Tweeter
From Wikipedia, the free encyclopedia
A tweeter is a driver 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 reach up to 30 to 35 kHz. the name is derived from the high pitched sounds of some birds.
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[edit] Operation
Nearly all tweeters are electrodynamic drivers, using a voice coil suspended within a fixed magnetic field. Tweeters 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. The coil is attached to a diaphragm, so the motion of the coil creates motion in the air which we hear as sound. Additional functional diagrams and details are listed under Loudspeaker.
Modern tweeters are typically different from older tweeters, which were usually small versions of woofers. As tweeter technology has advanced, designers have come to realize that woofer/midrange style cone shape and density were less than ideal. 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; 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.
All tweeters have electrical/mechanical properties that influence or even force particular crossover choices in a loudspeaker system using that tweeter. There are no standards or commonalities for tweeters, so a given tweeter may work well in one application and not at all well in another. In short, this means that one tweeter is not likely to sound right if installed with a crossover suited to another tweeter. It is important for designers to know and understand these parameters in order to build a satisfactory high frequency section in a loudspeaker system.
[edit] Dome materials
All dome materials have advantages and disadvantages. The four properties designers look for in domes are light weight, stiffness, rapid damping, and lack of coloration/ringing. Exotic materials like titanium and beryllium are light and stiff but can have ringing problems. Materials like silk will ring less but is usually heavier for similar size and shape domes, and are not as stiff. But there are good tweeters and bad tweeters made with all types of dome materials. While there is a lot more to driver construction than just dome material, dome material choice is often an important selling point, so designers often use the most exotic dome materials, add assorted additional features (eg, lenses or phasing plugs), too often without regard for other complexities. Some dome tweeters have exotic designs such as putting a fixed wave guide (also known as a phase plug) over the middle of the dome, attempting to reduce directivity at high frequencies, at least in part.
A common misunderstanding is that spending more money for a tweeter will produce better sounding speaker systems. There are many very good speakers which use inexpensive tweeters, and some expensive tweeters in loudspeakers not renowned for quality reproduction. In the right hands, an expensive or exotic tweeter may produce a better speaker, but doing so effectively is often difficult. And, in any case, some expensive tweeters do not sound good to most listeners.
Bigger is not always better for tweeters. In general, the smaller the tweeter, the better it will disperse sound at the higher frequencies. However, the smaller the tweeter, the worse the lower frequency performance and power handling will be. Conversely, the larger the tweeter is, the more poorly it will disperse sound at upper frequencies, but the better it will handle power and lower frequency performance. No tweeter design is well suited to all uses.
[edit] Ferro-fluid
Ferro-fluid is a suspension of very small (typically 10nm) magnetite 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 home audio tweeters. They are usually not referred to as tweeters, but as "high frequency drivers". Key design variances 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 coupling of the driver diaphragm to the horn throat. Generally, a home tweeter used in a PA/instrument application will fail in short order, often from voice coil burnout.
[edit] Fixing dented tweeters
Tweeters have a tendency to get poked and dented. A metal domed tweeter is generally ruined once it is dented, but a soft domed tweeter can be repaired.
- Method 1: Gently stick some masking tape to it and pull it out.
- Method 2: Use a vacuum cleaner and hold your fingers between the pipe and the tweeter so the vacuum never fully engages the cap. Be careful as some vacuums can suck the dome right off. This is very tricky.
- Method 3: Buy two new voice coils from a tweeter parts seller. Voice coil replacements come with domes. You will have to know the part number of the tweeter and the tweeter will have to be from a common stock. Buy two because you should replace both at the same time.
- Method 4: Make a tiny pin hole in the dome and bend the pin so you can pull the dent out from the inside.
- Method 5: Remove the tweeter from the speaker and blow air into the back vent hole (on the back in the center of the magnet, if it has one).
- Method 6: Remove the dome/voice coil assembly from the magnet assembly, where this is possible, then push dome out from rear
- Method 7: "Kiss" the tweeter, sealing around the dome with the lips, then sucking.
- Method 8: Press down onto the tweeter until it pops out.
[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 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.
A variation is the ring radiator in which the 'suspension' of the cone or dome becomes the major radiating element. These tweeters have very different directivity characteristics than do standard dome or cone tweeters.
[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.
[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.
[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.
[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 exhibit useful directional properties, with very wide horizontal dispersion (coverage) and very tight vertical dispersion. These drivers can 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.
Some manufacturers have opted to make wave guides that constrain the hemispherical output of an ordinary tweeter into a more focused beam, approximating a ribbon tweeter. However this approach doesn't produce the same output as a true ribbon, which will almost always have a better high frequency response than a standard cone type tweeter. This is because the entire ribbon is immersed in the magnetic field, whereas a standard dome or cone tweeter has only its voice coil coupled to the magnetic field. This leads to an inability of the dome or cone to track the signal input in the same way as the ribbon, creating different distortions. Ribbons will function at higher frequencies than conventional circular tweeters, without their breakup modes (ie, cone or dome distortion).
[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 or 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 are supplied with a high voltage power supply to provide the high voltage. 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 disphragm 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, 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 being one of the chief advocates. 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] Plasma 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 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. They 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. 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. Currently there appears to be only one manufacturer still making and using them, in Germany. They 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, these 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.
There is at least one German DIY 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.
