Slightly off on a tangent here but;
Was charged to soundproof a nightclub a few years back so found an acoustics chap and we had a meeting on site. Like most I had pre conceived ideas about egg boxes on walls and foam all over the place..........
Wrong.....
Open cell foam/felt/insulation/egg boxes/buggywool and anything else spongy only stops echo and does very little to stop the penetration of noise.
Solid stuff and as thick as you can go, reduces noise. Think of a sheet of normal plasterboard then look at a sheet of sound board. Looks the same apart from its thickness. Then try and carry a hundred sheets of the stuff up a flight of stairs. Its not only thicker but denser and quite a bit heavier.
As daft as it sounds 'Flashband' the alternative to lead flashing, that never works, is fantastic for killing sound on sheet steel. Almost the same make up to a very expensive and well known car soundproofer thats available.
Thick solid rubber is also very good but I save that for another web site I frequent.
Dense, heavy and thick against the bodywork covered in soft absorbent material in the cab.
Its the future............ :clap2: