Seriously guys, let's think about this for a minute. First of all, it's not my theory that 25% coverage with a constrained layer damper toward the center of the panel is all you need for vibration damping - it's the accepted standard in every field but aftermarket automotive. That's how it's done in much more critical applications than what we do. To be fair, up to 40% coverage can have some added benefit, but the 100% coverage with multiple layers doesn't hold any water.
One of the things that led to this craziness is the famous tap test. Tap on a panel and keep applying vibration damper and tapping until the panel doesn't ring at all anymore. There are two problems with this approach. First, once you are done tapping, that panel will never experience anything similar again. Second, once you are tapping on a layer of vibration damper and then another and another, you are eventually cushioning the impact of the tap. It has nothing to do with why you are applying vibration damper. The tap test is a great way to identify resonant areas of a panel
before you apply vibration damper, it isn't a useful way to determine that you have done enough.
The other misconception is that applying constrained layer vibration dampers significantly stiffen vehicle panels. Think about this. In most cases we are starting with steel. How much stiffer is it going to get from adding flexible butyl and aluminum foil? Even thick aluminum foil? This is like breaking your arm and instead of getting a rigid cast, you decide you can accomplish the same thing by applying a few thousand Band-Aids.
The problem with applying a constrained layer vibration damper on top of another constrained layer vibration damper is that the second layer is damping the first, the third is damping the second. To be effective, the vibration damper needs to be in direct contact with the panel. If the first layer is doing its job, the additional layers aren't doing anything.
Here's what Dynamat says in their FAQ:
You can apply Dynamat in patches (25-50% area coverage) to keep a specific panel from resonating or you can apply it over an entire area (such as the floor, doors, or trunk) to create a sound barrier and vibrational damper.
This is important. Applying any of these products at 100% coverage starts to create a barrier. You need a barrier to block noise form the engine, exhaust, tires and traffic. The more layers you apply, the better the barrier you are building. This actually seems like a good idea - I've done it myself, in the past - but it falls apart when you start to look into what makes a good barrier. The answer to that is simple - mass and not being tightly coupled to the substrate. Constrained layer dampers are certainly tightly coupled to the substrate. They also turn out to be pretty inefficient and very expensive ways to build a layer with sufficient mass to block sound.
Let's consider what happens when you add a liquid or paste on top of a constrained layer damper. All you are doing is reinforcing the constraining layer (foil). A properly designed CLD is going to have a sufficiently strong constraining layer to start with.
This isn't to say that liquids and pastes don't have their uses. They do. If you've ever handled cured liquid vibration damper, you'll notice something immediately: they are very stiff. When applied directly to the sheet metal, they actually do stiffen it - that's how extensional dampers work, completely differently than the way constrained layer dampers work. Applying a liquid or a paste on top of a constrained layer damper makes it impossible for the liquid to work the way it is supposed to, sort of like driving in deep mud.
Liquids and pastes are a good choice for someone who is going for extreme SPL and doesn't mind throwing a few dB's away to the vibration damping mechanism. For all out SPL, where nothing else matters, you want to stiffen the panels with no vibration damping. All of this comes down to defining your goals and picking the approach that will get you there. No single material can do everything and satisfy every need.
