Modular Room-w/in-a Room HT? Think's a good idea?

Lowrider57, I actually have a couple of different rooms which I could use at my home, including a garage space. The idea would be to at least have the base partition structure fit in, say, a rather small room/space - 11'x12' , for example - with maybe at least a few inches of air space on each side of the partitions , once assembled. That would allow for some ventilation options, and connectivity flexibility to electrical outlets, openings, entrances/obstructions, and so forth in the existing room. Yes, it's true that this narrow space separation would NOT be ideally conducive to isolating bass energy, as that really requires more space between the wall spaces. HOWEVER using very thick, heavy, somewhat flexible rubber mat material as the outer partition wall boundary material just might knock enough of the bass impact down, to perhaps half the effectiveness you'd othewise have just playing bass in an attached room in any given home. Gotta be an improvement. Then, of course, if you could assemble the same in a larger space, you would get the added benefit of depth/distance between inner/outer room boundaries, which definitely would improve bass and mid range/upper energy isolation, for even better performance, yes!
Basically, the system could be either fixed predetermined size, to fit in any given space you need, or could maybe be adjustable, by fitting more sidewall panels, with some sort of adjustable ceiling system engineered into it???
Anyway, just thoughts and ideas. But I can tell you right now, that, in any typical room in a house/apartment/condo, ANYTHING would be better than simply sticking equipment into the room and expecting to either isolate internal or external sound from the outside or the rest of the home!!! That's a given.

Agreed. That's why recording studios are built that way; with sonics the priority and keeping out external sound. Those rehearsal rooms are not concerned with keeping the true sonics of the instrument, the concern is a quiet room inside and out.
I think, if you've got the budget, it could become a good music room. The alternative is constructing secondary drywall with sound absorption and wiring which amounts to building an entire room, which may cost more.

That's why a consultation with a contractor who has built studios may be a good way to start.

Lowboy, good sound inside has got to be a relative term at most, up to some level of interpretation. I think, for all intents and purposes , the home AV/HT market has pretty much got down what makes for a good sounding overall space in any given relative room dimension and design, and how to treat it at this point. Simply going to any PMI LTD, Lucas THX, Rives Audio website, and similar, etc, can get anyone understanding what amount of diffusion, absorption, reflection, re verb, bass absorption vs relative size, etc and so forth is needed to build into any space for MORE THAN adequate sonics inside, say, hidden panels and corner/bass traps??! That part would be easy.
As for what makes for "keeping the true sonics of an instrument"(?) Lowrider57, I'm not completely sure what that means to you..or to anyone else for that matter. Perhaps, coming from some pro-audio/studio background, you have your own tastes in room acoustics? Couldn't help you there. Ask any number of theater builders vs Audiophiles vs audio engineers, vs home theater enthusiests vs pro magazine reviewers, and so forth, I'm sure you'll get varying options and tastes. And, yet, simply realizing that smaller spaces have less bass absorption vs mid/hi freq energy - which requires more diffusion around room plus treating first order reflections, and corner bass traps, is a must. Medium sized spaces need similar but more absorption mixed in on sides and ceiling. Larger spaces absorb much more bass and therefore need more midrange/hi freq absorption to balance out, plus some diffusion all around. ..basic.
We could go on for days here though.
My point is that that could be worked out formula (with some customer modification(??) upon assembly if desired, again hidden inside fabric pannels internally in any structure, maybe bass traps in corners, and basic. Still MUCH MUCH better sounding than any drywall painted echo chamber or untreated standard domestic living space...I assure you

IMO, the market would be even smaller than targeting audiophiles. I think for many of us, the journey is as important as the end goal. Now my wife, on the other hand, might prefer I spend the money and be finished.

Then again, if the only space I have to play with is in the attic of the detached garage, I might be very interested in installing a sound room complete with base traps and customized wiring.

Ultimately, it all comes back to the price.

I believe targeting audiophiles is probably not the market, actually. I think audiophiles are happy with their $5K pre-amp, speakers and amp's of choice at the present, and some records and spinner plus tweaks, mostly. I don't even think a dedicated construct-able sound-room even would be in their equation, honestly.
However, anyone looking for a full on custom home theater/entertainment space in their home might strongly consider such if it was marketed to them effectively, say, through their local custom installer/AV retailer. If a customer was showing interest in a dedicated space setup, then the custom retailer would stear them to one of their rooms with said "Modular Room system", and show them the benefits, and why said customers NEEDS such a product! And, as you said, Pgawan2b, if the pricing was reasonable and right, it might then work...especially for basement homes back East, and such like. I think it could work, really..and I think more need could be met here than one might think actually. Maybe maybe...