Tweeter
From Wikipedia, the free encyclopedia
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 reach up to 30-35kHz.
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[edit] Operation
Nearly all tweeters are driven by a voice coil which is suspended within a fixed magnetic field. Tweeters operate by applying current from an amplifier to a coil. The electrified voice coil induces a magnetic force which works against the fixed magnetic field. As current from the amplifier changes, the coil moves in and out. 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 very different from older tweeters, which were usually small versions of woofers. As tweeter technology advanced, designers realized the cone shape and weight was less than ideal given the forces at work. 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 all about trade-offs. There are many challenges in tweeter manufacture such as stopping the dome cleanly at each end of the in/out cycle, loudness which requires the dome to move farther in and out, and ringing when the dome moves faster than it can handle. There are challenges with keeping the dome centered as it moves and a maintaining a stable electrical environment for the amplifier. The list of tweeter design challenges goes on and on. Tweeter design is about effectively converting an electrical amplifier signal to mechanical air movement with nothing added or subtracted.
All tweeters have electrical/mechanical properties that dictate the crossover components for a given finished loudspeaker. There are no standards or commonalities to tweeters, so a given tweeter may work well in one application and not in another. It is important to know and understand these Thiele/Small parameters in order to build a satisfactory loudspeaker. In short, this means a tweeters may not sound right if installed with crossovers designed for another tweeter.
[edit] Dome Materials
All dome materials have advantages and disadvantages. The three properties designers look for in domes are light weight, stiffness and lack of coloration/ringing. Exotic materials like titanium and berylium are light and stiff but can have ringing problems. Material like silk will ring less but can be heavier and not as stiff. It is all about trade offs. 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 alone can go a long way toward selling speakers, so sellers often use the most exotic dome materials and dress up tweeters with fancy lenses or covers - often without regard for other complexities. Some dome tweeters will have exotic designs such as putting a fixed wave guide (also known as a phase plug) in the middle of the dome. The theory behind this is it now gives the tweeter material two points of suspension for less cone break up or distortion and more detail.
The best tweeter is the one that works best in your application. A common misunderstanding is that spending more money on tweeters will produce better sounding speakers. There are many very good speakers which use inexpensive tweeters. Generally, the more exotic and expensive the tweeter, the harder it is to work into a good speaker. In the right hands, an expensive or exotic tweeter will produce a better speaker, but tuning can get tricky and is not for the amateur.
Bigger is not always better for tweeters. The smaller the tweeter is, the better it will disperse sound at the higher frequencies, however the worse the lower frequency performance and power handling will be. Conversely, the larger the tweeter is, the more poorly they will disperse sound at upper frequencies, but the better they will handle power and lower frequency performance.
[edit] Ferro-Fluid
Ferro-fluid is an oil mixture with very finely ground iron in it. Many tweeters have Ferro-fluid between the coil and magnet to help cool the coil and reduce unwanted vibration.
[edit] Professional sound applications
Tweeters designed for public address (PA) and instrument applications are similar in makeup to home audio tweeters. They are not referred to as tweeters, but "high frequency drivers" instead. Key design variances are: cabinets are built for regular shipping and handling, tweeters often have horns attached to allow for higher sound levels and greater control of sound dispersion, voice coils are more robust to withstand higher voltages. High frequency drivers in horns are usually referred to as "compression drivers." Generally, a home tweeter used in a PA/instrument application will fail in short order. A PA/instrument tweeter used in a home application will not have as much detail.
[edit] Fixing dented tweeters
Tweeters have a tendency to get poked and dented. A metal domed tweeter is pretty much 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/voicecoil 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.
[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 are:
- The cone is very light and thin so that it can move rapidly.
- The suspension (or spider) is less compliant to suppress movement at lower frequencies.
- The voice coil is made of thin wire to reduce weight and inductance.
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, clock radios, boom boxes.
[edit] Dome tweeter
A dome tweeter is constructed by coupling 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 with a rigid suspension. This tweeter typically does not have a frame or basket, but a simple front plate attached to the magnet assembly.
[edit] Piezo tweeter
A piezo (or piezo-electric) tweeter contains a piezo-electric crystal as the transducer. It operates by the flexing of the crystal in response to the voltage applied across the crystal's surfaces. The flexation is the mechanical analog of the electrical signal impressed upon the crystal and thus the crystal converts electrical energy into mechanical (and hence acoustic) energy. The piezo tweeter does not use magnetism to operate and does not contain a magnet. They are very cheap and very hard to destroy because of the crystal construction, however they do not have the sound quality capabilities of more conventional tweeter designs.
[edit] Horn tweeter
A horn tweeter is any of the above tweeters coupled to a flare or horn. The component is usually called a compression driver and should be specifically be designed for horn use. The horn improves the off-axis response of the tweeter by aiding in greater dispersion (reducing directivity) of the tweeter. It also improves 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 in frequency the horn will have control of the sound. There are different types of horns, including radial and Constant Directivity (CD). Some will argue that although horns are efficient, they add a coloration and very forward "in your face" sound.
[edit] Ribbon tweeter
A Ribbon Tweeter uses a piece of very thin diaphragm made by i.e. kapton which support a planar coil frequently made by aluminium deposit, suspended in a powerful magnetic field (typically Neodimium magnets) to reproduce high frequencies. The development of ribbon tweeters has followed the development of ribbon microphones. High power versions of ribbon tweeters are becoming common in large scale sound reinforcement line array systems, that can serve audiences of thousands. This is because the ribbon tweeter exhibits specific directional properties, with very wide horizontal dispersion (coverage) and very tight vertical dispersion. These properties mean that the devices can be stacked vertically, making a high frequency line array that will project high sound pressure levels much further than conventional tweeters.
Some manufacturers have opted to make wave guides that channel the hemispherical output of a regular tweeter into a more focused beam, approximating a ribbon tweeter. However this approach doesn't yield the same output as a true ribbon, which will almost always have a better high frequency response than a standard tweeter. This is because the entire ribbon is immersed in its magnetic field, whereas a standard dome or cone tweeter has only its voice coil coupled to its magnetic field. This leads to failure of the dome or cone to faithfully track voice coil movement, creating phase distortion called breakup. Ribbons will function at higher frequencies than conventional circular tweeters, without breakup modes (distortion).
[edit] Planar-magnetic tweeter
Some companies will use a planar-magnetic tweeter, or a quasi-ribbon. While not as advanced as a true ribbon, planar magnetic tweeters are generally cheaper. Usually a thin piece of mylar or plastic is stretched that has a voice coil wire running numerous times vertically on the material between bar magnets. This uses a concept similar to the ribbon and electrostatic tweeter, except there is no voltage needed like an electrostatic.
[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 front and back element with holes in it on which positive and negative electrostatic charges have been applied. The output of the amp goes to a thin piece of metallized mylar which then pulsates with the music. Electrostatics have no even-order harmonic distortion because of their push-pull design. They also have no phase distortion. The design has endured for decades, but because of high cost and inefficiency is not widely used.
[edit] AMT tweeter
An Air Motion Transformer tweeter runs by pushing air out perpendicularly to the mounted position. The shape of its driver is a folded pleat of mylar around aluminium struts held in a strong magnetic field. The AMT tweeter is seldom used in modern loudspeaker design, Precide of Switzerland being the main advocates of its continued use.
