Tweeter
A tweeter or treble speaker is a special type of loudspeaker that is designed to produce high audio frequencies, typically from 2,000 to 20,000 Hz. 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.
Operation
Nearly all tweeters are electrodynamic drivers using a voice coil suspended within a fixed magnetic field. These designs operate by applying current from the output of an amplifier circuit to a coil of wire called a voice coil. The voice coil produces a varying magnetic field, which works against the fixed magnetic field of a permanent magnet around which the cylindrical voice coil is suspended, forcing the voice coil and the diaphragm attached to it to move. This mechanical movement 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 is heard 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 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 tradeoffs. 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, allowing 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. Tweeters contribute to a well-balanced and rich audio experience by focusing on the higher sound frequencies.
Tweeters can also work in collaboration with the woofers that are responsible for generating the low frequencies or bass.
Some tweeters sit outside the main enclosure in their own semi-independent unit, such as "super tweeters". The latter plugs in and swivels to adjust the soundfield depending on listener position and user preference. The separation from the baffle is considered to be optimal under the theory that the smallest baffle possible is optimal for tweeters.
Range
Most tweeters are designed to reproduce frequencies up to the formally defined upper limit of the human hearing range ; some operate at frequencies up to approximately in between 2 kHz to 20 kHz. Tweeters with a greater upper range have been designed for psychoacoustic testing, for extended-range digital audio such as Super Audio CD intended for audiophiles, for biologists performing research on animal response to sounds, and for ambient sound systems in zoos. Ribbon tweeters have been made that can reproduce 80 kHz and even 100 kHz.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.
Ferrofluid
is a suspension of very small 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.Professional sound applications
Tweeters designed for sound reinforcement and musical instrument applications are broadly similar to high fidelity tweeters, though they are usually not referred to as tweeters, but rather 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 from 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 112 dB/watt/meter. This is substantially more efficient than other tweeter construction.
Types of tweeters
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, or good damping properties or both;
- a suspension that is stiffer than for other drivers—less flexibility is needed for high frequency reproduction;
- small voice coils and light wire, which also helps the tweeter cone move rapidly.
Typical of the 1960s/1970s-era was the CTS "phenolic ring" cone tweeters, exhibiting flat response from 2,000 to 15,000 Hz, low distortion and fast transient response. The CTS "phenolic ring" tweeter gets its name from the orange-colored edge suspension ring that it has which is made from phenolic. It was used in many makes and models of well-regarded vintage speakers, and was a mid-priced unit.
Cone tweeters have a narrower dispersion characteristic that is the same as a cone woofer's. Many designers therefore believed this made them a good match to cone midranges and woofers, allowing for superb stereo imaging. However, the "sweet spot" created by the narrow dispersion of cone tweeters is small. Speakers with cone tweeters offered the best stereo imaging when positioned in the room's corners, a common practice in the 1950s, 1960s and early 1970s.
During the 1970s and 1980s, the widespread introduction of higher quality audiophile discs and the advent of the CD caused the cone tweeter to fall out of popularity because cone tweeters seldom extend higher than 15 kHz. Audiophiles felt that cone tweeters lacked the "airiness" of dome tweeters or other types. Nevertheless, many high-end cone tweeters remained in limited production by Audax, JBL and SEAS until the mid-1980s.
Cone tweeters are now rarely used in modern hi-fi usage and are routinely seen in low cost applications such as factory car speakers, compact stereo systems, and boom boxes. Some boutique speaker manufacturers recently have returned to high-end cone tweeters, especially recreations of CTS phenolic ring models, to create a vintage-sounding product.
Dome tweeter
A dome tweeter is constructed by attaching a voice coil to a dome, 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, through 38 mm. The overwhelming majority of dome tweeters presently used in hi-fi speakers are 25 mm 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.