Go to…. Overview Dome materials Cone materials Surround materials Basket materials Horn diaphragm materials Horn materials Stuffing materials Magnet materials

Materials for dome (tweeters and midrange) drivers:

Early domes tended to be made of hard plastics or soft treated fabric. Later on, metal domes arrived. Initially, only the soft domes really stood out as generally superior. Since that time, metal domes have improved as have soft domes, while hard non-metal domes have pretty much disappeared. There is still much lively theoretical debate over whether soft (usually treated fabric) or metal domes are best. The truth is that the debate is now mostly religious - many modern soft dome designs can perform as well as the best metal domes, and vise-versa. Often, the debate has degenerated into a question of taste - soft dome advocates say that metal domes sound too bright, while metal dome advocates say that soft domes sound muffled or "mushy" (whatever that means). What is true is that metal domes tend to have resonances above the audible range. Although too high to hear, these resonances can color the sound. This is true of any highly rigid material, even the Kevlar® inverted domes used by Focal. This can be cured in the crossover design, but it is something to consider when selecting a metal dome driver. Popular materials for dome drivers include: Treated fabric. 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 ration is mostly dependent on the material used to treat the fabric. Aluminum. The majority of metal domes use aluminum diaphragms. Aluminum 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 aluminum for an equivalent sized dome. It is, however, much stronger than aluminum and has slightly better internal damping. It's also significantly more expensive. As with aluminum, chemical or ceramic surface treatments (e.g. Focal's "Tioxid") and/or coating are sometimes used to boost damping. 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. Part of the reason for this is that the manufacturing process has to be rigidly controlled under what amount to clean room conditions. Although not a factor in completed drivers, beryllium dust or vapors, which are produced in the fabrication process, are among the most highly toxic, carcinogenic substances in all metallurgy. 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. Kevlar®. Although Kevlar is most typically used in woven form for larger coned drivers (see below), 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 this. Diamond. Accuton has begun producing drivers with their trademark inverted cone diaphragms made of diamond rather than ceramic. The process for growing these diamond diaphragms in the lab is proprietary. Extremely light, strong, and rigid and with an astronomically high sound propagation velocity, theoretically, diamond is an almost perfect material, but these drivers are obviously very expensive! Because of resonance problems and cost considerations, only two manufacturers (LPG and MBquart) try to make dome midrange drivers from metal. Those drivers aren't recommended, though.

Materials for cone drivers:

In the beginning, there was only paper. Paper has inherently good and bad parameters. The good news is that it has an excellent stiffness to weight ratio and quite decent internal damping. The bad news is that it's quite hygroscopic. Left untreated, paper will absorb atmospheric moisture, with a corresponding degradation in its stiffness. Within the past two decades, there has been an explosion of new synthetic materials used in driver cone manufacture. The first was polypropylene, which has since been joined by a host of other materials and multi-layer sandwich structures. Following is a summary of base materials (i.e. non-sandwich/composite materials) used in driver cones. 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: Carbon fiber paper - used by Scan-Speak (see below). Polyglass (glass microsphere coated paper) - used by Focal (see below). Slitted paper - used by Scan-Speak (see below). Kevlar® paper - currently used primarily by ATI/Dulcet/HiVi, and used in the new Shiva subwoofer (see Adire, in Section 2), Kevlar paper appears poised to become a significant technology. Polypropylene (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. Vantex/Comtex: 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. Carbon fiber 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. Kevlar® 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 (Focal, Eton, and Audax) use it as part of a sandwich structure. Aluminum 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. Increasingly, some vendors are now appying elastomeric coatings to the back of the cones to increase damping. A notable exception to the preceeding comments is the technology is used by E.J. Jordan to produce wide/full-range drivers with aluminum cones. Using a curvilinear profile and a cone material that is closer to aluminum foil than the typical cast aluminum cone, Jordan effectively uses progressive decoupling in the cone to minimize resonance effects. Magnesium cones are a staple of Seas (see Section 2 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. However, the same sorts of coating technology which have been successfully aplied to aluminum drivers have begun to be applied to magnesium drivers as well. Still, getting good results out of these drivers is beyond the capabilities of many DIY'ers. These should be considered for use by experts only (or in kits designed by experts)! Following is a list of sandwich/composite and/or proprietary materials in common use. Many of these are proprietary and apply to only or more manufacturers. Where this is the case, the manufacturer is noted. More information on specific manufacturers may be found in Section 2 below. Carbon paper. This technology, used only by Scan-Speak and Usher, 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-Tronic, 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 (Versatronics 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. CSX. A proprietary Peerless technology, the CSX cones are used in their top of the line drivers. Results have been generally reported as excellent. Damped Polymer Composite (DPC). A proprietary Morel technology, DPC is used in their top of the line woofers and 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 Versatronic (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. 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. This is an unusual paper technology which currently appears in only one Scan-Speak driver, and even that one is not generally available (yet) to the DIY market. 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. Another Focal proprietary technology, The W 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. Structurally, 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.

Materials for cone driver surrounds:

The outside compliance 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 to absorb mechanical energy which would otherwise reflect back through the cone to cause resonances. Two types of surrounds are in common usage: 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. 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 5), many of the recommended drivers use more flexible rubber or foam surrounds for greater Xmax.

Materials for cone driver baskets:

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 2. 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. Currently, you can find polymer composite baskets in models from Audax and Focal, and the number of vendors using them will only continue to increase.

Materials for horn compression driver diaphragms:

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 CTS (previously 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.

Materials for horns:

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. Historically, almost all horns were made of metal or wood. Sheet metal is a terrible horn material for reasons which should be obvious - it's thin, undamped, and rings like a bell. Cast metal horns are better, but often suffer from the same symptoms.Good wooden horns could be very good, but if done correctly, can be quite expensive. In the case of wooden horns, "done correctly" means fabricated from a block of wood, usually made of laminated layers, rather than fabricated from thin sheets of plywood. In recent years, many vendors have made the move to various plastics. These can offer excellent performance at affordable prices. Just as with speaker enclosures, horns can be gauged by the "knuckle test" - rap on the horn and see if it make a "ping" sound or a "thunk" sound ("thunk" is good, "ping" is bad). Summarizing the available recommended materials: Cast aluminum is relatively lightweight and can prove to be acoustically inert if thick enough. Even more than absolute thickness, also look for variable thickness. ABS plastic horns are easy to manufacture at low cost, and offer generally good sonic performance. Reinforced fiberglass horns are somewhat 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). Many of the structural thermoset plastics are foam core moldings. Foam core moldings are created by injecting a hot expanding plastic foam into a cold metal mold. The outside forms a dense "skin", while the interior is expanded foam. The resulting material is lightweight, has high strength, is sonically neutral, and offers high impact resistance, Wood horns are generally available only for midrange drivers. Done right (see discussion above), they can be superior to most other materials.

Stuffing materials:

As noted in the Appendix A, both sealed enclosures and TL's are typically stuffed with acoustic treatment, although for different reasons. In a sealed enclosure, the stuffing's thermal characteristics are most important. In a TL, the stuffing's efficiency at moderating the speed of sound in the line are most important. With these in mind, here follows a summary of various stuffing materials: Fiberglass. Used as insulation, Fiber glass is also among the most effective materials used for stuffing sealed enclosures. Fiberglass is useful only for frequencies above 100 Hz. Its greatest disadvantage is environmental - it irritates the skin and mucus membranes. The American Lung Association has some interesting things to say about it. Owens-Corning's new Miraflex® is a fine strand fiberglass also originally designed for use as insulation, but offering less skin irritation than standard fiberglass. Available data suggests its properties in both sealed enclosures and TL's are better than standard fiberglass, and possibly superior to most other materials. Unfortunately, it's also somewhat more difficult to find than other materials. DuPont's Dacon® II synthetic fiber was designed for use in pillows and other upholstery applications. It's a good all-around material for either sealed or TL enclosures. Dacron® II is useful down to 60 Hz. DuPont's Hollofill® II synthetic fiber was designed for use in insulated clothing. All Dacron® II comments apply. Long fiber wool is the traditional material used for stuffing TL's. Due to their similarity, it's also presumed to be the most suitable for TQWT's. It's most useful characteristic is that it's useful down to about 40 Hz, about 30% more effective than Hollofill II. The downside is that it must be mothproofed if you expect it to last for any significant length of time. It's also not a particularly consistent material since its qualities depend on the sheep which grew it. Acousta-Stuff TM is a proprietary polyamide fiber originally sold by Mahogany Sound, specialists in TL supplies. It's claimed to be almost equal to wool in effectiveness in TL's, but with the added advantages of greater consistency and inherent insect resistance. It's also often used instead of fiberglass, or even Miraflex, for sealed enclosures where its environmental friendliness more than compensates for its slight performance disadvantage when compared to fiberglass in sealed boxes. Acousta-Stuff is also sold by Parts Express. Open cell foam is an inexpensive general purpose alternative to the fibrous stuffing materials listed above. It's most common form is the familiar egg crate pattern used for packaging. It can used as a wall treatment in ported enclosures (also PR and bandpass designs), or as a stuffing material. As with all stuffing materials, it loses effectiveness below 40 Hz. Surprisingly effective, open cell foam and Dacro® pillow stuffing share the honor of being the most often used stuffing materials in common use.

Magnet materials:

Although the design of a driver's motor is of much greater significance than the magnet material (see the Motors listing in Section 1), it would be well to briefly review the most common materials. Back in the 1950's and early 1960's, Alnico or AlcomaxTM magnets were common. 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). 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. A class of magnetic alloys made using the rare earth elements neodymium and strontium are the most recent additions to the driver designers arsenal. 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. Morel has recently introduced a line of drivers using hybrid technology. 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.

©1998-2007 by Bob Stout, all rights reserved

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