I concur with Danno1 above on McMAster-Carr Supply. Go to
McMaster-Carr and type in acoustical insulation and go to page 3425. It explains the NRC and STL numbers that rate the sound deadening capability of their insulation materials. We just bought a lead and foam combination insulation roll from them to control the sound from an on-board 40KW diesel generator on a large cargo truck. It will be installed on the personnel side of the generator enclosure, which is already lined on its interior with a 2-in. thick foil-faced thermal fiberglass insulation board, also purchased from McMaster. They've got EVERYTHING!