Al, it's both mass AND heterogeneous materials.   (BTW sandbags rely on
internal movement of the sand particles to dissipate vibration)

An added mass will disrupt and alter the wave/vibration as it travels across
the metal.  Asphalt roofing rolls will do this admirably, and cheaply,
especially if you put enough on.  I know, I've used them myself.  It makes the
'clang' sound duller.

The mixture in the Q-pads (and others of this nature) have particles of
different density imbedded in them.  As the metal's vibration hits them, the
particles vibrate at a different rate from the asphalt.  This internal motion and
friction changes the wave energy to heat energy.  The sound is eliminated.

So, while the roofing rolls will change the vibration energy, the Q-pads
transform it to heat, thus eliminating it.  Any insulating will help, the Q-pads
are just a lighter weight, more efficient solution.

I will be doing side-by-side comparing next week when the stuff shows up.  I
have seen store demo's with Dynamat on a desk clerk's bell.  A small piece of
the stuff kills all the sound except the initial 'thunk' of the clapper.  Neat
trick.

PS:  The asphalt roofing rolls are a good alternative, just be careful where
you put them:  I had one catch fire (electrical short) a couple weeks ago, and
it burned a little TOO well for my liking.  Think Napalm.  Glad I was
standing there watching it, it would have gotten ugly.

Bill Marshall
85 GL Tiico "Pandora's Box"
Aurora, IL