Did you know you can improve the sound-barrier between two rooms if you stagger the wall studs? This is great for between bedrooms or a bedroom and bathroom. To stagger the studs, use a 2x6 at the bottom and top of your framing, instead of the usual 2x4s.

Or if you're working with an existing wall, use one-inch furring strips to widen the existing frame. Add studs 16-inch on center as usual, lining up the studs against one edge of the floor frame. Add a second set of studs half way between each of the first studs, lined up on the opposite edge of the floor frame. You'll use twice as many studs, but your drywall won't be attached directly to the other side, and sound vibrations won't travel as easily through the wall.