Jim Thiel: A Coherent Source Page 4

Atkinson: The first drive-units you designed and made in-house were the passive radiators in the CS2 2 and CS3.6.

Thiel: Yes. And that was really very simple—not many parts involved and not many variables. The woofer for the CS7 was the first driver that we manufactured completely in-house.

Atkinson: And now the design loop is very much more efficient because you can model the drive-unit on your computer, your CNC machine tool produces the molds or whatever from your computer model, and you can actually go to many more iterations?

Thiel: Very many more. Especially in the development of the geometries for the diaphragms. At last count, we'd actually made 77 different experimental versions of the CS7's lower-midrange unit. In most cases the differences involved diaphragm geometry, but in other cases the voice-coil and magnet-system geometries were different. We were able to go through so many iterations that we can end up with a design that is much closer to what I want.

Atkinson: And that's in terms of wide drive-unit bandwidth . . .

Thiel: Yes.

Atkinson: . . . low distortion . . .

Thiel: Yes.

Atkinson: . . . and high excursion?

Thiel: Yes.

Atkinson: And the three-way CS6 is the first Thiel speaker where all the drive-units are manufactured in-house?

Thiel: That's correct. We manufacture the woofer, the midrange, and the tweeter.

Atkinson: The CS6's midrange/tweeter coaxial unit looks superficially similar to the one you used in the CS7, which was sourced out-of-house.

Thiel: It is similar to the unit used in the CS7 in that they're both coaxial midrange/tweeter units that use a 1" metal-dome tweeter and a midrange unit with an anodized aluminum diaphragm. However, the CS6 tweeter is quite different. It's a completely new unit with a very long magnetic gap. It incorporates many of the design features that until this time we've used only in woofers: the use of copper in the magnet system, the use of rubber rather than plastic for the tweeter surround, the use of Kapton as the voice-coil former material, the use of aluminum wire in the voice-coil. The result is that this new tweeter has substantially lower distortion and substantially higher output than anything we've made before.

Atkinson: One of the problems with a coaxial unit is that the tweeter sees a symmetrical environment in terms of dispersion and reflection, the result being some uneven behavior in the mid-treble. How did you solve problems like this?

Thiel: Sitting back in the throat of the midrange or woofer cone is not a good environment for a tweeter—its energy is altered and colored quite a bit. So we needed to develop a diaphragm geometry for the midrange driver that did not compromise the response of the tweeter. We made the flare angle of the midrange cone much shallower than is normally the case. The tweeter is not mounted in the throat of a typical, rather deep cone, but is in the center of a very-shallow-flare diaphragm. The problem then is that a cone with that shallow a geometry does not retain adequate strength and therefore exhibits undesirable high-frequency resonant behavior. The fundamental breakup of this diaphragm, even though it's rather small, is at about 2kHz.

What we ended up using is a sandwich construction: we have a front aluminum diaphragm with the best shape for the tweeter's energy to see, and then an aluminum rear cone with a normal conical shape, which is usefully stiff. And they're connected with an intermediate cast layer of expanded styrene foam. This allows the composite unit to have equal or greater strength than a normal diaphragm but still present an optimal shape to the tweeter.