This really requires two things. First, sound travels through everything in the room...not just the wall. It travels through the floor, the ceiling, any common airducts, etc.
Your best solution will probably be to make a wall, with a gap, as mentioned. The best way to do this (seeing you are using a full range audio system, would be to make a concrete cinder block wall using Helmholz resonators. You can get these from Proudfoot, they call them the Sound Blox. While not extremely cheap, they are very reasonable. If you cannot use these, use cinderblocks, and after the wall is completed, cut vertical slices through the outer wall of the cinderblocks every 24 inches or so. These act as a makeshift helmholz resonator. Then, apply drywall to the outside of this. Make sure to leave a gap of at least 6 inches between the existing wall and the new one.
If you want to go all out, then you need to float the floor as well. After your wall is complete, get some neoprene floor covering (expensive), and lay it atop of your existing floor, up to the new wall. Then put a layer of fiberboard subflooring and plywood flooring. Top with carpet, and you're set. In a residential area, there's not much you can do about the ceiling, but with the wall you should get an STC rating of about 40 (40 db down on the other side) and with the floor STC 50-55. Hope this helps!
Brian
Your best solution will probably be to make a wall, with a gap, as mentioned. The best way to do this (seeing you are using a full range audio system, would be to make a concrete cinder block wall using Helmholz resonators. You can get these from Proudfoot, they call them the Sound Blox. While not extremely cheap, they are very reasonable. If you cannot use these, use cinderblocks, and after the wall is completed, cut vertical slices through the outer wall of the cinderblocks every 24 inches or so. These act as a makeshift helmholz resonator. Then, apply drywall to the outside of this. Make sure to leave a gap of at least 6 inches between the existing wall and the new one.
If you want to go all out, then you need to float the floor as well. After your wall is complete, get some neoprene floor covering (expensive), and lay it atop of your existing floor, up to the new wall. Then put a layer of fiberboard subflooring and plywood flooring. Top with carpet, and you're set. In a residential area, there's not much you can do about the ceiling, but with the wall you should get an STC rating of about 40 (40 db down on the other side) and with the floor STC 50-55. Hope this helps!
Brian