There are three types of deadener products, each with different objectives and results.
1) CLD: constraining layer dampeners
These are a foil liner placed on top of a viscoelastic layer, typically butyl or asphalt based. The rubber base is to help absorb vibrations and the foil layer creates another layer of metal on top of the stock piece which you are applying it to. This has the effect of stiffening the panel, increasing the resonance frequency. It does not eliminate it and only marginally reduces it. It does NOT block sound.
This is your dynamat, fatmat, ss damplifier, peel & seal, etc products.
For the most part, these products are worthless. They ad extra weight without any real benefits. To use this type of product properly, you have to figure out which panels are vibrating AFTER the install and treat them reactivity. It's been found only covering about 25% of the surface area of the offending panel in the middle of the panel is the best trade off in install vs results.
Most people go crazy with this stuff and build up layers upon layers and covering every surface of the car. This is only a waste of money.
2) Foams
These are liners of, as one would expect, foam. Foams actually block noise transmission (road noise, outside noises, engine noise). However, because of the thicknesses required, they only really work on high frequencies. They work because they are very porous. Sound waves get trapped and scrambled in side the air cavities in the foam.
They come in two varieties, open cell and closed cell. Open cell works better because they allow more easy access for the sound waves to enter. The downfall of open cell is that they hold water. Closed cell work well, not as well, but reject water.
Foam is also what you use to prevent panel vibration noises (buzzing). You can use something as simple as weatherstripping from Walmart to isolate the buzzing pieces.
3) Mass layered barrier
This is your low-frequency sound blockers. They act to simply to add mass to block noise in the lower frequencies. The denser the better. Typically lead based mass-loaded-vinyl (MLV) is used because lead is extremely dense and has a fair amount of mass. The price isn't too bad either.
To effectively use MLV, you first apply a foam later, ~1/4", then lay the MLV over top, trying to cover as much surface area as possible. You can use duct tape or CLD strips to seal between cut pieces. Overlap MLV layers if necessary, or cut to fit to exact size.
Doing this creates what's called a floating barrier. Airborne sound waves get trapped between the metal of the car and the MLV, diffused by the foam in between. You do not want to add CLD to the top of the floating barrier. The fact that it's flexible further helps diffuse sound waves
When deadeneing the car, your first objective is to understand what type of noise you're trying to prevent. If it's airborne noises, your best bet is MLV and a floating barrier. IF it's structural noises (suspension clunking) or panel vibrations, target it with CLD.
How I recommend to deaden a car is first, pull the whole interior out. Target around the strut towers, wheel arches (where water sprays) and kick panels with CLD, not a lot. Test how much is needed while driving the car. Then under the head liner and a-pillars, put a layer of 1/4" open cell foam. In the center of the roof (if you don't have a sun roof) a single layer of CLD would be a good idea. Next, line the floors with MLV. Go as high up the fire-wall as you can and pull the dash if you can. Use 1/4" closed cell foam and 1lb/sqft lead based mlv. Do the same with the doors. Afterwords, install the interior back in the car. Fix any panel buzzing with weather stripping. Then while driving the car and the stereo blasting, check for panel vibrations that are producing sound. There probably won't be too many, if any. Treat as necessary with CLD. To go a step further, line behind all plastic panels with open cell foam matting (think upholstery foam cubes cut to size). Under the dash, put in a layer of open cell foam matting if you have room.
At this point, the car should be pretty damn quiet.
