controlled.
[edit] 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 film 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. The concept is most similar to that of electrostatic tweeters, with the advantage that there is no DC voltage field needed as in electrostatics, nor arcing, nor dust attraction.
[edit] Electrostatic tweeter
An electrostatic tweeter operates on the same principles as a full-range electrostatic speaker or a pair of electrostatic headphones. This type of speaker employs a thin diaphragm (generally plastic and typically PET 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 transformer 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.
[edit] AMT tweeter
The Air Motion Transformer 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 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.
[edit] Horn tweeter
A horn tweeter is any of the above tweeters coupled to a flared or horn 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 than more common types of tweeters (see above). Properly used, a horn improves the off-axis response of the tweeter by controlling (ie, reducing 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.
[edit] 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 plasma 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 ozone, 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 speaker and plasma arc loudspeaker.
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-Voice 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 Plasmatronics 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.
[edit] Repair
Some tweeters are prone to damage, and their repair is part of the work of repair shops and maintenance crews.
Dome tweeters are often little protected in domestic speaker cabinets, and are vulnerable to dome denting. Whether a dented dome works acceptably or not depends on whether the distortion makes the voice coil out of round. Domes are undented by various methods, including:
- vacuum cleaner nozzle
- sticky tape
- bent pin
- removal & refit of the dome assembly, enabling access to the rear of the dome
Paper cone tweeters are sometimes prone to tearing of the paper cone. However these are usually old tweeters with acceptable but uninspired performance, and low value, and repair is usually considered not worthwhile. Cones are sometimes repaired with a small piece of plasticised paper (e.g., vinyl record lining paper) and a flexible glue, though this adds weight and thus affects high frequency performance. Glue alone adds less weight but is more prone to failure.
Electrostatic tweeters can suffer holing of the membrane due to arcing. Whole membranes are replaced if in poor condition, but the membrane resistance requires matching for proper performance. Either OEM film is used, or charcoal is applied to bare plastic film and polished off to reach the required resistance.
Horn tweeters occasionally need debris removed. It is either fished out with a hook or the horn is removed.
Tweeter voice coils are not often rewound, as tweeters are usually not high price items.
[edit] References
[edit] See also
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