Stiffening panels raises their resonant frequency while deadening them is an attempt to lower it. So you do want your deadening to be flexible and massive so it can lower the resonant frequency and not raise it slightly simultaneously.
Flexible and massive = 2 steps forward
Massive and stiff = 1.5 steps forward
Not so. Deadening is not an attempt to lower the rf of a panel. Thats the point of Mass loading.
Though strengthening a panel will slightly raise the Res Freq of a panel, adding mass, weight or strength to a panel will make it harder to vibrate with the same given energy. The amount that it gets raised by the foil being used by sound deadening mats is very small.
This is the basic concept behind mass loading dampers which are not an efficient way to treat vibrations. The added mass does help when other damping methods are used however.
Constraint layer dampers, for example, contain 2 parts; flexible adhesive and a stiff constraining layer, usually foil.
The flexibility you are looking for is in the adhesive layer. This is actually elastomeric, not flexible. The more ridged or stiff the foil is, the more control it will keep of the adhesive layer and the more vibrations the butyl will convert in to heat. This is why Hushmat recently stopped using mylar as their constraining layer and switched to a .127 micron foil. It works better. The thicker the foil, the stringer it is, the better it works. The raising of the resonant frequency due to the strength of the foil is exponentially reduced by the adhesive layer proportionately to the strength of the foil.
Reduce the thickness of the foil and though the resonant frequency might change slightly the efficiency of the butyl gets drastically decreased. The trade off is great and mandatory.
ANT
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