. One full layer is waaayyy more effective than 25%. Two full layers is more effective than one. A simple knock test can prove it to you. The mfr follows ASTM guidelines for tapping test when determining the amountof factory damping. In acoustics we also used tapping tests as well as accelerometers.
Simply rap you knuckles on the outer panel, listen to the difference as you add layers and compare to an untreated panel. resonance is clearly heard.
I apply layers until the panel sounds the way I want it to. Some areas don't need a full layer. Some need several. Depends on the size and thickness of the panel.
I'm sure you don't mean to give the impression that vehicle manufacturers are tapping on the sheet metal while they try out applications of vibration damping, at least not in the way we do it for aftermarket installation. I've seen hundreds of gutted vehicles and haven't seen a single OEM application that used multiple layers. The reason is simple - subsequent layers primarily damp the previous layer, not the sheet metal itself. Extremely inefficient. Where you see full, or close to full, coverage from the factory, it;s there to serve as a vibration damper and a barrier.
Tapping on sheet metal is a good way to determine which areas are resonant before you start. Tapping while installing will always lead you to apply too much. What you need to be listening for is decay in the resonance, not the frequency of the sound you get when you hit the panel. After 25% coverage, weighted toward the center of the panel, the decay will be instantaneous to your ears so it becomes impossible to tell if you are making further progress.
You
will hear the resonant frequency of the panel drop as you add more mass. This can be satisfying and has led many of us to apply way too much material but it's a psycho-acoustic trick of the mind. It isn't practical to add enough mass to lower the resonant frequency below the audible range - quadruple mass for
each octave drop, so all we're getting is a more pleasing sound in response to mechanical excitation - not what we are looking for in ordinary use.
If you think you need more than 25-30% coverage with a vibration damper, you either need to add a barrier if you are trying to block sound or you need a better way to reinforce the sheet metal if you are fighting panel distortion under high SPL. If we had the same capacity to analyze the vibration modes of a panel we could get by with less than 25% because we'd know exactly where to place it. We don't, so we have to use a bigger target.
