The barrier layer used to be LEAD in many products, but with the push to get lead out of the environment, and the cost of lead these days, vinyl is more common.
IIRC the science of it is that high frequency noise can be absorbed by foams, but low frequency noise consists of much higher energy levels (think of how you literally feel subsonic and bass noises) and you can only stop that energy by abosrbing it with something that has high mass (lead sheeting) and takes a lot of energy to move.
Depending on what your noise source is generating, one or the other may be more effective but the stuff with the "mass" layer generally should absorb more noise. It isn't just to block odor or moisture, it is there to absorb energy.
As shogan says, those lower frequencies may come through things like your engine bed. In theory you have rubber engine mounts, which stiffen as they age and should be replaced every 5 years or so. Those also dampen vibration and noise. In theory.
And plugging up every hole, every noise pathway, and using the right hangers/adhesives, is supposed to make a huge difference, regardless of the material.
Last edited by hellosailor; 08-04-2011 at 03:59 PM.