Tweeter
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A tweeter is a loudspeaker designed to produce high frequencies, typically from around 2,000 hertz to 20,000 hertz (20,000 Hz is generally considered to be the upper limit of the human ear). Some tweeters can manage effectively flat response up to an octave or more higher (30 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. Today, most tweeters are dome shaped and made of a vibration damping material such as silk, or an extremely light and rigid material such as titanium.
Tweeter design is intended to effectively convert an electrical amplifier signal to mechanical air movement with nothing added or subtracted(sometimes referred to as transparency.) The problem is difficult, and real-world tweeter design involves trade-offs. There are many challenges in tweeter design and manufacture such as stopping the dome's motion cleanly at each end of the in/out cycle, properly handling high level signals which require the dome to move farther in and out, and ringing in which stored energy is radiated after the drive signal stops. There are also challenges with keeping the dome centered as it moves, handling large amounts of power in a small voice coil, and with maintaining a stable electrical environment for the amplifier.
[edit] Dome materials
All dome materials have advantages and disadvantages. Three properties designers look for in domes are low mass, stiffness and damping. Exotic materials like titanium and beryllium are light and stiff but can have ringing problems due to a lack of damping. Materials like silk will ring less but are usually heavier for similar size and shape domes, and are not as stiff.
In general, the smaller the tweeter, the better it will disperse sound at the highest frequencies. However, the smaller the tweeter, the worse the lower frequency performance and power handling will be. As always in loudspeaker design, there are inevitable tradeoffs with design technologies.
[edit] Ferro-fluid
Ferro-fluid is a suspension of very small (typically 10 nm) magnetic particles in a very low volatility liquid (often a fluid silicone preparation). There are multiple viscosity and magnetic property variants. Many tweeters have ferro-fluid in their magnetic gaps to help conduct heat away from the coil, conduct the magnetic flux to the voice coil, and to damp unwanted vibrations, all while helping center the voice coil in the gap to prevent binding.
[edit] Professional sound applications
Tweeters designed for public address (PA) and instrument applications are broadly similar to high fidelity (home audio and studio) 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, kapton and mylar, each having its own audible characteristics. The diaphragm is glued to its voice coil former which is often made from a different material than the dome itself since it must cope with heat without significant dimensional change. Kapton is a very popular material for this application. The suspension is usually a continuation of the diaphragm and is glued to a plastic circular frame in turn fastened to the magnet, often with machine screws. The diaphragm is generally shaped like an inverted dome and loads into a series of tapered concentric rings called a 'phase plug', which matches the acoustic loading across the surface of the diaphragm, preventing phase interferences (cancellations, even comb filtering) between different points on the diaphragm surface. The phase plug exits into a slightly tapered tube through the magnet which typically provides some acoustic gain before itself loading into the horn throat proper. The horn flare controls the coverage pattern, or directivity, and also adds 6dB or more of amplitude (or gain). A modern well designed horn and compression driver combination has an output sensitivity of between 105 and 118dB/watt/meter. This is substantially more efficient (and less thermally dangerous to a small voice coil and former) than any 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 (eg, ceramic cones in one manufacturer's line), or good damping properties (eg, silk or coated fabric) or both.
- the suspension (or spider) is less compliant than for other drivers because it's not 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 sign in modern hifi usage outside of Bose's 201 and 301 loudspeaker models.
[edit] Dome tweeter
A dome tweeter is constructed by attaching a voice coil to a dome (made of coated cloth, 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. Typical sizes are 1 inch or perhaps 1.5 inches. There are even dome tweeters with 3/4 inch voice coils.
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 (and hence acoustic) energy. While piezoelectric tweeters are relatively cheap, and rugged when compared to typical voice coil tweeters, most are not capable of the same level of linearity and accuracy of reproduction compared to high quality conventional tweeter designs. Piezoelectric tweeters are most commonly found in inexpensive stereo and public address speakers, where cost and reliability are more important than accuracy. In some high-end stereo speakers, most notably the Dahlquist DQ-10, piezoelectric tweeters have been used as supertweeters, to reproduce frequencies beyond the limit of most dynamic speakers, although this is not a common practice.
In many cases in lower cost equipment, typically portable stereos (boomboxes), flat disc piezo tweeters are fitted that have an audio output that is too small to hear. The devices are fitted to enable claims of 2 or 3 way speaker systems. That the output is inaudible is probably a good thing, since these are resonant discs unsuitable for use as genuine tweeters.
[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 Mylar 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 Mylar), 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 Mylar or equivalent) around aluminium struts held in a strong magnetic field. The AMT tweeter is seldom used in modern loudspeaker design, Precide of Switzerland and ADAM of Germany being prominent vendors currently. 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, Dr Oscar Heil. They are capable of considerable output levels and are rather more sturdy than electostatics or ribbons, but have similar low mass moving elements.
[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, or to more tightly couple the tweeter diaphragm to the air for higher efficiency, or both. The tweeter in either case is usually called a compression driver and is quite different than more common types of tweeters. 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 the tweeter can work as the horn will provide coupling to the air at lower frequencies. There are different types of horns, including radial and Constant Directivity (CD). Horn tweeters are often said to have a somewhat 'different' sound than non horn loaded tweeters; some regard this as better, others as worse. Horn designers note that poorly designed horns, or improperly crossed over horns, have predictable problems in the accuracy of their output. Many companies have horn loaded tweeters, but avoided using the term. The names include "elliptical aperture" "Semi-horn" and "Directivity controlled" They are nonetheless, a form of horn-loading.
[edit] Plasma/Ion tweeter
Because ionized gas is electrically charged and so can be manipulated by a variable electrical field, it's 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, 1980's German made Magnat "magnasphere" speakers were banned from import to the USA. 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 1980's, the Plasmatronic 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.
Currently there appears to be only two manufacturers still making and selling plasma tweeters, Acapella Audio Arts, in Germany[1][2] and LANSCHE AUDIO in Germany[3] This tweeter is extremely expensive, and has been favorably reviewed.[4][5][6] Note that there is at least one German DIY Audio plasma tweeter design which claims maximum sound pressure levels in excess of 115 dB, and without special machined cells, or horns, or dedicated gas tanks.[7]
[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
- kiss and suck
- 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 (eg 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. Its 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
- ^ Acapella ION TW 1S
- ^ Welcome
- ^ LANSCHE AUDIO - manufacturing for finest High End and CORONA plasmatweeter
- ^ The Acapella Audio Arts Campanile Loudspeaker
- ^ SoundStage! Network Ultra Audio - Archived Article
- ^ Acapella Audio Arts Violin Loudspeakers (High Version) by Tony Maresch
- ^ Ulrich Haumann's DIY PLASMA TWEETER
- ^ Diego Barone - Plasma Tweeter
[edit] See also
