One studio I built I put neoprene acoustic decoupler strip under the bottom timber. A few years later it got demolished. That foam had compressed from it's 6mm down to a thin hard (and pointless) hard lump!
Loads of people swear by green glue. I used it once and I vowed never to repeat it. The damn stuff is messy and yukky - and I'm not certain it did much. After all, you can't build a room, test it, then re-build it with the stuff to hear the difference? One studio I did was free-standing with all it's sides exposed. Inside and out were 2 x 9mm plasterboard on the timber studs, but withe an extra layer of 12mm MDF on the inside - because not only is it heavy and dense, it resists flight cases bashing into it, and doesn't damage easily with office type chairs - plus, you can screw into it anywhere - so guitar hangers, shelving, brackets, that sort of thing can be done anywhere.
What I did when it was done was go around the outside walls and the ceiling with a mic connected to headphones and play very loud music inside. The idea was to see how much leaked through the walls and ceiling. The results were very odd, and not what the books on the subject suggested. The leakage through the 2 layers of PB on the outside and the 2 layers of PB and one layer of MDF on the inside was very, very low. Nothing above 200Hz I could detect, and the very bottom - kick drum and 5 string bass B string detectable but not an issue at all. The ceiling which was just 2 layers of PB on the inside and 2 on the outside topped with ½" chipboard - was nearly as good. The snag was at the top edges where ceiling and walls met. Somewhere there there was midrange escaping. I could not localise it on the outside, but making noise on the outside and listening inside worked - It was where the layers of ceiling plasterboard touched the top of the walls. These had been cut to match the angles and the sound escaped through the tiny gaps, and with the ceiling joists on the top of the top timber, in to out were in some places just two layers thick, based on the exit route through gaps.
I cured this in the next studio by not putting the ceiling up the same way. Did the walls, and then put the first ceiling layer on top of the walls, and then the supporting joists on top - so the wall/ceiling joint is the same thickness as the rest. Since then, I now always do this and leakage is minimal.
