I'm with GregP on the rockwool. A while back I was specifying a generator and went to have a play with it (this thing was an absolute monster - I seem to remember that it was a V24 diesel - it was bloody load, I know that). As the enclosure that it sits in has to let air in and out, but also had to keep the massive amount of noice low, they used acoustic enclosures at either end of the generator enclosure. All this consisted of was rockwall sandwiched between perforated steel sheets (holes about 2mm wide and close to each other). The whole sandwich was about 50mm thick and sandwiches were equally spaced with an air gap between them (again about 50mm spacing between sheets). The whole enclosure was about a 3m cube. You could stand at one end of it and shout your head off, the person standing on the other side couldn't hear a single word. Being the sad git that I am, I found it absolutely amazing (pity that the rest of the gen set was a steaming pile).