CT - a few general thoughts here. I am aware that Richard uses the term ’pistonic drivers’ and what he means my it, and agree that it is highly desirable; but there are many designers who don’t want it. There has been considerable convergence of thought around accuracy of reproduction since the 1970s when Richard and Jim were making breakthroughs. I say breakthroughs because at that time it was not widely accepted that ’truth in reproduction’ was a desirable thing. In fact, most designs were developed to deliver their particular euphonic flavor. Within that context, the Thiel / Vandersteen "everything matters, and accuracy is the goal" perspective was unique then, but quite widely accepted now. Note that Dunlavy and a few smaller designers also honored full coherence. The marketplaces of ideas and commerce have not generally supported the extra difficulties of coherence.
In the context of ’getting everything right’, drivers must move pistonically. And as time goes on, more and more do so. I don’t know whether RV has any actual corner on that claim, beyond coiling the term and using it in his promotion. But he does share his design values with John Dunlavy and Jim Thiel, requiring flexural rigidity. I’ll take a little memory trip here. Thiel’s coherence awakening came after the 01 and 02 (1976) which had normal third and second-order crossovers (all polarity-correct.) The 03 development began in that normal way, but veered to first order in an organic procession (which I’ve mentioned here before.) That development of a phase coherent system required the development of more pistonic drivers. Note that many (normal) designs use the flexural cone delay of the circumference lagging the apex as a way to mitigate the reverse-polarity negative going onset transient of the upper driver from the typical second-order, inverted polarity crossover. In other words, a floppy driver causes less of a particular problem than would a stiff driver in that common second-order/ inverted polarity design. But that advantage evaporates with a coherent first order crossover, while keeping the significant time, phase and harmonic distortions of that floppy driver.
So, back to 1978, the final 03 is coherent, but the drivers are ordinary. Next product in development is the 04, a 6.5" coherent two-way. Jim developed a double-cone woofer for that product, which was patentable (and eventually was Thiel patented.) The front cone is a curved flair and the back cone is straight, both driven by the same voice coil and to the same surround attachment. The double cone triangulates the weak circumference, creates double propagation paths (introducing self-damping), uses the air captured between the cones as an averaging medium - all producing a very stiff pistonic system of light weight and low cost. Alas, before the finalized 04 was introduced, one of the cones was discontinued, and no workable mates could be found until we grew enough to commission our own cones.
Please pardon my spotty memory, but as I recall, the first product to incorporate that double cone geometry was the woofer of the CS2.2 introduced in 1990. It took that preceding decade to develop the sources, solutions, patent and manufacturing infrastructure to make the concept a reality. That geometry is used in the CS3.6 midrange and probably other Thiel drivers. I left Thiel in the mid 1990s and don’t have first-hand information of the further trajectory of that double-cone solution.
RV’s solution using carbon fiber over a balsa core is very effective, along with very expensive.
As far as Jim developing his pistonic drivers, I propose that he made steady progress toward that goal throughout his career. In the early 90s, we conducted in-house cone development that significantly improved specific rigidity via material and profile and taper geometry. I further propose that Jim's statement pistonic driver, was the CS3.7 / 2.7 midrange with its radial corrugation diaphragm, which represents a conceptual and technical breakthrough. The 3" diameter voice coil bisects the radiating area such that there is equal mass distribution on either side of the motive force. In other words, the driving force is balanced rather than propagating from a voice coil at the apex to a passive rim at the surround. In cross-section, the (half) diagram is a T with the wavy diaphragm on the top and the vertical stem being the voice-coil former - driving the diaphragm from its centerline. The corrugations keep the diaphragm from bending for a near perfect piston. I would love to see comparative Klippel pix of the Thiel driver vs the Vandersteen driver. I bet they are both world-class and that the Thiel driver costs a fraction of the Steen. I hope that Jim’s driver might live on. I consider it his crowning achievement.
