You seem to be confusing your terminology here: the BACK wall is the one BEHIND you as you sit at the mix position. There are no speakers in the back wall (unless you have a very strange 5.1 setup). The FRONT wall is the one in FRONT of you as you sit at the mix position. The one where the main speakers are located, which might or might not be soffited.
Ok, regarding your question: If you are soffit mounting your speakers, that implies that you are using an RFZ design concept, or maybe CID or NER, or some such. In turn, that implies that the room is not a rectangle, and has more than six sides. Therefore, simple room mode calculators that work on length, width and height are not appropriate for determining room modes accurately. Since the front baffles of the soffits basically displace part of the front wall of the room, and also angle it, you no longer have a six-sided rectangular room, and the modal response can no longer be calculated with simple calculators. If you have your side walls splayed too (very probable, with RFZ design), then things are even worse. And if you have angled your ceiling, of if you have a hard-backed angled cloud, then that adds more fuel to the fire.
That said, they can still
approximate the modal behavior to a certain extent, as long as you understand the limitations. As long as your room is reasonably close to rectangular, you can use the average length, width and height to get a ball-park idea of how the modes will behave. For rooms like that, I usually do several different calculations using various combinations of the minimum, maximum and average dimensions in each direction. If all of those combinations look good on paper, then you can be reasonably certain that you won't have terrible modal issues.
a "soft" soffit should maybe use the rear wall?
If the soffit is "soft", then it isn't a soffit at all! It's just a bass trap with a speaker embedded in it. A true soffit has a massive, hard, and rigid front panel. It HAS to, in order to act as an infinite baffle.
But even some rigid heavy soffit designs I've seen on here still have ports or vents in the top and bottom, so I'm assuming at least some low frequency energy does actually get to the rear wall?
In John's soffit design (one of the best out there, actually), the entire bottom section of the soffit (below the speaker shelf), is a bass trap, with either hangers or just filled with absorption. There is a gap at the bottom of the front baffle, in order that the bass trap is exposed to the room, so from that point of view, yes, some bass energy does get behind the lower part of the baffle. There might or might not be a vent slot higher up on the front baffle, but since that slot acts somewhat like a Helmholtz resonator, you won't find much of the spectrum getting through there. And since the internal cavity is filled mostly with insulation, there's not a lot going on in there, related to the room. It is mostly related to the speaker itself.
My specific case is that I have a room who's finished dimensions will be 6'-10" Tall by 10'-11" Wide. In order to fit the Sepmeyer's ratio, I need the rear wall to be 15'-11" - but from where? I can actually fit both measurements in my available space so I just need to know where to take that measurement from.
You don't need to go nuts about room ratios. Room mode prediction is just one of many, many tools that are used in room design. And modal behavior is just one of many factors that need to be considered in room design. It's an important aspect, yes, but not the only one by any means. I'd be a lot more concerned about that extremely low ceiling, than about hitting a specif ratio. I would look for ways to get that ceiling higher, and get more total air volume in the room, rather than getting too focused on room ratios. A 6'10 ceiling is a much bigger issue to be worried about: I would concentrate on fixing that first, then worry about ratios later.
- Stuart -
