I had a HUGE leslie trip a number of years back, and spent way more time in the studio after hours than any healthy human should, doing nothing more than playing with different leslies and micing options.
There is no set speed on a real leslie. The horns and lower drum are belt driven and generally have been through way more than they were intended. As you use them, the speeds change. The temperature and age of the belt alone can affect the speed noticeably.
Even if the speeds matched, they wouldn't really match. The top horn is generally a dual horn (2 facing in opposite directions which actually spin) so you will hear 2 pulses per rotation. The bottom speaker doesn't spin. Instead a drum around the bottom speaker spins. There is only one hole in the drum, thus only one pulse per rotation. The bubbling warbling sound (or however you want to describe it) that brings me and so many other unlimited joy and shivers down the spine is from the inherent mismatch. In other words the rubbing between the high and low waves.
These speeds differ greatly from cabinet to cabinet.
The acceleration on the top dual horn and the lower drum is also quite different the top and bottom as well as between different cabinets. The mere physics of the design dictate that. It's all about unsprung weight. Any motorcycle riders in here will know what I'm talking about. One of the ways to increase speed and mobility is to decrease unsprung weight. The main weight of the top horn is in the center where the magnet is. When the weight is more centralized, it's simply easier to get it up to speed. The drum on the other hand has more weight on the outside thus has a harder job to get up to speed. Try getting a rope and tying a 2lb weight on one end and a 1lb weight on the other. Now try holding each weight in turn and spinning the other over your head (make sure no one is standing near and that you know how to tie a good knot). Which was easier to spin and got up to speed faster. Same applies.
Not all leslie have 2 speeds. Some I've used are merely on and off and a brake to slow it. Whether the brake was original or not I don't know. These brakes, like everything else about leslies, varied in effect from cabinet to cabinet as well as between top and bottom.
The real magic of the leslie is not the set speeds, but the transitional speeds and sounds they create. Thats why you'll see quite often hammond players who know what they're doing constantly changing the speed. There are obviously times when you would just set it and forget it, but when they're really working it, they're searching for and trying to stay in that transitional stage of acceleration/deceleration.
I really like the idea of going for the B4 sort of emulation. It seems in my experience to really match a true leslie sound. The acceleration, slow speed, fast speed, as well as mic balance are crucial to getting a real sound with the flavor you so desire. I think a really good modification to the B4 is adding the brake such as Hughes and Kettner did.
I'm sure that would take a lot of processor, but if your using a leslie. I can't see that you would be able to use much else. A real leslie sound is pretty big and all absorbing, but it's not really a speaker type so much as an amp type. Pretty much every leslie I've ever used is in fact a tube amp in and of itself. That's why you can but a keyboard into it (or a guitar for that matter
). I like turning the gain up to full and wailing on a big rotator. Tubescreamers work well into them as well. Maybe it could be a power stage as well as a cabinet to get the real feel.
Personally I would use a sim like this to no end. There is reason that most tone serious guitarist won't settle for one of the stomp box emulations and instead cart these hernia inducing cabinets from gig to gig. Some have given up carting a wardrobe sized box around, but pretty much all who have admit that it's a real compromise in sound quality. I reckon that if something like what I said above were available, the number of guitarist using a leslie would jump dramatically. Many would say "I'd never use that", but I believe that to be merely because it wasn't a real option that could be explored. I think the same could be said of a number of the tweak-able options available on the AFX. Many had never thought to have or could afford the mods, but now find them indispensable.
To clarify:
-NI B4 sound customization (speeds and accelerations and mic balancing....possibly ambiance which is pretty important and hard to get with just reverb)
-H&K (not the gun) brake
-Tube gain stage
-3 button midi controllability (on/off, fast/slow, brake on an intermittent switch)
Forgive me if i've rambled, repeated myself, or missed something. It's now 7am here in Sydney....I am yet to go to bed, and I have a 5 hour drive ahead of me to get to my gig tonight. Did I mention my kids will soon be up and bombarding me with "Daddy, breakfast please".
Leslie....the first and still the greatest chorus ever.
