Ben, go here -
http://www.saecollege.de/reference_mate ... 0Chart.htm
scroll down to "brick veneer" and look at the second example.
Now, look at my modification (attached) this should exceed STC 63, and the brick will help improve the lower freq's not taken into account in the STC method (good for drums)
Just remember about proper caulk, and that there should be close to total separation physically between the brick and the inner wall. If this isn't possible due to something I'm not aware of, you would need to get some Resilient Channel and mount the two layers of wallboard on the channel for isolation.
On dimensions, your length is such that a wider room (16 feet) would allow nearly exact duplication of one of the Sepmeyer ratios, using an average ceiling height of 10 feet. (1:1.6:2.33) - If you don't want to, or cant, do that then you should keep the average ceiling height at 10 feet. That would give you almost 3,000 cubic feet in the room which should sound pretty good. There would be a slight "hole" in room response at around 32 hZ, but basically a pretty even sound.
You would still need to do some wall treatment after the room is up to control flutter echo (parallel walls)
The ceiling needs to have at least 2'9" rise in 13 feet in order to have a 12 degree slope, which John has found is about minimum to eliminate flutter echoes - this will leave only the walls to treat for that problem. If you made the high side 12 feet and the low side 8 feet, you would average 10 feet, control flutter echo, and probably simplify the building somewhat.
On windows, you should use double WINDOWS, each of which is double glazed. The windows will be your weakest link in the sound isolation. You should frame inner and outer windows separately, as shown in the pic here but without tilting the glass.
http://www.saecollege.de/reference_mate ... indows.htm
If you can, find two windows that are heavy and two that are slightly lighter in mass, and use one of each for each pair of facing windows. You want the glass in one half of a double window to be a different mass than that of the facing window, for less leakage.
Oops, almost forgot to attach the modified wall pic... Steve
Soooo, when a Musician dies, do they hear the white noise at the end of the tunnel??!? Hmmmm...