this goes a lot to how external and internal sound is handled - they are very different and are dealt with very differently - these mats and such are generally used to handle sound INSIDE the car, and not to prevent sound from entering the car, which is the problem i see and am dealing with in my car
when in doubt, i always go with physics - blocking or dampening low frequency sound (below 100hz) from entering a space (not the same as dampening sound already in that space) involves preventing the resonating membrane from oscillating - this is usually done by either bracing or adding mass to that membrane
1lb/sq ft is not a lot of mass, and i would be surprised if it did anything significant without a very serious bond to the membrane
in studios, we have to screw the heck out of anything we are trying to stop low frequency sound on, cannot allow any air gaps, and have to make sure that those membranes we are working on do not make contact with other more susceptable ones
i don't see an easy answer, but good luck
94 Midnight Metallic Blue Cab Porsche 968 w/deviating cashmere/black interior and WAY too many mods to list - thanks to eric for creating
www.968forums.com
"It isn't nearly as expensive to do it right as it is to do it wrong."