I was at work last nite when i realized something, drywall can really cut fown noise transmission. In this case we are talking commercial grade drywall that was installed to make a fire rated wall. This particular wall was built up to be several inches thick. One layer was a drywall like board probably a bit over an inch thick with a green colored facing material. Then there was some metal spacers and three layers of more conventional drywall but this stuff was much thicker, im guessing about 1" thick. While the wall was built for the insurance company to satisfy fire safety concerns the construction did kill a lot of noise from the area.

Im not really involved in commercial building construction so I'm not sure where the materials are purchased. Ive never seen such materials in home centers, or common hardware stores. So you might have too look around a bit. The same green faced boards where also used to line transformer vaults and the larger switch gear rooms.

The boards by the way are very heavy as you might imagine. In any event the idea comes to mind that several layers of 5/8" drywall would likely kill sound as effectively. Document the use of fire rated materials and it might even save you some insurance money. The nice thing here is that dryway is relatively cheap, readily available and easy to work with. Drywall of course sucks in manufacturing areas due to the ease of punching through the thin stuff but that would be less of a problem when built up with 2-3 layers of 5/8" panels. .