[PILOON]

I have done some soundproofing on aircraft over the years.

Large (thin) metal pannels can 'drum' and many aircraft mfgs simply make an 'X' with a tarry type material on large pannels.
An X with a tube of caulking or a dense tape will improve or deaden that type of nuisance.

Dense foam that is self adherant attached to an engine cowling will cut out many engine noises.

Covering the firewall with dense self adhesive foam (about 1/4") will deaden engine compartement noises.

3M makes a dense self adhesive 1/4" foam coated with lead that is super efficient to deaden most engine derived noises. Bell helicopters use the product to kill turbine noises in the Bell 206"s and is sells as a 'hush kit'

I once did my cesssna with all these 'tricks' and could then hear the valves clacking as all other sounds were deadened.

On my tractor roof (large flat pannel) I glued 1/4" foam followed with a vinyl fabric. This deadened any drumming sounds and also prevented condensation plus kinda insulated heat and cold transmission to the cabin.

For a muffler you could always wrap it in fiberglass (fireproof) a-la hot water tank insulation approach which should cut out the 'tinny' echo sounds.

It all boils down to frequancy and what is best for that particular frequancy range.

Low frequancy wants to be deadened or absorbed (felt, dense foam even tar paper)

Higher freq's want lead or denser plastics.

Tentest is also a great sound absorbant.

My home gas generator is housed in a plywood box with old carpeting stapled to the inside walls of the box. You can now hear the valves clacking but not the other noises as the carpet absorbs most of the middle to higher pitched sounds. (this works great for compressors and pumps also( Just allow enough openings for air circulation.