I've used products by SOUNDCOAT with great success. They have different
density and multi-density materials for varying frequency ranges. Their
products are intended for use in engine compartments and harsh industrial
environment applications. East coast: 516-242-2200, West Coast:
714-979-9202. Call them to get a distributor near you. The distributors
aren't beholden to SoundCoat; if they carry another product that will suit
your app better, they'll let you know. The stuffs not cheap and you have to
buy a minimum quantity, which is always more than you need. Go in with
someone that has the same need and split the order.

Good luck!

-Steven Dodson
Kneeland, CA
"Inga" the 87 Syncro (grounded)