Making a "room within a room" in the attic? any downsides?

[R2D2]

i've got a space in my attic that i would like to make a soundproof'ish studio for listening to music, watching movies, and most importantly, playing guitar.
The space would be 15' x 10 (LxW) and i'd like to make it a room within a room. my main concern is; like many attics, i have a sloped ceiling/wall (the roof), What i was wondering was if i would need to make this a double wall as well since its going to go up towards the roof and out? Also, would there be any point in making the floor double "walled" like the walls? im not a drum player, bassist, or a bass aholic when it comes to music, so would there be any risk of audio leaking downstairs below the room?
I was thinking, for simplicity's sake, i'd just build a "room within a room" chamber on the walls, and not worry about elevating the floor or lowering the ceiling.

 

[GatorPigDog]

If you haven't yet, I'd see how bad the sound transmission is between the attic and the spaces you're worried about before doing anything. There's diminishing cost to performance returns beyond just adding insulation and layer of drywall to the walls and ceilings in terms of sound control. You can browse some different wall assemblies in the link below, however, these only show STC ratings which don't account for Low Frequency sound transmission.

Sound book