soft dome versus hard dome tweeters

Just play some violin music and that will give you your answer.

@knotscott  Depends.  You should ask the fans, but one approach is to use a coaxial or coaxial-full range driver.  If you are willing to accept the beaming above a certain frequency though you can get get most of the benefits of being without a crossover at all.

Yep - there's simply no "perfect" solution or approach to desgning full-range speakers. All have drawbacks. Some constraints are more important than others, and this is driven by how our brain / ear system, works plus add some variation for our differences as individuals.

If you want a more "perfect" approach you can try single-driver headphones - this eliminates the crossover, eliminates the room (you have ear interactions but it's "simpler"), and renders true full-range all from a single driver. You can pick your driver material - hard or soft - plenty good examples of both exist (I prefer soft). On paper, the headphone experience should be amazing. BUT, it's simply not as satisfying as a good speaker setup - even the really exotic and pricey headphone gear - and I've tried. There are multi-driver headphones too, and some of them are very good, even dating back to 1970's AKG K340!

The Tannoy coaxial (DC) is based on the idea that SOME coherence between the 2 drivers is necessary, but not 100%. They're phase coherent, but only precisely at the crossover frequency. They are NOT time aligned. Their 15" woofers are probably beaming at the 1.1kHz crossover (even worse for the 15" tulip waveguide models at a higher crossover frequuency), but the horn tweeter helps cover for it, and at least the radiation pattern is symmetric (coaxial arrangement). IIRC there was a study that claims human hearing can appreciate the affect of EITHER phase coherence OR time alignment, but that having both netted no additional benefits. The brain is wild, man. 

@erik_squires 

Thanks for the WAWB reference. I looked that up on DIYaudio. One guy talks about the excellent balance and sharp percussion. This is what I'm noticing on my system lately. With  my CD horn it wasn't really doing those things to full capacity until I recently just EQ'd the heck out of it using an FIR filter. The raw response droops considerably at both ends, but I was afraid to "overdo" the EQ. Now it's flat as a board all the way down to the 600Hz LR 48 dB crossover point, and it follows the crossover target with near perfection. And, the phase is flat all the way through. That much EQ required as much as -15 dB from the peak at around 2 kHz. It seemed too extreme to me, but it was the right thing to do. 

Speaking to the beaming on top with the WAWB, I've been toying with a Dayton aluminum dome midrange, which has a breakup peak at about 12 kHz. That thing beams on top too, and I think I understand why hard domes beam like that. The middle of the dome is just too loud compared to the edges, and the middle part of the dome is relatively flat. The sound off the dome at all points radiates perpendicular to the surface to make spherical waves, but with the center being too loud, the pattern doesn't look nearly as good as one would hope. I think one solution might be to somehow reduce the efficiency of the dome at the center. A way to do that might be to make it leaky in the middle by drilling holes. This will reduce the overall efficiency as well, but it might be worth it if it improves the dispersion. The 2" dome could be putting out something up to over 16 kHz with half way decent dispersion if the hole drilling works. And of course the resonance would need to be notched down.

I bought an old Dremel drill press from my neighbor. I'm ready to drill, but wondering if it's worth it. It might do something really bad and render the driver useless. If it works as well as I'd hope, it'll reach down to 600Hz without horn loading. Maybe 400Hz with horn loading. That's just a little lower than I'm already getting with my JBL 2426H on its horn.

@asctim DIYaudio is the best place to discuss mods to drivers, but I suggest you want a phase plug instead.  :)

Both metal and soft dome tweeters have improved over the years. But loudspeaker design, possibly more than any other aspect of hi fi, is about system performance. As an example, the Focal inverted dome titanium tweeter sounds radically different in Focal and Wilson speakers (of the relevant era), respectively.