Sound walls are made by offset studs on double thick walls. The studs from one side don't transmit mechanically to the sheathing on the other (which acts like a drum). 1' of air space might even be excessive, but Ed's got the right idea.

My solution for the shop here was to sheathe the entire outside with 1.5" pink high density foam as an underlayment to the insulated siding. Combined with the R19 and 5/8" sheetrock, it's very effective (and has an R value approaching 30). I can barely hear my air compressor on the other side of the wall from it. All seams were taped with house-wrap red tape.