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Re: Acoustic Insulation for engine room?
voyager, my understanding is that the closed cell foam, if it is high density, will stop high frequency noise, much as ear stopples will. If you've ever used these you may note the effectiveness differs greatly from one brand (foam type) to another.
If you want to stop low frequency noise, you do that by converting the energy into heat, and that requires mass. That's what lead sheeting is for. The high mass of lead absorbs large amounts of vibrational (acoustic) energy and converts it into heat. You can't do that with anything unless it is DENSE which is why they still use lead.
Physics rules, there's no other magic solution. There are some high density plastics like "sorbothane" and others but you will find them as pricey as lead. Bottom line, you can pay the piper, or listen to the the pipes.
Or get some of those expensive Bose active headphones.