Sean: "The Soul driver is a variation on the original coax first used in our Union, a short-lived but praised performer.
"We also used it in the Cube. We are very happy with our coax, especially the changes we made to it for Soul Six. As you mentioned, in loudspeaker design power transfer functions are multi-faceted. In the voltage domain Soul Six is a pretty average 8Ω load but yes, the field strength in the gap is very high and the coil underhung, tightly wound and the gap also is pretty darn tight. It does a comparatively very good job at converting electrical into acoustical power. It also does other things well like distribution of that audio power in the room and how well timed bass, mids and highs are with each other throughout the acoustic power field. The first step of good amp⇒voice coil-⇒membrane power transfer is the inductive design. It's what you do with the magnetic power with an eye on mechanical aspects, bandwidth and dynamics. Sticking to magnetics, you want your voice coil as the main element which the amp acts upon to generate as close to an ideal magnetic field as you can.
"The voice coil for the Soul VI is underhung thin wire fully and linearly immersed in very strong static flux. This is a unique design and our custom part. Now you must put all that magnetic power and reactance into usable motion without loosing the desirable bits while damping out the distortion. Again, in the real world there's nothing ideal. If we transform any thing, there will be undesirable fallout. Our coil combines with a paper/Mylar former where the wire turns and their insulator cross-link with the Mylar which bonds to the paper. This nowadays is a fairly common way of doing things particularly with high-power high-efficiency designs. But this close to the moment of signal transformation, everything makes an audible difference – the type of paper, temperature used for bonding, adhesives and their applications – seriously, everything is critical in this assembly of the voice coil. We're not yet controlling its motion, the spring/damping forces and how well motion maintains and propagates into and through the membrane into the air. That's somewhat Phase II of driver design. That's where we look at the membrane's dynamic behavior and its interaction with the air load it sees. These aspects of a loudspeaker must coincide in a single design concept so signal from one end can communicate to the far end. It's a mesh network and dynamic."
On Soul VI vs. Druid VI cabinets and Eminence vs. Radian tweeters. "These are good questions. I really love the design, performance and consistency of the Radian 850 and 950 tweeters. These are very large-format tweeters which precludes them from fitting within the design requirements for our 10" coax. One of the problems is their 2" mouth exit. For the coax we use a 2" Ø voice coil to get good efficiency with a magnet structure designed for consistency, long-term performance and temperature stability plus bandwidth/power distribution targets. The Eminence 151 ring radiator is pretty fantastic from 6kHz to about 27kHz and a nice compact package with a coil very near its mouth. Fired through the main driver's pole within a horn design that keeps flux-to-gap strength as high as possible, it also addresses specific acoustic/mechanical concerns. Mechanically, the 151 is off the shelf. It attaches directly to the back of the T-yoke using rubber of dual density and low durometer, with the final mating vulcanization. The acoustic mouth continues a compound largely parabolic expansion through the 10-incher into a final conical mouth via the whizzer. Materials as well as shapes are really important here. Horn loading amplifies any design flaws as well as rewards moments of brilliance, accidental or otherwise. Getting horn loading to match up with direct radiators is simply a pretty hard thing to do. For us getting the impulse response within the bandwidth/distribution goals as high as possible was critical for the sense of realism and life.
Super tweeter high-pass filter.
"Nano tech is the same as in Druid VI's main driver. The key ingredient are 200nm carbon tubes supplemented by tungsten-carbide nano spheres. The core of our membrane continues to be cotton as it has fantastic natural strength-to-weight ratio and stellar damping. The nano infusion is on the cone's skin. There are some differences to the Druid VI driver as this whizzer is a straight rate and a touch shorter and the attachment to the voice-coil former assembly more rigid and a touch lighter. Two different speakers skinned two different ways. With Druid we wanted a very specific monolithic look. For Soul, a higher level of bracing and build-out was required to keep cabinet talk in check and the driver's mechanical interface as rigid as possible. It all combined to make this naturally woody-sounding shape a whole lot less woody. With Soul's form factor we get a much more natural expression of how the Griewe design—the internal acoustic behavior of the cabinet and how that interfaces with the room's low acoustic impedance—works. The linear expansion of the four side plates also plays well to the rigidity and damping qualities we wanted. Soul VI is still interestingly complicated on the inside but we designed it to be pretty easy to assemble. We use the main side plates as forms for the internals, lay up wet and then allow to cure for a few weeks. There's still a good bit of acoustic damping inside but because the bulk of internal standing waves and expansion rates within the cabinet were addressed from the start, less is used away from the areas behind the driver.
Review pair in break-in bay.
"One thing I failed to mention on the Druid VI cabinet is that while very rigid and braced to the nines, it too is of pretty low mass. It's the far ends which are heavy, the big aluminum plinth and the massive magnet on top. This lowers the system's physical resonant frequency and gives the drivers a solid platform to push against. But as mentioned, the needs of Druid's shape and those of Soul are very different. Our high-pass uses a Jupiter paper/wax copper 1.0µF cap with a parallel coil on the tweeter so a 2nd-order twin-pole filter. We scrapped the chip resistor in the last minute since it turned out that the bulk of 151 tweeters don't need the pad. Those 151 drivers which measure on the high SPL side which would need the slight pad are culled for other usage."