sorbothane is practically a gel. it's the softest polyurethane i know of, and is generally found in shore duro A30-70. A30 is basically jelly, while A65 is fairly firm. that said, the shore durometer rating understates how soft sorbothane it. sorbothane has very very high yield strength (ie, excellent memory), meaning that it can deform a very large amount before it degrades and holds the deformed shape.
practically, what this means is that it is very good at isolating high amplitude vibration. the energy of the compressive waves is dissipated into the sorbothane. the way to think about it is that the vibrations are potential energy propagating through some solids. when they hit the sorbothane, the potential energy of the wave is converted to mechanical energy which is limited to non-permanently deforming the sorbothane.
typically, the other way to dampen the vibration of, say, a panel of bulk material is to make it heavier by eg attaching an asphalt/rubber composite to the panel. this lowers the resonant frequency of the bulk material to frequencies low enough that they do not propagate, or are eg inaudible. dynamat, etc. are all formulated for this purpose.
