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|Posté le: Mar 22 Avr 2014, 19:20 Sujet du message: SPEAKER DRIVER MATERIALS USED FOR MANUFACTURE
|DOME TWEETERS & MIDRANGE
Aluminium. The majority of metal domes use aluminium diaphragms. Aluminium has an extremely high stiffness to weight ratio, but poor damping characteristics. Some vendors also chemically treat the metal or apply coatings to improve its damping characteristics.
Titanium. Titanium is slightly heavier than aluminium for an equivalent sized dome. It is, however, much stronger than aluminium and has slightly better internal damping. It's also significantly more expensive. As with aluminium, chemical or ceramic surface treatments
e.g. Focal's "Tioxid"and or coating are sometimes used to boost damping.
Fabric Treated . Often silk, but other materials are also in common use, hand-treated fabric is used as the material of choice in some of the most respected tweeters in high-end sound. Fabric domes have high internal damping. Their stiffness to weight ratio is mostly dependent on the material used to treat the fabric.
Beryllium. Used only by TAD-Pioneer in its professional drivers, beryllium is the ultimate metal for use in diaphragms. It stiffness to weight ratio is the highest of any known metal. Although its self damping is not inherently superior, its stiffness usually raises its resonances to well over 50 kHz, obviating the requirement for special surface treatments. Beryllium is also one of the most expensive materials from which to fabricate diaphragms.
Polymide. Polymide is the last of the hard dome materials. It is used in only one recommended driver, at low-cost 3/4" dome tweeter from LPG.
KevlarTM. Although Kevlar is most typically used in woven form for larger coned drivers Focal still makes two Kevlar dome tweeters. For more information of Kevlar, its description is discussed in the information of cone materials which follows this.
Ceramic. Accuton produces tweeters using thin aluminum oxide ceramic diaphragms. Used correctly, ceramic can be an almost perfect material. Although it's not as fragile as many assume, fabricating ceramic diaphragms suitable for use in tweeters is quite tricky. For more information of ceramic diaphragms, its description is discussed in the information of cone materials which follows thi
Ceramic composite. RCM Akustik produces tweeters using a composite of ceramic and some otherwise unidentified material. Most comments about Accuton's ceramics apply to this material as well.
Paper cones still perform amazingly well, proving they're not as low-tech as many assume. Light and stiff, using an almost infinite variety of fibers, paper can be engineered to meet almost any requirement. Treated/ coated paper cones largely address the issue of degradation due to atmospheric conditions. The effectiveness of these treatments varies, depending on the specific technology. If properly done (a big IF), paper remains one of the best possible materials for cones. Some of the better paper technologies include:
Paper - is used by Scan-Speak Vifa Peerless Vifa and most speaker manufactures
Carbon fiber paper -as used by Scan-Speak
CSX - Composite sandwich cone - as used by Peerless
HDS series - Composite 5 layer pressed paper - as used by Peerless
Polyglass - Glass microsphere beads coated paper as used by Focal
Slitted paper - used exclusively by Scan-Speak and their clones
Kevlar paper - currently used primarily by ATI/Dulcet/HiVi, and used in the Shiva subwoofer Kevlar paper appears poised to become a significant technology.
(PP) has become almost as ubiquitous as paper. It's reasonably rigid, has inherently high internal damping, and is totally non-hygroscopic. Without a particularly high stiffness to weight ratio, many PP cones are reinforced with fillers such as mica, talc, carbon black, acrylic, Miraflex (fiberglass), and Kevlar. Unadulterated PP is obvious by its milky white translucent appearance. Generally a very good material combined with very mature technology. PP cones are manufactured either by thermoforming or injection modeling.
Although not currently available in any drivers available to the DIY community, a new form of PP developed by the University of Leeds (UK) called variously Vantex or Comtex is potentially the most significant new technology to come along for quite a while. Researchers developed a process to form a material which consists of aligned PP fibers in a matrix of thermoformed PP. The resulting material therefore consists of approximately 80% molecularly aligned PP fibers in a matrix of 20% PP resulting from melting and cooling the "skins" of the fibers. Currently, only Wilson Benesch is using Vantex/Comtex drivers in its proprietary loudspeakers. Vantex/Comtex is currently being evaluated for automobile body parts due to its strength, low cost, and recyclability. It's also being tested for use in aircraft radomes. Vantex/Comtex appears poised to become a significant PP technology.
has become a popular material for cone designers. Depending on the vendor, you'll find it as part of a high-tech composite, woven and formed, or used as a coating over paper. This suggests a material which is somewhat hard to work with and which still needs some research to figure out the best way to utilize. Still, many of the carbon fiber coned drivers do offer excellent characteristics.
is the fiber used to make body armor for military and police use. Many speaker vendors have incorporated Kevlar into their cones, usually woven and formed, either alone and coated, or as part of a sandwich construction. Many of the same comments for carbon fiber also apply to Kevlar, although I've received many more caveats for Kevlar drivers than for carbon fiber ones.
The problem with Kevlar is similar to that of metal - lightweight and rigid, it tends to ring like a bell! The internal damping of Kevlar is enough to damp some of these high frequency resonances, but the effect can still clearly be heard, as well as seen in the data (especially waterfall plots), requiring the careful use of notch filtering. It's also worth noting that several of the three vendors with the most positive recommendations using Kevlar (Scan-speakFocal, Eton, and Audax) use it as part of a sandwich structure.
cones are produced by a number of manufacturers. Metal cones exhibit the least distortion and coloration in the passband of any cone material, combined with excellent group delay characteristics. The penalty, as noted above, is undamped resonances and severe breakup modes in the upper stopband. Typically used as a woofer material (i.e. well below its resonances), you may still find it necessary to use a HF notch filter to fully tame an aluminum cone.. Whether the advantages of metal cones outweigh the problems is a matter of taste. The DIY'er should definitely think twice about using aluminum in anything other than a woofer or subwoofer application.
cones are a staple of Seas (see Section 3 below) top of the line Excel series. All comments made for aluminum apply to magnesium, only more so. Magnesium is lighter than aluminum, with comparable strength and stiffness. It also has even less internal damping. What is true is that getting good results out of these drivers is beyond the capabilities of many DIY'ers. These should be considered for use by experts only
Sandwich - Composite Cones
Sandwich/composite and/or proprietary materials in common used by Peerless . Many of these are proprietary and apply to only or more manufacturers.
Carbon paper. This technology, used only by Scan-Speak, uses a carbon fiber surface treatment applied to a paper cone. The results are generally excellent making this one of three premier paper technologies currently available.
Carbon fiber reinforced PP. This technology, apparently used only by Versa-Tronics, tries to combine the best characteristics of both materials. The results are mixed. Again, whether this is due to the material technology or the vendor (Versa-Tronics is not usually considered a high-end vendor), is debatable.
Ceramic. Accuton produces a line of proprietary drivers using thin aluminum oxide ceramic diaphragms. Similar technology is also used in several highly regarded high-end systems.
Damped Polymer Composite (DPC). A proprietary Morel technology, DPC is used in their top of the line woofers ad mid/bass drivers. Results have been generally reported as good.
Fiberglass. Some manufacturers have fabricated cones both from woven fiberglass in a matrix of unspecified polymer, as well as fiberglass filled papers. This is only used by a few companies, One company which uses it with excellent results is Seas. On the other hand, another company using fiberglass extensively is Versa-Tronics (which normally isn't considered a high-end vendor) with mixed results. The principle disadvantage of fiberglass is weight.
Expanded foam. Used only by Cabasse, this is an adaptation of the same material used to produce cases for many consumer products. If you've ever seen an older Apple computer, you've probably seen an expanded foam case. The unexpanded foam is injected into a highly polished mold. As it expands, it forms a hard surface wherever it touched the mold, combined with in internal structure of very lightweight foam. This would appear to be an ideal cone technology, but, to date, only Cabasse has managed to use it effectively.
HD-Aerogel (HDA).An Audax proprietary material, HDA combines carbon fiber and Kevlar in a matrix of acrylic. In the past, some of the Audax HDA drivers have been well reported while others have caused problems. As the technology has matured, Audax has been able to achieve a level of performance and consistency that was lacking in their first-generation HDA drivers. The key to this is in Audax's part numbering system where the digit(s) following the material designation ('Z' in the case of HDA) represents the design number. In the past, Audax changed specs without changing the part number. The most recent drivers clearly designate their generation - up to 18 in the case of the 6.5" HDA drivers.
Hexacone. This Eton-proprietary technology combines two layers of woven Kevlar over an inner layer of honeycomb Nomex. Nomex is the same material used to make fire-retardant suits for race car drivers. Eton's Hexacone drivers exhibit properties similar to Kevlar, having vary high stiffness to weight ratio, but with underdamped breakup modes in the stopband.
Neoglass.A Focal proprietary technology, Neoglass consists of a polymer cone (probably PP) with a surface treatment including microscopic glass microspheres. Neoglass drivers can be found in several highly-regarded high-end systems.
Nomex-carbon fiber honeycomb. A proprietary technology, this material is used for a unique line of flat cone woofers
Polykevlar. Another Focal proprietary technology, Polykevlar consists of two outer layers of woven Kevlar over an inner layer of their glass microsphere technology. As with Neoglass, Polykevlar drivers can be found in several highly-regarded high-end systems.
Polyglass.Yet another Focal proprietary technology, Neoglass consists of a paper cone with a surface treatment that includes microscopic glass microspheres. As with the other Focal glass microsphere technologies, Polyglass drivers can be found in several highly-regarded high-end systems. Polyglass is the second of the three premier paper technologies in use today due to its stiffness, light weight, and excellent damping.
Slitted paper. An unusual paper technology which appears on Scan-Speak drivers and used extensively the new revelator series of Scan-Speak bass mids available from WES Components , The cone starts out as a more or less conventional paper cone, then diagonal slits are cut into it. The cone is then glued back together and coated. The result is a cone as light and stiff as paper, but with controlled discontinuities along the glue lines to eliminate internal resonances. This is the third of the three premier paper cone technologies.
TPX. TPX is an unspecified polymer material (a plastic, reportedly polymethylpentene) used by both Audax and Seas. Seas' version specifically mentions PP as well. The presumption is that Seas uses an alloy of PP and TPX.
W sandwich.The "W cone" A honeycomb material is used in most of Focal's top of the line drivers. It's pedigree was as an exclusive material used in Focal/JMlab's high-end Utopia series loudspeakers for several years before being offered to the public. Srtucturally, W cones use a thin layer of foam material sandwiched between outer layers of "woven glass tissue".
XPP. XPP is a Seas proprietary clear plastic used in their latest "T" series drivers. Although quite new, initial reports of this material are highly favorable.
SPEAKER DRIVER SURROUNDS
Where the mouth of the cone is attached to the frame is called the surround. Its purpose is twofold; to provide compliance so that the cone can move, and also to absorb mechanical energy which would otherwise reflect back through the cone to cause resonances which show up as peaks in the frequency response .
Foam surrounds are available using a variety of expanded foam materials. From a driver engineer's viewpoint, foam is almost ideal, extremely compliant with high damping. Unfortunately, foam has one major problem. It tends to rot over time and exposure to atmospheric contaminants. The problem is exacerbated in climates with high heat and humidity or locales with high atmospheric pollution levels. Newer foam materials are claimed to be much better in this regard. Still, if you use a driver with a foam surround, you'd be wise to begin checking it for signs of deterioration after about 5 years, then annually thereafter.
Rubber surrounds are also available in a variety of materials, the most popular being Santoprene and butyl rubber. Not as compliant as foam, rubber also lacks foam's damping potential. But, it also doesn't rot.Rubber
Pleated fabric surrounds are typically seen in high efficiency drivers, usually intended for sound reinforcement or musical instrument use. With such an "accordion" surround, Xmax is typically quite limited. Another characteristic of such drivers is a typically high Fs, limiting their usefulness in hi-fi applications. For these reasons, you will find few, if any, drivers with fabric surrounds in the LDSG. Even in the high efficiency drivers section (Section 7), many of the recommended drivers use more flexible rubber or foam surrounds for greater Xmax.
The frame of a cone-type driver is called the basket or (not surprisingly) the frame. Aside from simply supporting the structure of the driver, the basket design is critical in allowing the back wave from the to cone to escape into the enclosure. Another important characteristic of the basket design is that it not have any mechanical resonances within the driver's passband. Many DIY'ers apply some sort of damping material to their baskets to minimize coloration due to basket resonance. The final consideration is dimensional stability - the basket should be rigid enough to withstand normal operation and handling without deforming. Two types of baskets are in common usage:
Cast aluminum or magnesium baskets are more expensive, but generally regarded as superior. One primary benefit is that the thick cross sections and thermal conductivity of the metal help dissipate VC heat more readily. Resonances are usually (but not always!) above the working range of most cone drivers. Where it especially shines is in its structural integrity and rigidity. Cast materials don't bend. If subjected to enough force, they'll break, but never bend.
Stamped steel baskets are considerably less expensive to produce than cast aluminum baskets. Their physical properties are, however, inferior in almost every way. They tend to have lower frequency resonances and they can be bent during shipment or by careless handling. Another problem with stamped steel baskets is that, since they're fabricated from a single sheet of sheet steel, they can't have the variable thickness which contributes to a cast basket's rigidity and physical openness. A side effect of this is that if you build in sufficient structural support, more of the back wave of the cone will be blocked. For stamped steel baskets done right, also check out Morel's line of woofers and mid/bass drivers, as noted in Section 3.
Polymer composite baskets are relatively new to the world of serious DIY drivers. The cost to produce such reinforced plastic baskets is cheaper even than steel, so the manufacturers have a lot of incentive to make it work. Primarily available now from Audax, the number of these will continue to increase.
HORN COMPRESSION DRIVERS
Horn systems for midrange and tweeter use, separate the driver from the physical horn. The drivers are usually referred to as compression drivers. There are several popular materials used for the diaphragms of the compression drivers.
Phenolic is a decidedly low tech (and low cost) material primarily used in bargain drivers for commercial sound (i.e. sound reinforcement) applications.
Aluminum is the most popular material in use today, aluminum diaphragms offer the best balance of light weight, rigidity, and low cost.
Titanium costs somewhat more than aluminum, but is now used in many high end compression drivers.
Beryllium offers the utmost stiffness of all metals. Beryllium is potentially the ideal material for compression drivers. For the downside, see the discussion of its toxicity in the discussion above of dome materials. Only TAD-Pioneer uses it.
Carbon fiber has the same advantages (high stiffness at an affordable price) for compression drivers as for direct radiators.
Piezoelectric drivers are available primarily from Motorola. Unlike all other types of drivers (compression or direct radiator), piezoelectric drivers appear to the amp as a capacitor, resulting in a different crossover design strategy.
Once you have selected a compression driver, you have of a horn system. The other half is the horn itself, usually referred to as simply "the horn" or the "horn/lens". The latter term is also used to describe a physical horn with an attached acoustic lens like JBL uses on some of its compression drivers. Popular materials for horns include: Aluminum which is cast is relatively lightweight and acoustically inert. It's also somewhat more expensive than other horn materials.
ABS plastic horns are easy to manufacture at low cost, and offer generally good sonic performance.
Fiberglass horns that are reinforced can be more expensive than ABS horns, but can offer better performance if done right.
Other plastics used for horns include structural polyurethane (good) and high-impact polystyrene (not quite so good).
Wood horns are generally available only for midrange drivers. Done right, they can be superior to most other materials
Alnico or AlcomaxTM magnets. Alnico is an almost perfect material for speaker magnets - extremely powerful, yet with a high Curie temperature (the temperature at which a permanent magnet loses its magnetization). Unfortunately, world events conspired to make it economically unfeasible for most designs. One of its primary constituents, cobalt (the "co" in Alnico), is produced in limited quantities from only one or two nations who have raised the price more than tenfold since the 1950's. Today, you can still find Alnico magnets, but only in very expensive specialty drivers (e.g. Lowther wide range paper cone drivers).
Alnico 500 as used in the very expensive PR7 ribbon tweeter or TAD pioneer (pioneers pro division)
Ceramic/ ferrite magnets are now the most common. Although much less powerful than Alnico, ceramic magnets are quite economical and also have a high Curie temperature. Their obvious drawback is that ceramic magnet assemblies are necessarily large to achieve useful flux levels. This is especially unfortunate for home theater applications where the large magnet structures generate strong stray magnetic fields which must be shielded if they're to be used around TV's.
Neodymium and Strontium are rare earth alloyed elements and are the most recent additions to the driver designers toolbox. Rare earth magnets can be as powerful as Alnico, yet still acceptably economical. Their largest drawback is their low Curie temperature which can restrict their use in high power drivers. Rare earth magnets are extensively used in magnetically shielded designs since their small size allows them to be contained totally within the motor structure, thereby minimizing the external field. Used also for their low profile magnet assby in confined spaces like car doors etc
Using both neodymium and ceramic magnets in the same structure provides many of the benefits of rare earth magnets (especially small motor size), while remaining economical to produce.
"Ils rabâchaient ainsi les mêmes arguments, chacun méprisant l'opinion de l'autre, sans le convaincre de la sienne." Gustave Flaubert - Bouvard et Pécuchet (1881)