Mass, Massive, Massless

Written by Bill Leebens

I’m fascinated by materials, which I ascribe to a childhood obsession with spider webs: how could something that’s barely there, be so strong? In college I focused on materials science and internal combustion engines as part of a program in mechanical engineering (and a double major in journalism—go figure).

I mention this because when it comes to the study of audio, I‘m drawn more to the physical and mechanical aspects of design than to electronics. Mechanical bits like structures and transducers are just easier for me to understand, and while I have a basic understanding of circuit types, I just don’t find them that interesting. But an FEA analysis of a tweeter diaphragm or a tonearm, interferometry studies…NOW we’re talking. And I can geek out on reports of new materials like nobody’s business.


Diamond domes

Lab-grown diamond tweeter domes from Accuton/Thiel & Partner.

As many have pointed out elsewhere, mechanical design in audio is a study in contradictions: an ideal speaker material would have zero mass with infinite rigidity; a tonearm moving across a record should have infinite mass to control the cartridge while simultaneously having zero inertia (or zero mass and infinite inertia, in order for the moving elements of the cartridge to work without interference from the arm….aaaghhh!). Clearly, these are contradictory requirements, and it’s impossible to achieve both sides of the equation. The best designs are the best-balanced compromises.

Colin Chapman, the founder of Lotus, was a brilliant structural engineer who often valued lightness over strength. The result was that Lotus usually either won races—or went out with a catastrophic structural failure. Chapman was once faced with a dilemma in the design of his first closed passenger car, the Elite: it needed 2” more headroom. Raising the roofline 2” would spoil the aerodynamic profile, and lowering the seat 2” would weaken the monocoque structure. A helpful bystander suggested, “why not raise the roof 1”, and lower the seat 1”?”

An apoplectic Chapman replied, “I can’t do that! That’s a bleedin’ COMPROMISE!!”


Colin Chapman would not have done well in audio design, where almost all designs are the result of careful choices and compromises. I say almost all as there are products where a designer is smitten with a new material or obsesses over one aspect of performance, to the detriment of everything else.

Let’s look back to the Infinity Black Widow arm: an early application of carbon fiber, Black Widow was designed to be ultra-light, so as to be compatible with high-compliance cartridges of the period, like the Shure V15 and ADC XLM. The resultant small-diameter tube was almost as whippy as the tip of a flyrod. Later tonearm designs made use of principles known to the designers of common drinking-straws: a tube of larger diameter with lesser wall thickness can be both stiffer than and lighter than a smaller-diameter tube with greater wall thickness.

Philosophies even differ wildly in turntables. Many view high mass as a necessity to ensure isolation from feedback and to provide a stable platform and smooth performance. Others argue that higher mass results in greater stored energy, and thus, in blurred sound.


Compare the hefty slate base of Oswalds Mill Audio's Tourmaline (photo: Cynthia van Elk)...

…to the lightweight skeletal base of Rega’s  RP 10.

Logically enough, no one in audio has obsessed over mass as much as speaker designers. Early designs made use of the materials available—for the most part, paper for cones and phenolic for the diaphragms of compression drivers. Paper cones were stiffened by tapering, adding ribs, and varying the thickness of the paper itself. Paper cones were naturally light, and well-designed ones resisted break-up; when they did break up or wear out, however, the results were ugly. Many still prefer the characteristics of paper cones (which tend to have lower-q resonances) than stiffer but more-resonant plastics, ceramics, and so on.

The ultimate in low-mass speakers is of course the plasma or ionic drivers we discussed in Copper # 18. The opposite can be seen in the heavy cones often found in subwoofers, where the need for stiffness, coupled with massive magnet assemblies, make lightness less of a priority.

What’s the point of all this?  If you look back at the history of audio (as we often do here), you’ll see that almost every conceivable material or configuration has been tried at one time or another. It seems to me that those who believe in One True Path in audio—and boy, there are a lot of folks out there who do— are missing a lot of interest, and possibly missing out a lot in enjoyment, as well.

But that’s just me….

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