Hi Platinum,
I am a complete newbie, but here are my thoughts:
Careful with mics close to that upper wall. Search "comb filtering" effect if you aren't familiar with it.
Are you using acoustic caulk in all the openings/cracks throughout? It's different than regular construction caulk.
Very important from what I understand: "where air goes, sound goes", was what I got.
I saw someone on Harmony tell you Rockwool
is OC 703. Not so. It is acoustically similar in equal thickness, but handles VERY differently. Rockwool would be perfect to put inside your walls (instead of pink fluffy) as it is much more dense, but is kind of "crumbly" it breaks apart easily.
703 is quite rigid (it would stand on end on its own) and is nice to place ON your walls (when wrapped in fabric). 703 is what they put in commercially made bass traps.
Rockwool only costs about half as much as 703. I believe it's also known as "Acoustical Fire Batt (AFB)" brand name in US is "Roxul". I am using all three materials in different locations in my build.
More mass = more sound stopping. e.g. two layers of drywall is better than one.
Might you be better with a couple layers of drywall on the outside with Green Glue between and an "inside-out" wall (for saving space on the inside of your booth)? Just a guess on my part. That is what I am doing in the front of my CR and on one wall of my vox booth. You get the deadening of reflections without loosing any floor space.
What are you doing to stop sound coming through the "roof" (slanted wall)behind you? That might be an issue, or it might be a real good place to create (at least a partial) inside-out wall? It should help with any comb filtering also.
Search for that here and you'll find lots of info.
Could you remove the door to that small space and fill it, line it with insulatation, cover with a fabric panel to create a bass trap there? Save interior space?
Look at double glass window construction on this forum before you start putting that together. There is a lot of experience here in that.
These links that explained a lot to me:
http://www.saecollege.de/reference_material/index.html
Somewhere there is a chart that shows different wall construction configurations on this site, but I can't find it right now!
STC Ratings
Studs with one layer 5/8 drywall one side = 28 (no insulation)
Studs with one layer 5/8 drywall each side = 35 with insulation = 38
Studs with two layers 5/8 drywall each side = 41 with insulation = 45
Anyone know where the little chart that shows them side by side is?
As far as ventilation, lined duct that makes 4 - 90 degree bends, bigger rather than smaller duct so the air moves slower (less noise). Get the fan itself away from the booth. If you
really sealed up the booth,you'd need some way for air to "enter" also. The problem is that unless you do the same thing with the air intake (lined duct that makes 4 - 90 degree bends, not cheap) you quickly defeat the soundproofing you are tryin to do.
I am struggling with same issues for my booth and I haven't worked it out yet myself. I am thinking of two flexible duct runs with a fan on the end of one, outside the booth.
If it's not truly airtight, I imagine this stuff is much less critical.
Look at the Braun web site for fans...
This is a ceiling fan but its quiet and about $80:
http://www.broan.com/display/router.asp ... tID=100225
I am using this for venting out my tracking room:
http://www.broan.com/display/router.asp?ProductID=742
I am hoping I can find an "in-line" fan that is small, but I haven't yet.
~ Bill
(Just kidding)