Materials Science

Written by Bill Leebens

I’ve danced around this subject before, with my discussions of tone and wood. Back a million years ago in my college days, I majored in mechanical engineering, focusing on internal combustion engines and materials science. I was a bit obsessed with the magic of the stuff things are made of, and the intervening decades have only produced even more magical stuff.

In the words of my favorite philosopher, Yogi Berra, “You can observe a lot by watching”. Similarly, you can tell a lot about the stuff things are made of by tapping on them with your fingernail. Granted, it’s not as catchy as that Yogiism, but it’s true, nonetheless.

Audiophiles know this: everyone does the infamous knuckle-rap test to see just how dead that speaker enclosure is, and more than a few of us have suffered bruised knuckles from it. We know that tapping on a paper speaker cone will create a pup-pup-pup sound, wheraeas pinging an Indian brass bowl with a nail will cause it to ring like…well, a bell.

What do we learn from those exercises? The paper cone and brass bowl demonstrate the extreme ends of the spectrum of resonance, low-Q versus high-Q. In the obtuse and obfuscating language of engineering texts, “Q” stands for “quality factor” which represents how well damped a circuit or resonator is. When you tap that paper cone, the impulse—sound—dies out pretty quickly, which would indicate a low-Q, or overdamped, resonance. That brass bowl, ringing for minutes—that would be an example of a high-Q, or underdamped, resonance.

E.J. “Ted” Jordan, the British speaker designer, was known for eloquence of expression as well as for the elegance of his designs. In his 1963 text, Loudspeakers, Jordan defined Q as ” the magnification at resonance”. If you can wrap your head around that brief description, that’s a pretty good definition.

The first loudspeakers a century ago had cones made out of paper because that’s the stuff they had to work with. It was readily available, and when molded into a cone shape, could be surprisingly stiff and strong. Just don’t poke a hole in it—then it all goes to hell. Many of us have had the misfortune to wreck an old driver with a slip of the hand; I still wince when I recall a screwdriver mishap with the big paper-cone speaker in a ’36 Philco console radio.

Don’t think that the paper was simple writing paper; it was often reinforced/stiffened/damped with everything from wool, cotton, or linen fibers, to—in later years—glass fibers, glass beads, almost anything that was lightweight and stiff. Or not: different makers had differing ideas of what made up an ideal cone, and as you can imagine, the characteristics of paper made with cotton fibers and paper fleshed out with glass fibers were radically different. In broad terms, the former would possess a lower Q, the latter, a higher  Q.

The Altec 604: first introduced 60 years ago, still made by Great Plains Audio. Paper cone—but mellow? Nah.

In general, paper cones are chosen because of their low mass and their tonal characteristics, which are largely defined by the low Q of the cone. While many would characterize paper-cone drivers as “mellow”—the classic sound that defined rock’n’roll was the sound of tube amps and paper-cone Jensen, Celestion, and JBL drivers. Jimi Hendrix and Pete Townshend “mellow”? Really?

Current speaker systems that utilize paper cones might be thought of as “anti-audiophile”, emphasizing tonality and the gestalt of music rather than the hyper-etched detail often used to create impressive demos of…well, those audiophile faves. Such brands would include DeVore Fidelity, Auditorium 23, and Audio Note. Interestingly enough, those same companies tend to utilize cabinet construction techniques that go against the ultra-rigid norms of audiophile favorites: DeVore often utilizes bamboo, and Audio Note writes of their AN-E speakers, “…the cabinet is lightly braced and little internal damping is used. The cabinet is designed in such a way that it augments and supports the drivers in their task, not unlike the box of a guitar.”

Brands like Focal, Magico, Wilson, and YG generally favor “modern” driver materials like ceramics, diamond, and beryllium (although Focal just introduced the Kanta line which combines flax-cone woofers with beryllium tweeters?!?), combined with ultra-dense cabinet materials designed for maximum inertness. In the past, those drivers have been characterized as “tizzy”, but much of that has been alleviated in recent years due to a better understanding of the types of crossovers needed to best utilize them without aggravating their intrinsic high Q. Ted Jordan—mentioned earlier—developed a line of speakers in the ’70’s which had aluminum-cone drivers and glass enclosures. Jordan claimed that glass had a single, high-Q resonance which was easy to control or damp, whereas wood materials had broad, low-Q resonances which muddied the speaker’s response. Ted was long a pioneering designer, having shown a full-range electrostat at the same UK show where the Quad ELS was first shown—so who knows? Maybe he was right.

Synthetic diamond speaker diaphragms produced by Thiel & Partner/Accuton. Ideal? Maybe. Maybe not.

Audiophile discussions often degenerate into sweeping, black-and-white generalizations, such as “accuracy” versus “musicality”. Is one school right, and the other wrong? Not to wimp out, but just as in the analog-vs.-digital discussions, each side has its merits. But as with audiophiles on forums, designers tend to be acolytes of a particular school of thought, and  respond as Colin Chapman of Lotus once did: “I can’t do that! That’s a bloody COMPROMISE!”

I’m certainly not going to tell you what to like. Personally, I own speakers with driver cones/diaphragms made of paper, cloth, plastic, metal—and ionized air!  So, decide for yourself.

Are you HAPPY now?

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