Robin Marshall: A Modicum of Genius Page 4

The first bending mode of the dome, even an aluminum dome, is not what most people think. Most people say, "Here's our aluminum-dome tweeter, the first bending mode is at 23kHz." Not so. That's the second bending mode. The first bending mode is maybe at 200Hz. It's the second bending mode that's bothering them, which perhaps is at 23kHz. This is usually an oilcan mode, when the middle of the dome is going backwards while the outer edges of the dome are going forwards. It's not a rocking mode, which is not so much due to the dome—and whatever you make the dome out of—but is due to a suspension problem.

The first bending mode is when the outer edges just slightly begin to tip up. This is widely known. Don Barlowe, who is seriously underrated by I think almost everybody (footnote 2), has already written a number of papers about this. He described, in a paper on dome radiators he gave to the 50th AES Convention in London, the first bending mode as being low down.

Atkinson: You say that you think a cone tweeter might be a better way of going about it?

Marshall: Yes I do. Because when a dome goes into breakup, it's utterly, totally finished. Uncontrollable. That's it. There's nothing more to be had. When a cone goes into breakup, all that's happening, providing you can control it, is that the radiating area is diminishing. It's much easier to control that. There's a lot of work to do, of course. I wouldn't like to say that you can just take a sheet of paper and design a cone tweeter which is going to be a world-beater. But I'm sure there's a lot of scope. I shouldn't say this, should I? I should just go out and do it.

Atkinson: But can't you add damping to control the dome breakup, or use a material which has high intrinsic damping?

Marshall: Yes, but the damping makes things worse. You look at a soft-dome's frequency response—and that's how most people judge a tweeter—and if it's nice and flat, it's wonderful, isn't it? What it's not telling you is that the first worrying resonance, the second resonance, may be at 6kHz. It's heavily damped, it's very low-Q, but that means it's actually worse than if it's an aluminum dome. If you looked at it in the old-fashioned way of judging hi-fi in the 1970s and early 1980s, a low-Q resonance is great because you can't see it. But a low-Q resonance is far more worrying than a high-Q resonance.

Atkinson: Peter Fryer in the '70s (now with B&W), and now Floyd Toole at the NRC in Canada, have done work that indicates that low-Q resonances will be more audible than those of high-Q.

Marshall: Yes, Floyd Toole says that a high-Q resonance xdB down won't be as audible as a low-Q resonance much, much further down, two times xdB down. There's a lot of engineers, of course, who work purely on theory, on "Let's measure it. Listening? Oh, I've heard of that, but you know, how do you actually do that?" I think Peter Fryer did a lot of good work to begin with on resonances. And every engineer involved in audio, for God knows how long, has always said the best way to treat resonances is to damp them like anything; you know, make them very low-Q. Peter Fryer at least had the courage to say, "Mmm, not so." And then provided evidence to prove that wasn't the case.

Atkinson: A low-Q peak may not be nearly as high in amplitude, but there's a larger area under the curve. And you can hear it.

Marshall: Oh yes! That's where soft domes fall down, I think. They spit and sizzle at you, but when you look at the response you think, "I don't understand it. Why?" If you simply think about what the thing's doing, it's obvious, isn't it? This awful resonance in the audio band.

Atkinson: So what you're saying in effect is that you have to use a stiff dome made from metal or some similarly hard material such as a ceramic . . .

Marshall: . . . to take that resonance as high as you can. And then don't attempt to damp it. I suppose in a way Celestion fell into that hole with their copper dome. Didn't they try to damp the resonance electrically?

Footnote 2: Don Barlowe designed the Leak Sandwich cone, then went on to Rank Wharfedale's research department. He did a lot of very good work on groove deformation in gramophone records, writing a really far-ranging paper on that subject. A wonderful engineer.—Robin Marshall