Every advice I’ve pretty much ever seen from people like Ethan Weiner and other actual acousticians is that when you have the chance - when you’re building from scratch and on purpose as a studio - you build the walls “inside out”. If you actually need the mass of Sheetrock for isolation purposes, you put it on the outside surface or between the studs, then put your chosen insulation inside that with soft fabric to cover. This way you’ve already got absorbtion between sources and receivers in the room and the hard heavy reflective surface and don’t have to build any new absorbtion. If that’s too dead, then you can look at diffusion or just a few reflective panels which can be as simple as like a little wood paneling rather than all the work and materials framing and building broadband absorbers.
Mmm...I'm using the Rod Gervais "Home Recording Studio" 2nd Edition book for some basic guidance, and TBH, there's nothing in it that suggest doing an "inside out" wall structure build. All the wall build diagrams and options are about layers of drywall as your inside "leaf"...and then there's a section about treatment applications after that.
I think if you even tried that...if the build was being done by permit and with licensed contractors...you would have all kinds of code issues by not having a drywall as your inside layer.
And you're right...if you did the "inside out" wall, it would certainly be a pretty dead room...so then you're stuck doing "reverse" treatment to put some liveliness back and get a good balance. I think most people would be much more comfortable (even if they weren't dealing with a permit build) to do the more standard approach, and then apply treatment afterward.
For a serious commercial studio...it's another world. The building footprint is usually bigger and allows for both code building and treatment in the wall cavity.
Speaking of that...there was/is one acoustic supplier that I spoke with that was offering an acoustic solution that was built into the walls. They provided the panels, and then your contractor installs them into the wall framing..etc.
I asked...what about the need to install drywall to meet code...and he said, yeah, you would still have to do that. I then asked, so what's the treatment inside the walls going to do...
...and after that the conversation kinda went into some theoretical realm of the fantastic, that it would still provide some LF help.
Mind you...that system, just for the panels for a typical 20x30 room would have been over $25k...
...and you get to install it, not them, so if it don't work, it's going to be on your installer, not them.
Needless to say...I'm not going that route.
Can't speak for the smaller spaces...but in my studio build, the size of the space and the total volume will not require anywhere near the amount of trapping as a small space does. I'm going to focus mainly on diffusion to start...and then just fill in with some absorption as needed, but I will also include fairly hefty LF traps for the two back corners, which I already have (along with some broadband traps). I think I will end up with broadband traps more up on the cathedral ceiling than I will on the walls, as the saltbox shape of the room with the cathedral ceiling should push a lot of that sound up in there.
It will certainly require a bit of experimentation.