Originally Posted by 02Zetechse
A actual deadener with the foil constraining layer deadens in a different manor than mass loading a panel to change its resonating frequency ...
Not all sound deadening materials come with foil. The foil's primary purpose is to act as a heat barrier. The rubbery nature of these materials and the fact that they are stuck to the surface with an adhesive dampens the resonance along with the added mass they provide that lowers the resonant frequency.