Jacques Mahul of JMlab: Inverted domes & otherwise... Page 4
Mahul: [laughs] Actually, Jonathan, we do produce, each year, a JMlab wine.
Scull: You see, I can be very intuitive.
Mahul: [laughs] Yes. In fact, here is a bottle of JMlab Sauternes for you and Kathleen.
Scull: Jacques, thank you very much.
Mahul: Last year it was a cognac!
Scull: Well, Jacques, I see I'm going to love this assignment!
Mahul: [laughing] My pleasure, Jon-a-ten.
Jonathan Scull: Jacques, I'd like to learn more about that inverted-dome tweeter...
Jacques Mahul: Yes, it's a very special wide-dispersion design. You know, many people think the [positive] dome tweeter has better dispersion characteristics than the inverted dome because of its shape. But in fact, it is exactly the contrary. Very quickly, to explain: with a positive dome, the coil is positioned outside the dome.
Scull: Yes, to the rear...
Mahul: But with an inverted dome, you have the coil in the middle of the dome. The mechanical coupling is much better. Consider that the coil of a [positive] dome tweeter is attached tangentially—it's outside, on the circumference. As a result, you have no angle created between the coil and the "base" of the hemisphere—they lie in the same plane. In this way you actually lose a significant amount of energy.
With the inverted dome—the coil in the center—there is such an angle created between the dome and the coil. The energy transfer is more efficient, so you lose only perhaps 45 or 50% of the energy between coil and dome. And this is very important, as it brings out all the micro-information in the music.
Scull: What's the function of the...that's a phase plug suspended over the tweeter on a small vertical bridge?
Mahul: Yes, it optimizes phase coherence and extends the frequency response at the very top end of the treble.
Scull: And the actual material of the tweeter, Jacques?
Mahul: It is pure titanium, vacuum-coated with a composite of various materials and mixed with a oxide of titanium. That's what gives it the characteristic dark gray color.
Scull: And the magnets?
Mahul: Ah, the magnets. They are made of a special neodymium that we have made in France. We have, in fact, only 20 kilos of this material. It has a very high saturation threshold that's capable of creating a magnetic field of nearly 21,000 gauss—2.1 Tesla! That's the same or even higher than any number of large-motored woofers.
Scull: To the effect...?
Mahul: It gives an extremely fast risetime and very good damping characteristics. The stronger the magnetic field, the better the damping, you know.
Scull: And you use a D'Appolito drive-unit configuration?
Mahul: Yes, the tweeter is positioned in a line making an arc with the two midrange drivers above and below. Having the tweeter at the center allows fine adjustments of the phasing, so we achieve a dispersion characteristic very close to that of a point source. The main lobe is located perfectly along the axis, minimizing the "daisy" effect.