Spenceroo, I agree that there are not many competitors to SP Tech that use waveguide technology.
KEF and Pioneer TAD Home Audio and Tannoy and Gradient and Nomad (and probably a few others that I can't remember offhand) use coaxial drivers where the woofer cone acts as a waveguide for the tweeter. The problem is, it's not physically possible to get an ideal waveguide shape (along with throat transition and smooth mouth flare into the baffle) with this format. Still, it's possible to get a significant improvement in radiation pattern control with a good coaxial configuration.
Amphion uses a fairly shallow, wide-pattern waveguide and I think they are pretty close, conceptually, to what SP Tech is doing - but Amphion does it in a much narrower cabinet and so they won't have good pattern control down as low as the SP Tech device. YG Acoustics uses an even shallower waveguide in their designs, so it's probably only a marginal improvement over a flat baffle.
For a while Earl Geddes was manufacturing and selling large-format 90-degree waveguide-based speakers here in the US. Geddes is the source for modern waveguide theory, and I believe the SP Tech waveguide draws on his foundational work. At the moment, Geddes' designs are being manufactured in Thailand for the prosound market and he does not currently have US distribution.
The Emerald Physics CS2 uses a 75-degree pattern waveguide for the high frequency section mated to a dipole woofer section via a DSP crossover, with bi-amplification required. The larger prototypes shown at RMAF 2006 and CES 2007 used a 90-degree waveguide.
Yours truly uses a 90-degree waveguide based on the work of Geddes. I think the SP Tech waveguide is 110 or 120 degrees, but otherwise what we're doing is conceptually similar - though we have different ideas about crossover points and a few other details.
There are several prosound companies (Genelec comes to mind) that use waveguides in the design of their monitors.
I'm sure there are others that I'm inadvertently leaving out. By way of disclosure, I either presently am or have been a dealer for several of the lines mentioned in this post.
Duke
