2 Car Garage conversion - LA - question about leafs etc

Longtime reader, first time poster. :) First off, let me say thanks for all of the valuable information i have already gained from all of you. i am always impressed by your 3d sketchups, progress photos, and the like. I apologize for asking what might be a dumb question but every time I read about this i just get a bit confused. I have a finished garage that has painted drywall already on all of the walls (except of course where the garage door itself is). i want to build a room within a room inside this garage (a foot away or so inside the existing walls). my main question is about the leafs. since the existing walls are already finished - when i build a new set of walls inside these original walls, do they just need to be drywall on the inside (or double or triple drywall) on studs with insulation? or do i need to put drywall on the "outside" of the room within a room as well? this is what i am confused about considering that the garage already has finished walls. any advice would be greatly appreciated! thanks in advance!

You might do yourself a favor and install sheetrock to the existing interior finished walls before you build anything else. Many are not aware of the reduction in sound that this can do for them, since it all reads like black magic. Doors, windows and the ceiling if not treated exactly like it is one of the walls, are the places that sound leaks in and out, so these areas cannot be over emphasized on the importance of adding mass, isolation plugs, seals and gaskets, etc. to beef up these potential acoustic holes. Many who are in this situation have been known to remove the existing sheetrock from the walls. Skill and a sawzall and strict attention to wiring in the walls are needed to extract the sheetrock in order that you can use it again to mass up the exterior sheathing. Others have not gone thru this effort and simply build the framing inside the proposed area installing the mass onto the interior side of the new framing. But to answer your question directly, the sheetrock, in a mass air mass configuration, only goes on one side of the framing, in your case, the interior side of the framing. What you have is a coupled wall assembly with mass on either side, possibly coupled with a facade (brick/stucco) with not much more than maybe 37-40 on the STC scale. The proper method to achieve a mass air mass configuration is to remove the existing interior sheetrock, mass up the exterior sheathing, frame up a new wall minimum of one inch from the other framing, and finish the interior side with two layers 5/8" sheetrock.

Thanks alot, X! Very detailed answer. Does space between the existing wall & my new walls make a difference? As in, what if I needed to leave space to walk around all sides of my structure? So perhaps 2 feet on every side? Does that have any effect on the leaf construction? Thank you!

more "air" will help with isolation, more is always better up to a certain distance. If you have to do this then do it since it helps you. The part where it can hurt you is if this air and the exterior wall become part of a live walk-thru and do not remain sealed up at all times. You could have a door or whatever might be existing, windows, etc. but these would need to be sealed properly as well and remain closed when you are in your recording space since this would be a breach of the mass/spring/mass assembly and will reduce the transmission loss of the overall assembly.