Does this include behaviour under power at relatively low frequencies? You cross a 1" exit driver at around 900Hz-1kHz which is considdered very low in pro-sound circles. How does the loading of an OS compare to say an exponential (or others) for high-SPL use (assuming same mouth area)?

In sound reinforcment the situation is clearly different. One is always trying to operate these devices at their upper limit. So I would not take a 1" Comp-Driver down to 900 Hz. In most of my designs for pro, I use a 2" driver down to 1.6 kHz or so and then there are sometimes three of them per cabinet. The pro world and the home world are quite different.

Loading of a waveguide, or horn, is a totaly over-blown concept. For all practical purposes any shape with the same throat and mouth areas will have about the same loading. Some may have a little more somewhere and another a little more somewhere else, but they will all be within about 1 dB of each other. I pay no attention what-so-ever to loading. I only care about wavefront formation, internal reflections and diffraction and the resulting polar response. I can easily EQ any "loading" differences, but I cannot correct any of these other properties after the fact - they must be corrected in the design.

So one gets 1.4"-2" drivers that have a nearly planar wavefront or correctly curved wavefront at the throat to enter into a (presumably, or at least in one direction) narrow dispersion horn. Any suggestions as to suitable drivers? (The BMS 4550 is known to be a very, very efficient driver that can be crossed very low for instance and outperforms for eg the B&C DE700 acording to some German designers but you have commented in response to Tom Danley somewhere that they don't strike you as suitable)

3 large format compression drivers per cabinett would be some very narrow dispersion waveguide like in a line-array presumably?

Yes, you are correct. I have done a lot of work in this area.

Each desired polar response dictates a mouth wavefront shape. This seems obviuos, right. The OS waveguide will only ever yield an axi-symmetric wavefront and hence axi-symmetric polar pattern. To me this is nearly ideal since that is the pattern that all piston drivers have (except some obscure ones - ovals, square). So matching waveguide directivity to a piston dictates an OS waveguide.

But with multiple piston drivers at low frequencies a non-axisymmetric pattern could be achieved which would require a different waveguide. There are two possibilities here; prolate speriodal; and Ellipsoidal. The first is rectangular and the other is elliptical in cross section. In fact the OS wavguide is a special case of the Ellipsoidal. These three waveguides all require different shapes at the throat - rectangular, elliptical and circular. It is possible to design an adapter, but this is less than ideal. Ideally the phase plug should create the proper shape at the compression drivers exit aperature. Hence a specific phase plug for a specific application.

I applied for a patent on interchangable phase plugs several years ago. After much discussion with the USPTO, it looks like it will be issued within the next year.

Correct - thats the determining set of factors, but its really the Vd required at the maximum expected SPL. If this can be accomodated then the driver is fine.

"Any suggestions as to suitable drivers? (The BMS 4550 is known to be a very, very efficient driver that can be crossed very low for instance and outperforms for eg the B&C DE700 acording to some German designers)"

Strange how German designers find German products supperior to Italian ones! For the most part I find the contrary - I like and use B&C compression drivers and I have tested some BMS units that were pretty bad, some are better. But honestly, most drivers are about the same. Take them appart! Do any of them look any different? You can practically interchange the parts. I use B&C because they work as good as much more expensive drivers (like TAD, JBL, etc.) at a fraction of the cost. Why pay for a brand name when there is no perceptable audible improvement?

I am not prone to buy a brand name as I know enough to be able to sort out good from bad, and I can, and have, tested a wide variety of these devices. Simply stated - brand does not buy you quality. Some models of each brand are good and some are not so good. In general, I have found B&C to be the very consistant at a low price - a very good value.

I use the B&C DE25, now the DE250 and I have tested more than a half dozen competitors. Some are comparable in performance, butnot in price, and none are comparable in price/performance, which is why I still use the DE25.

BMS does use a different topology in all fairness (I take it your dissapointment was with the 2" co-ax) and their drivers tend to be crossable very low.

18Sound goes on about a phase plug they use (in the 1480 and 2080 esp) which seem to be very well behaved in combination with their waveguides. Is their work related to yours?

My disapointment was with the coax which was obominable at the crossover. There single diaphragm drivers are about the same as everyone elses.

I am not aware of what 18 sound has done, I worked with B&C. Perhaps my work leaked out, but thats OK since the concepts are patented so it won't matter how they got the idea if its the same as mine.

I have found that most claims in this biz are meaningless. Big claims, little difference.

http://www.eighteensound.com/pdf/ND1060.pdf.

Thanks