A studio in the country where in the next 5 miles is nobody else has to do a lot less on isolating than a studio in the heart of a big city.
Not necessarily: Nature can be pretty noisy too. Thunder, hail, heavy rain, even wind can make plenty of noise, making it necessary to isolate just as much as in the city. It's also pretty hard to find an area out in the countryside where jet aircraft never fly overhead. Etc. Unless you live in the middle of the Atacama desert (where it has never rained in recorded history, and the wind hardly ever blows), then you are pretty much stuck with the need for isolation.
I see what you guys mean with the three leave walls choise of me, but now while those walls are build and I can't go back.
Actually, you
can go back if you want: it would just be sad and cost time and money, but it is possible. And I hate to say it yet again, but people here did warn you about that BEFORE you started building the walls.
But you seem to have two problems, not just one.
The first problem that you will have is your three-leaf wall, which you already know about: That problem is going to be in the low frequencies, around where kick drums, floor toms and bass guitars are. And the same area as where trucks, buses, aircraft, cars, motorbikes and other city noises are. Depending on the construction details (mass, air gap, insulation, etc.) and your recording needs, it might be a big problem, or it might not be a problem at all.
And the other problem you have is your non-decoupled inner leaf walls: The issue there, of course, is reduced isolation. Since your inner leaf walls are coupled to your outer leaf ceiling, you have a clear flanking path between leaves, so the best isolation you can hope for is about the same as you get with a normal single-stud wall, and probably a bit better than that, but not much. I'm guessing here, but I reckon your STC will be somewhere in the mid 40s, given the above issues.
It could have been in the mid 60's.
You can still fix this, but it seems like you don't want to.
I'm not trying to be a "jack-ass" here, as some think: I'm merely pointing out the harsh reality of what I see as potential issues in what you are doing.
You cannot make the laws of physics go away just by hoping and wishing that they will not apply to your studio.
- Stuart -
So lets hope it doesn't turn out so bad as you guys tell me it is. Because I'm getting sleepless nights of all these negative posts about my walls,... 
He doesn't seem to be reading your posts Soundman!
