@James: Have you actually seen inside one of those Noize Control boxes? - the only info I can find on them shows a grille in the horn, so cannot tell what is happening in the throat. I also note that their info does not include response and polar plots....
The polar plots of the HL14-25 horn didn't impress me much either, but not entirely surprising with continually curved side walls. Again, I'm not certain that the curved walls of the Noize Control tractrix can maintain a constant directivity vs frequency, unless someone can show otherwise with polar plots.
A wooden HF flare would not present much problem, as all the fancy throat geometry is already included in the driver, and all that is needed is a short straight-sided waveguide similar to the F1 Resolution top end. The main consideration would be the use of body filler in the corners close to the throat to give as smooth a transition from the round hole to square horn as possible. I have also found that bare timber can absorb HF energy, and a few good coats of gloss paint can make up to 6dB difference..
The F1 axehead plugs are very interesting. If you start with a flare using straight panels all around, and then calculate a central member to give an exponential area expansion (for high efficiency) as well as using it to effectively split the horn into two vertically stacked segment, you end up with something VERY similar in shape to the F1/Turbo axehead. I did try this (in timber - VERY time consuming to make from layered shapes) and found it narrowed top end response considerably. However, I was using a different throat geometry, so this probably made it worse, but it would seem that this style of plug will influence dispersion as well. LOTS of experimentation would be required to get this system right, and while Funktion-One would have had the neccessary time, resources and cash to develop this (and very well I might add), it would be beyond the scope of most DIYers. I think we should be able to achieve very acceptable results without having to go to this extent.
There are a few problems associated with narrow dispersion horns. We need to make the mouth width large enough to maintain pattern control at lower frequencies, and the sidewall angles will dictate the depth of the flare. In this respect, an 8" driver would allow a shorter horn than a 6.5" driver. A long flare with shallow sidewalls will tune to a much lower frequency than what would be normally required, naturally increasing low end efficiency at the expense of top end loading. Techniques such as a fast initial flare rate (achieved through phase plug geometry) feeding the lower rate can improve the top end, however an unnatural midrange response hump around 400Hz to 700Hz is a typical consequence. To offset this, a driver with a rising response characteristic (response increases with higher frequencies) will work very well.
I have been looking at a few different drivers, including the SN-8MB and the B&C 8PE21. Unfortunately, most 8" drivers available off-the-shelf are really designed as mid-bass woofers and not as dedicated midrange units. As such, they usually have low resonant frequencies and reasonably heavy cones. Drivers such as whatever is in the F1 resolution mids just aren't normally available without a special build. The raw response of both the SN8MB and 8PE21 would present a problem in the low-mid area. The Eminence Beta8 response is a little better, but I still had problems with a 500Hz hump in a previous design. Whilst I haven't modelled it, the small Xmax of the 8PE21 could also be a limit, depending on the lowest frequency it is taken. Both these drivers have similar sensitivities within our intended operating range, but is not overly high. The SN8MB is available here in Aus at a reasonable price, so I may grab one to play with anyway.
Most of my current work is based on the 6.5" Eminence Alpha6 - mainly because it is cheap!! (here anyway), however the importers no longer stock this model, so I will have to look at something else anyway. Whilst I have had good results with the Alpha6, there are other options to gain better performance. One driver that has caught my eye is the 18Sound 6ND410 neo 6". This beast has the desired rising response, high sensitivity and good power rating and will probably work very well despite a slightly longer horn requirement. I plan to get one of these for further experiments.
In regards to the low-mid section, I was wondering if the SN12B may be a better choice than the SN12MB, due to higher sensitivity and power rating.
Cheers
Graeme
PS: Do I get the prize for longest post