Cone Talk: Materials 

In this section of our web site I get the opportunity to pass along a few decades of accumulated wisdom to audiophiles interested in speaker design. Today I will deal with cone (diaphragm) materials, since much is made of this in "buff book" press and in manufacturers’ advertising. 

Indeed, to read some ads and articles, merely selecting Kevlar or treated paper or polypropylene for the diaphragm material of a given driver makes or breaks its performance. Truth is, good sounding drivers with excellent measured and audible performance can be made from many different materials, depending on the application. I have my favorites, and these materials turn up in the VMPS speaker products we make. 

I became a speaker designer in 1976 and have done nothing else for a living ever since. I feel fortunate, since many colleagues who started out when I did, or came into audio hardware design considerably later than I, have long since left the field. Many others operate companies funded by "OPM" (other people’s money) and never make a profit: ultimately, these operations go through tons of cash, repeated sales and changes of ownership, and depend on finding new investors on a regular basis just to stay afloat. Companies founded by designers (like VMPS) generally are sold or taken over by money men who decide, after a goodly while, that the firm would be better off without its progenitor; you find yourself on the outside looking in at what used to be your company. 

But I digress. This is supposed to be about materials selection for speaker diaphragms. In order to make an intelligent choice, consider the following unshakable physical laws:

1. A good diaphragm material must be infinitely rigid and virtually massless (e.g. concrete that weighs nothing). 
2. A good diaphragm material must transmit sound poorly (I know that sounds strange; more later). 
3. A good diaphragm material must be easily formable into a shape that is physically strong, e.g. a cone rather than a flat surface. 
4. A good diaphragm material must be lossy (I’ll explain). 
5. A good diaphragm material must be quiet (I bet you didn’t think about that one). 
6. A good diaphragm material must be durable and rugged. 
7. A good diaphragm material must be cheap (again I’ll explain). 
8. A good diaphragm material must be workable (you must be able to attach things to it, such as spiders and surrounds.) 

These are the eight basic tenets and the problem is a good material must fulfill all of them. It is not enough to be good at one and bad at two others. A perfect score is necessary. Of course some demands on our list contradict the laws of nature (rigidity and masslessness do not go together). Such a requirement is often overlooked by copywriters and almost always ignored by cost accountants. If you have one of the latter in your company, he’s going to make a lot of the decisions. VMPS has never had one, so I get to go crazy, lucky me. 

Check out your cone materials menu and you see the following: 
1. Paper. It comes in two flavors, plain and treated (i.e. coated with plastic).
2. Plastic. Usually polypropylene or polystyrene.
3. Fiberglass.
4. Kevlar
5. Graphite and/or carbon fiber 
6. Woven carbon fiber 
7. Honeycomb 
8. Metals (aluminum, titanium, and alloys) 

While we’re at it, check out surrounds (the annulus at the edge which helps center the 
cone and absorb traveling waves originating at the cone apex and terminated by the surround):

1. Foam 
2. Butyl rubber 
3. Natural rubber 
4. Textile, usually treated with glue 
5. Crinkling the edges of the paper cone (known as the accordion surround) 
6. Plastics, such as santoprene 

You’re going to have to make some decisions now. Let’s assume we’re building a woofer from scratch. We want it to be a very good driver: highly efficient, high in output, high in power handling, linear, low fundamental resonance (Fs). Any materials we choose must contribute to the fulfillment of these requirements, as well as our laundry list from the start of this article. So, let’s examine the candidates one at a time. First, we will presuppose that we’re making a cone woofer. Why a cone? Because it is an inherently more rigid shape, and we are going to depend on rigidity (stiffness from point A on the diaphragm to point B) for good performance. Without rigidity, we could not propel the cone from its apex, as the material would simply buckle. A flat woofer would shimmy and bend if we tried to propel it from its center, and centering would become a real problem. Plus, a cone has the added benefit of more surface area than a flat surface, and it can move more air. Moving air is the essence of creating good bass response.

Again, back to our bill of materials:

1. Paper. All in all, not a bad material. Paper can be made into any thickness, has good stiffness-to-mass (what’s called Young’s Modulus), and has been in use since 1925 to make speakers. Treat it with plastic and it becomes more rigid and less likely to change due to atmospherics (e.g. absorb moisture, rot from UV exposure etc). Many modern woofer cones are paper or treated paper, particularly in drivers generating the first octave and second octave of bass (16Hz to 64Hz). Our first requirement (R1 from now on) from paper rates a grade of "B". Requirement 2 (R2 in future) for "transmit sound poorly"-- sounds like an oxymoron--however, gets paper a grade of "D". Why should a cone transmit sound poorly? Because when you transmit sound, you excite the resonance frequencies and breakup modes of the material. You generate a phenomenon called "diaphragm noise" (see R5). Paper is a very loud material. Treating it with plastic helps, but adds considerably to its mass. Since paper’s noise spectrum lies at higher audio frequencies, you probably don’t want to be able to look directly at a paper cone woofer. If you can see it, you can hear its high frequency audible noise. 

Back to our list. Paper fulfills R3 (formability) handsomely, grade B+. However, during the conemaking process, if you make paper pulp in the morning, by the time you form your last cone from it in the afternoon its properties have changed a little. Paper would get an A in R3 if you didn’t have to worry about consistent quality from batch to batch. As for R4, lossiness, paper scores a D+ to B depending on thickness. Lossiness is related to sound transmission and diaphragm noise, but is a category in itself. The more acoustically dead the cone material, usually the lossier it is. The ideal material would be 100% lossy and contribute nothing to the sound beyond the compression and rarefaction of the air from cone motion induced by processing the input signal electromagnetically. Since paper is not very lossy, it is again important not to be able to hear its noise spectrum. R 5, quietness, has been pretty much covered by the above, grade: D for paper. R 6 durability and ruggedness, grade B for both with paper. R 7, cost, is where paper shines. A good paper cone can be manufactured in quantity for one dollar or less in a 12" size, with a surround. Without the surround, the paper cone costs pennies. Grade: A+. Finally, paper has excellent workability: handling it, gluing to it, stacking, storing, shipping it: grade A. All things considered, I give paper and treated paper a C grade, depending on thickness. 

2. Plastic. When polypropylene cones became available in 1984, we switched to them immediately. The advantages over paper were obvious both sonically and otherwise. R 1 (rigidity/mass) scores a "B" for plastic. R 2 (sound transmission) another solid "B", along with R 4 (lossiness). As for R 3 (formability), the grade is "A+". There is nothing you can’t make out of plastic, as you well know. R 5 (quietness) is another "B", since polypropylene in particular generates considerably less noise than paper, although it suffers from a unique "plastic" coloration.. R6 (durability) scores an "A": they’ll be digging up pristine unfossilized undegraded polypropylene a million years from now (maybe that’s not so good). R 7 (cost) is a "C". Good poly cones in the 12" size with surrounds cost $3 to $4, a lot more than paper but still reasonable. Only on R 8 (workability) does plastic fall down. Polypropylene is particularly difficult to glue to, and stay glued to. The industry has worked very hard on this over the past 15 years with good effect. Overall grade for plastic: "B". 

3. Fiberglass. An excellent material in many ways. R 1 gets a "B", R 2 a B+, R 3 (formability) a C. It is difficult to form fiberglass into a good cone, since it must be both woven and thermoformed to be at its best. Many cheaper fiberglass cones simply add a stiffener rather than heat-treat as is really necessary. R 4 (lossiness) depends on how the fiberglass had been manufactured. With the stiffener added, fiberglass is noisy (R 5 grade C); thermoformed, it is relatively quiet (grade B). R 8 (workability) is C, marginal; things you attach to fiberglass tear loose easily, since the material shreds. Durability (R6) rates A. Cost (R 7) is high: $4 to $6 per cone. Overall grade: B, but due to its cost, you don’t see much fiberglass in woofer cones. 

4. Kevlar. A polymer, Kevlar is currently the rage with some manufacturers who tout its "bulletproof" strength. Which is true. A woven material, Kevlar is strong axially (you can’t poke your finger through it). Radially, the stiffness you’re interested in to maintain cone shape and rigidity, Kevlar gets an "F". Kevlar must be permeated with a stiffener which makes it noisy (R 5 grade C), less lossy (R4 grade C), and less formable (R3 grade D). So much adhesive is added to most Kevlar cones that what you listen to is primarily what I call The Sound of Glue. Because of the stiffener, Kevlar suffers from pronounced breakup modes which are treatable only with the dreaded "notch filter" in crossover, which destroys phase integrity. Kevlar rates a "D" on R7, cost, with cones costing about as much as fiberglass. One of my least favorite materials, Kevlar gets an overall "D" grade from me, inspite of a "B" grade in R 1, stiffness-to-mass ratio. 

5. Graphite and carbon. We are now getting places. Just like those expensive golf clubs on TV, graphite can be injection molded into a cone. R 1 grade A, for excellent rigidity and stiffness to mass. Also B+ grades for R4 and R5, and a B- grade for R 2 (as a homogeneous material, graphite transmits sound better than a woven material which of course is not homogeneous). R 6 durability rates an "A", as does R 8. On R 7 (cost), not so good: at least as expensive as Kevlar and fiberglass. R 3 (formability) is of course an "A". Carbon is often added to plastic for increased rigidity and improved stiffness-to- mass at minimal cost. 

6. Woven Carbon Fiber. Since this is the material in premium VMPS woofers and cone midranges, I obviously think highly of it. R 1 is a B+, R2 an A+. R 3 (formability) is the bear since it takes formidable manufacturing technology to weave WCF into a tight cone, making R 7 (cost) damn near prohibitive. We buy 12" woven carbon fiber cones in Japan at a very high price, at least three times that of Kevlar or fiberglass. We then have to assemble them ourselves into woofers, since no one else in this country is foolhardy enough to make 12" WCF woofers with proper big magnets (excellent lossiness--R 4-- makes WCF the least efficient cone material resulting in lower woofer sensitivity). You need a bigger "motor" (magnet structure/voicecoil) with WCF to achieve the same sensitivity smaller, cheaper motors get with other materials. However, WCF is the champ for R 6 (ruggedness), R 8 (workability: everything sticks to it!), plus great R 2 (poor sound transmission, due to high self-damping properties plus the nonhomogeneous material). Overall grade: A. 

7. Honeycomb. Nearly 20 years ago we were approached by a company offering aluminum honeycomb sandwich as a cone material. Grade A for R1, R2, R4, R5, R6. Grade F for R3, R6, R 7 and R 8. Now making a comeback, I don’t see a great future for this material. Overall grade: B-. 

8. Metals. You’d think that any material with sound transmission properties like aluminum and titanium (grade F on R 2, R4, and R5) could never find its way into high fidelity drivers, but it has, due to outstanding R 1, R3, R6 and R8, plus a B grade on cost R7. Yes it’s rigid and light: certainly the best pistonic motion in wooferdom comes from metal cones. But the noise? The sound transmission (put your ear to the rail to hear that train a- comin’)? The complete lack of self-damping? Give me a break. Grade: D. 

If you’ve read this far, here’s a brief treatment of surround materials: 

1. Foam. The old standby, foam is still of use today in many applications. It is more formable (R 3 grade A), workable (R 8 grade A), lossy and quiet (R4 and R 5 grade A). It is R 6 durability that cost foam its preeminence as a surround material. Sensitive to fungus and UV, foam rots in 8 to 12 years. Inspite of making modern surrounds with built-in fungicides and with UV resistant formulations, foam gets a bad rep nowadays. Actually, sometimes foam is the only way to go, say when you want stiffness with low mass (thin walls). We use foam surrounds only when they work better than rubber. Overall grade: B. 

2. Butyl rubber. The synthetic rubber surround is very popular because of high scores everywhere but cost and stiffness to mass. A good material except where you need the virtues of foam. Grade B+. 

3. Natural rubber. By now you’ve realized that when we use a certain material in our best drivers, there’s nothing better. Grade A lossiness, durability, R2, R3, R8. High cost, and Grade B stiffness-to-mass. Because of the unfavorable cost (three times as much as foam or butyl), natural rubber is not used much by anybody but us. Grade: A-. 

4. Textile. Found mostly in sound reinforcement applications because of low cost, almost as cheap as the accordion surround, below. Poor R 1, R 3, R 8; good R4. Breaks down with time and provides poor centering. Overall grade: "C". 

5. Crinkling the edges. This allows up-and-down cone motion while providing no other necessary qualities. Strictly for sound reinforcement and el cheapo drivers. R 7 (cost) A+. Grade D or worse on everything else. 

6. Plastics. Santoprene and other hybrid polymers have been promoted recently as combining the virtues of foam and butyl rubber. Due to high R 7 (cost), not very popular. Jury is still out, because availability is virtually zero. Most cone makers have never seen such surrounds. 

That’s it for today, hope you learned something. 
Brian Cheney