What Jon said. Also, the weight of the material applied has something to do with reducing vibration. Even a blob of tar in the middle of a panel will change it's natural frequency and reduce the sound generated somewhat, preventing the panel from drumming. That being said, the heavier the material used will make it perform better to reduce sound. I used a stick-on neoprene foam without a foil layer, and while I don't have a comparison, it's reasonably quiet. I covered the inside of the outer door skin, the entire roof, behind the gas tank. Getting it in the doors was an issue, as I almost got my arm stuck and was thinking I would have to call the Fire Department to extract me.
