Basement studio impact of specific construction compromise

I would like to understand the sound isolation impact of a construction compromise I am considering. This is for a single room basement project studio in my home in Massachusetts. My sound isolation needs are primarily about preventing sound getting upstairs. I have learned from reading here that I cannot isolate just the ceiling in any meaningful way - I know that I must construct sound isolation for the room as a whole in order to isolate to the upstairs. However I assume that the final achieved isolation may be different in different adjoining spaces, and a compromise that weakens the isolation to the outdoors or the rest of the basement while having minimal impact on isolation to the upstairs would certainly be acceptable in my situation. I am building a room-in-a-room with an independent ceiling supported on the new interior walls. The floor is going to be the existing concrete (polished). I understand that the floor represents a large flanking path, but I hope this will have minimal impact on isolation to the upstairs. Is this a reasonable assumption? I plan on using double layer 5/8" drywall with green glue on the interior of the new walls/ceiling. The basement room in question is only below grade to the front of the house (it is a walk out basement). At the back of the house the concrete wall comes to 31" above the slab, and the exterior wall above that is standard wood frame construction (including windows for natural light in this room). My primary question here concerns construction of the new wall at the back where the 31" concrete ledge is. Optimally I would assume I would just consider this part of the exterior wall and build the new wall with the base-plate on the floor. However I would like alternatively to build the new wall on this ledge. This would mean that the bottom 31" of that wall would be directly attached (via pressure treated 2x4 attached flat to the vertical surface of the concrete) to the concrete (single leaf) whereas that wall would be a separate second leaf from the exterior wood wall above the 31" point. (I hope that description is clear) The reasons for wanting to do this are to maximize the available width of the room, and to minimize the resulting window sill depth to the exterior windows. By the way, I plan to build solid core "doors" the same size as the windows into the interior wall. These would be opened to let natural light in and closed when isolation is needed. Thanks for any comments!

Here are some pictures to illustrate what I'm asking about. I'm hoping that in constructing the wall on the concrete ledge that the concrete ledge would be functioning as an extension of the floor to some extent and therefore it won't make a great deal of difference compared to building the wall on the concrete floor in front of the ledge. Has anyone come across something like this before?

"This would mean that the bottom 31" of that wall would be directly attached (via pressure treated 2x4 attached flat to the vertical surface of the concrete) to the concrete (single leaf) whereas that wall would be a separate second leaf from the exterior wood wall above the 31" point. (I hope that description is clear)" If you are framing room-in-a-room on first look, this can be done. Where it would get under scrutiny is exactly how big a room are we discussing. In order for your framing to support not only itself but a ceiling it really needs to be floor up with no framing members shortened. But if you install full sheets of plywood, same thickness as your sheetrock, standing vertical on either side of every wall corner you can reinforce the framing to be stronger. You could also benefit from a larger air space on that wall and simply frame it up from the floor up which would be even stronger. As always, you should consult someone in our area that can verify your techniques and methods will not be adverse to your safety, structure or have any potential liabilities only someone closer to you might be able to help you with.

xSpace wrote:

In order for your framing to support not only itself but a ceiling it really needs to be floor up

Thanks for your response. I've decided to take that advise. I've also started a new thread with more details on my build - it can be found at http://www.johnlsayers.com/phpBB2/viewtopic.php?f=1&t=14605