Wow this is great news for me thanks! Very usefull stuff, I allready started building so now I still have time and room to correct things.
I have only 1 question left though
The wall I'm going to make now, I mean the orange wall in my drawing, Do I make this a standard double leaf wall ( 2 sheets of gypsum on both sides of the wooden frame with insulation in between ) or do I leave the back open?
I do have other people in the house with whom I live, so I was thinking about just making a closed box ( double leaf on a single wooden frame )
Ofcourse the inside of the control room will also get treatment.
Thanks for commenting guys, I really apreciate this.. I'll start posting some pics the next days.
The only wall where you MIGHT want to put dry wall on both sides is the wall that divides your space from the rest of the house. The other three walls you want to leave open in the back. Since you were budgeting for 4 total layers of drywall, I'd take one of those four layers and add it to the INSIDE of the orange walls/ceiling. You would still want double layers on the wall that separates your living space from the recording room.
I would suggest that you actually build two walls to divide off your space. One would be the part of the purple wall that divided off the space in your original design. I would make it run all the way from one cement wall to the other so that your studio is in sort of a containment vessel. This would give you a double door dividing off your space and better isolation than you'd get just drywalling both sides of the orange wall.
If that is not practical, I would strongly suggest making that part of the orange wall (and only that part) out of something like 2x8s instead of 2x4s. Better yet, 2x8 top and bottom plates and then staggared (sp?) 2x4 studs vertically. Cover both sides with AT LEAST two layers of 5/8" dry wall.
IF I drywall both sides of the orange wall instead of leaving them open, would this make MUCH of a difference or just a little bit ( let's say 3 layers on the outside and 2 on the inside with rockwool slightly compressed in between )?
I do mainly dance/R&B/HipHop productions, and I love to crank up my Genelec 8050's , they have some pretty heavy low
The problem you have is that the cement wall isn't going anywhere so if you drywall both sides of the orange wall (all the way around), you are creating a triple leaf and you will make things WORSE not better.
Do all of your layers (as many as you can) on the INSIDE of the orange wall. Study the references on this site and you will see that it's not so much how many layers of drywall you put up but WHERE they are. Unless you plan to remove the cement walls (not recomended) you will ALWAYS have to count it as one leaf.
I know it doesn't look like it makes sense but they physics has been PROVEN to be correct in labratories.
I have only 1 question left though 
