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content derived from http://www.mgaguru.com/mgtech/electric
Research from the suppliers of these materials indicates that the single biggest reduction in noise is achieved by dampening the cars floor-pan then followes by the front firewall between the passenger and engine compartments, then the inside of the wheel arches, the door skins, the rear side body panels and finally the boot area including the lid. One should bare in mind that anything thicker than 6mm is going to make the door panel almost impossible to fit onto the door frame (at least on this car). And the after effect isn't that pronounce either.