The disappearance of the traditional amplifier

Shadorne, these are two great points, and I agree that they are very significant potential benefits of the "short open waveguide" approach. But they're not constant-directivity (which was my main point), and since as it does indeed very much depend on what "driver" you have to begin with . . . these behave fundamentally very much like a standard direct-radiating driver.

But as far as the cone vs. a dome to "maintain uniform spherical wavefronts" that's the whole problem, neither of them deliver any kind of wavefront that's consistent with frequency. Cones, domes, inverted domes, ring-radiators . . . they can all exhibit profound differences in their application and execution, but they are all of a similar ilk in their inability to deliver a consistent wavefront independent of frequency. The compression driver differs in the fact that it (at least aims to) acheive this goal.

I enjoyed Mr. White's article to which you kindly provided the link, but the main problem is . . .

The theory behind the waveguides to be described is that a dome driver produces what is fair approximation of a spherical wave over its piston range

I simply can't conceive of this as being valid . . . I wish my knowledge of physics and my mathematical skill was sufficient to expound on this further, but I think it reasonable to say that it would be hard to build a consenus on this among those who do have competencies in these areas. Further, his calculations are based on the idea that the dome behaves as a point source . . . which is certainly impossible except perhaps for an extremely narrow range of frequencies.

After all, if a dome behaved as a point source, then simply screwing it into a baffle of appropriate size would produce absolutely perfect directivity characteristics, and we wouldn't need waveguides at all.

But they're not constant-directivity (which was my
main point), and since as it does indeed very much depend on what
"driver" you have to begin with . . . these behave fundamentally
very much like a standard direct-radiating driver.

I agree absolutely. "Constant-directivity" is indeed a term that
applies to compression horns rather than 'short open conical
waveguide". And the constant directivity in a speaker using short
conical waveguides is achieved primarily by limiting the drivers to covering
frequencies with wavelengths larger than the diaphragm diameter (this means
a three way in most cases rather than a more protypical two way
"CD" horn). The waveguide simply narrows the wide dome
dispersion so as to integrate the dome with the driver covering the lower
frequencies below the crossover.

After all, if a dome behaved as a point source, then
simply screwing it into a baffle of appropriate size would produce absolutely
perfect directivity characteristics.

In general this is true - a dome works very well as a point source...this is why
they are so popular as the standard tweeter in the majority of speakers (used
within a limited bandwidth of course as they do start to become directive
somewhere above about 8 to 12 Khz and also suffer from breakup like any
regular cone at even higher frequencies and, of course, they rapidly drop in
SPL output as you go low in frequency and exceed excursion limits - however
there is not much "music" above 12 Khz anyway and they make
awesome cheap tweeters )

Large domes for covering lower frequencies also have a nice dispersion and
sound great but they have proved much less successful than the ubiquitous
dome tweeter - mainly because they are expensive to build properly (you
need a very large voice coil/motor and rocking can be an issue due to lack of
lateral support/alignment for the motion ( so some designs resort to having
two spiders) - all factors that make large domes extremely expensive
compared to a regular cone so few designers use them (awesome but way too
expensive).

~~~~One other reason pro systems use amps in speaker cabs is damping factor, which rises enormously compared to using long runs of large copper, allowing for tight control over cones.

Just The Facts
By

Fbi - It also makes possible to eliminate in-series inductor that limits DF much more (to about 100) than cable itself (it has typical of 0.08 ohm).

Kirkus - Magnet structure is often "immense" in underhung motor speakers.

Here are some photos of field coils.