1. Why does John's soffit design require the monitor to be placed inside a box, and Thomas's does not? Are there drawbacks or advantages to either?
John's design holds the speaker very rigidly in place, since the speaker is attached to the baffle via the box. John's design is massive and rigid all around, keeping things very firmly in place. Thomas's design still keeps the front baffle very rigidly mounted, but doesn't need to be as tight on the speaker, since they are independent with his concept.
2. I see no provisions for ventilation using Thomas's design. Does filling the cavity with loose fluffy pink stuff allow enough ventilation as is? Or should an airway be provided?
You still need ventilation. I think Thomas does not show that for clarity, but you do still need it. You can't enclose a speaker that is generating hundreds of watts of heat in a small area with no ventilation and expect it to keep on working.
3. Should the bezel be as large as possible both horizontally and vertically? Ie. Floor to ceiling/wall to wall using Thomas's design? Or could a rigid backed broadband absorber be placed above and below the bezel? If so, how tall vertically would the bezel need to be before adding a little absorption?
That's a little tougher to answer!
The basic concept of flush mounting id to create an "infinite baffle" for the speaker. One that appears infinitely large acoustically. In other words, much lager than the longest wavelength that the speaker can produce. Obviously, in the real world that is impossible, since we are talking about waves that are dozens of feet long. So next best is just "as big as possible". But even that isn't strictly necessary, as you can see from the baffle-step equation. The real purpose of the baffle is just to prevent the wave from "wrapping around" behind the speaker (technically, it prevents it from radiating into full space and forces it to radiate into half-space), so as long as your baffle does that, you are fine. So if your baffle extends out to the side wall and front wall, you are fine horizontally, and if it extend up to the ceiling and down to the floor, you are fine vertically.
But even that isn't necessary: There's another issue involved here: the further away you go along the face of the baffle, the smaller the effect is, and the lower the problematic frequency is, so it gets to the point where level is low enough and the frequency is low enough that it doesn't really matter too much any more.
That's why John can do exactly what you suggest: set up a hard-backed absorber below the speaker to deal with reflections from the back of the desk. You could, in fact, do that on all four sides, starting a a decent distance from the speaker center. The further you get away from the speaker cone, the deeper the absorption can be.
The above applies to all soffit designs, not just John's or Barefoot's
With my soffit designs, I follow John's concepts for the part below the speaker, then I take the baffle up almost to the ceiling, but I do generally leave a gap of many inches, and I use that area for bass trapping. There's a "lid" on top of the area where the speaker is, of course, and on top of that lid I put a lot of absorption, then a fabric front on it. For high-end rooms, I borrow form both John and Barefoot to create a rigid box around the speaker but with suitably designed resilient pads all around, so that there is no contact at all between the speaker and the enclosing box: the speaker "floats" inside the box. I tune the mounting system to a low enough frequency such that resonance will not be an issue.
4. I've read that the monitor should be centered in the bezel and I have also read that it should not. Which is correct, or is that dependant on the flush mounting design implemented? If an offset is preferred, how great or little of an offset is ideal?
If you center it in the baffle, then there will be symmetrical focused lobing patterns forming for some frequencies. If you offset it from the center, that effect is reduced. A good rule of thumb is to center it about 2/5 of the width of the baffle, but that's also not written in stone. I used to think that centering it was ideal, but in reality it isn't, so you might still find some of my older posts from many years back, that say it should be centered. Ignore that!
:oops:
5. For example, if I created a bezel that was 48" wide and 24" tall, would the center frequency be approx 93 or approx 227?
I think you mean baffle, not bezel, and a baffle that was much wider than it is high would be unusual anyway! But the baffle step response is always related to the SMALLEST dimension of the baffle, so it will be the higher frequency.
- Stuart -