Updated: Oct 30, 2021
I want to take a moment to explain a very popular product, but from an installers point of view. Many conversations I have read in regards to sound deadening I see people focusing on price and brand name, but I worry that they may be overlooking some other factors that an installer like me would focus on. I try not to focus so much on the brand name, but more the type of deadener, how it will be installed, and the benefits of using each type of material. So before I start, I want to just mention some of the reasons someone might want to deaden their vehicle. Deadening will of course help lower road noise by blocking it out of the cab of the vehicle, but it can also help lower thermal/heat transfer from the inside to the outside of your vehicle as well. And the number one reason that I see most people using deadener: to reduce panel vibration. Without getting too technical, the following is a brief overview of the materials that I have encountered and how I have used them. Foil Backed Butyl - This is the most common product on the market. The main use of this is to absorb panel vibrations. The butyl is almost like a sticky rubber consistency and it absorbs vibrations very well. Usually has a foil or metal lining on the top side. This will lower panel resonance, and help make the outer door skin of a vehicle sound more "dead". If you were to tap your finger on it vs an untreated piece there will be a significant difference in the sound and resonance.
You will want to clean and dry the surface of the panel/vehicle before install. You can peel and stick this material, Installation is easiest when its not really cold outside. Be careful when placing it, if you don't like exactly how it lays down you can usually peel it back and reapply several times UNTIL you press the product down to adhere the sticky side. I use a special roller tool for this purpose, it will leave a different imprint or design on the deadener (as shown in the picture below, right side is placed without rolling, and left side is rolled) to let you know when it is pressed down properly. The pressing action helps the adhesion and performance of the material.
ABS or rigid thin material can be used to help block off some of the large access holes in the door panel or body cavities. Doing this will allow you to block even more road noise from entering the cabin, and give a good surface to place more product. Using a rigid material like ABS will allow you to make the panel removable which will be helpful if you ever need to service the door locks or window tracks. Making a panel like this can also help to make a larger and more sealed enclosure for the speaker out of the door itself which can improve the mid bass response. You do not want to do this with just the foil backed butyl because it is not rigid enough to span large spaces like a sheet of 1/8" or 1/4" ABS can. So usually this will go underneath the foil backed butyl, or layers of foam. MLV (Mass Loaded Vinyl) is a thin sheet that is great to go above the deadener and behind the foam. It will help block sound and helps with thermal insulation as well. Some brands might have this layer built in with another material. I have seen this come a few different ways. I have had to glue this into door panels in the past so don't be shy to use some contact adhesive. Closed Cell Foam is moderately dense. It can help with thermal and sound insulation. This is a great material to put over the top of the foil backed butyl for an extra layer of insulation. Usually this is available in peel and stick sheets as well but I have seen in some instances where you need to use your own contact adhesive. Open Cell Foam is less dense and looks more like a sponge or a cake. Open cell foam is used to absorb higher frequencies that would normally bounce back off of harder surfaces. I see open cell foam come in speaker baffle kits (to absorb and diffuse the backwave) and also I have noticed that with some component speakers they will include open cell foam tweeter rings that can be used when rear loading a tweeter into a factory cavity. This open cell foam will absorb any higher sound that goes into it, reducing the reflections and cancellation.