Josep, since decibels are a logarithmic value expressing a ratio, you can't just add 50 dB and 50 dB of STC value and come up with 100 dB of loss - If I remember correctly, it would be more like a total of 60-65 dB instead. Each time you double the mass, you only get 5-6 dB more Transmission Loss. Add to that the loss due to distance (which is not easy to figure except in free space) and you would probably get about 12 dB improvement with adding the second wall and room.
Another thing to consider is REAL loss, as compared to theoretical loss. Field measurements of walls are ALWAYS worse performance than laboratory tests due to things like unforseen flanking noise (a very real problem in your case, unless you're prepared to slab saw the masonry floor between your two brick walls) poor caulking practices, blobs of masonry dropping between isolated surfaces, cracks and porosity of masonry not being taken care of, etc -
I've not seen any specs on brick masonry walls, but here's some comment on double cement block walls that's kind of interesting -
http://irc.nrc-cnrc.gc.ca/practice/noi1_E.html
Another thing to remember when designing a studio - STC is nearly worthless as a measuring stick!!! You need to take things into consideration at least two full octaves BELOW where STC stops counting. The STC range of frequencies from 125 hZ to 4000 hZ was originally intended to be used for VOICE attenuation, not music. Today's music studios, if they're going to isolate well, need to consider down to at least 30 hZ for Transmission Loss. This is true both between studio and outside world, and between control room and tracking room if you want to hear what things sound like through your gear instead of through the walls/windows. It is a major pain in the ass to have to do test recordings of every little thing when setting up mics, especially for things like acoustic drums - it's much quicker if you only hear what's being sent to the board with little or no bleed through the structure of the studio.
Take for example a STC 63 wall made of gypsum wallboard - by the time you get down to 30 hZ, that wall will be lucky to attenuate the sound by 30 dB, probably more like 25 or so. If you're kicking the crap out of an acoustic drum kit, the kick drum is at maybe 110 dB at the mic, 105 at the wall, and 80 dB on the other side of this so-called 62 dB wall - that level is only 5 dB down from where most knowledgeable engineers do most of their mixing; not much attenuation when you're trying to hear what's going on tape instead of what's bleeding into the Control Room...
Gotta go for now, more posts to check out before bed time... Steve
Soooo, when a Musician dies, do they hear the white noise at the end of the tunnel??!? Hmmmm...
but take a look:
