Which damping mat is the best? The quick answer is, you probably don't need it in the first place and if you do get some, it's all the same.
If you do think you need it, lets look at the hype vs. science.
There are many, many brands of damping material. Dynamat which is one, if not the original damping mat brand on the market. It's a top tier brand and is one, if not the most expensive brand. Dynamat's claim to fame is that you get +3db or twice the amount of sound from your system by using their product.
This claim is supported by damping or deadening your listening environment to lower the road noise in your car, thus lessening the power requirement needed by your stereo system by half which is equal to -3db. First off, -3db is not very much at all, however this claim is sort of true but in order to achieve it, you have to use hundreds of pounds of it.
Lets look at the original low road noise listening environment, your home. Home audio systems can use a fraction of the power of a typical car audio system and still sound fantastic! The secret to great listening environment air isolation. Air isolation along with vibration / distraction elimination are really what make a system sound great whether it be home, car, RV or spacecraft. Because air is what transfers sound to you, if you want the sounds outside to stay outside, make your listening environment isolated from those sounds.
For homes, most use a typical double wall system with fiberglass insulation in between to hold still even more air in place. (Outer sheath and drywall inside surrounding fiberglass, straw, shredded blue jeans or newspaper). Other techniques like adding a vapor barrier as well as encasing the home in foam and then stucco or siding increase the isolation through multiple barriers to keep the outside out and the inside in. Windows of course increase their R value, (Thermal Resistance), if you use a double or even triple pane design for the best isolation or R value..
For cars, the same practices can be employed to greatly enhance your car's ability to isolate the air. Many of these techniques are already in place for high end cars. I'm not sure if Lexus still does it, but they used to make a big deal about actually laminating dual walls of steel with a soft center to reduce noise. This also helps keep the transfer of heat and cold to make it into the cabin as well. Mercedes Benz has been known to utilize double pane glass to aid in this as well.
Most car makers just use asphalt based damping material. I find it funny that many sound damping material enthusiasts will actually go through the effort of removing this damping mass to simply add their newer version of the same material.
How it works
Damping mats / sound deadeners works in two ways. The first is by changing the resonant frequency of the 'noisy' panel in need of damping. (Yes the material also turns vibration into a tiny amount of heat but the heat signature is very tiny, barely worth mentioning. The mat will not feel hot after a big bass session.) The mass, any mass for that matter will change the resonant frequency of the panel. The mass need only to be attached firmly enough to withstand the flex of the panel. This added mass simply lowers the resonant frequency, it does NOT remove it. When the resonant frequency is lowered to a frequency that is no longer audible or at least no longer annoying, mission accomplished. Any added mass does not add to your audio efforts. If you want to add to making your vehicle louder, you'll need to remove noisy devices / panels by either strengthening panels and or bracing. This eliminates the transfer of energy from inside the car to the outside. This is why having your buddies lay all over your car during an SPL run pays off. Your ride is not well braced.
Damping mats do not strengthen your vehicle, they only add mass. With this in mind, you can use an infinite number of devices and methods to add mass to your noisy door and body panels. Some brands, (Second Skin), would actually sell a device known as an, "SPL Tile". This was simply a normal, everyday and very cheap ceramic tile glued to your door panel. Did it work? Yes. However the video by Buzz was taken down due to the quick erosion of marketing advantage, (People realized the doucheyness of someone selling you something you can buy cheaper and without shipping down the street rather than online.), and replaced the product with simply a small section of the same damping mat that comes in rolls, the name stayed the same.
I actually pointed out this idea by making your own damping material on the DIYMA forum. Little did I know that the forum was owned by Ant, (Anthony Collova), who also owned Second Skin at the time. When asked if I included Second Skin as one of the products that just add mass and are overpriced, I agreed. I do apologize to Ant as I assumed that a forum called DIY Mobile Audio would filled with ways to DIY, (Do It Yourself). My post there was unfortunate and out of place. I just wish, this video, were in place before I posted. Now I know better and posting that information here is more appropriate.
So if you want to make your own damping material for your car or boat, the recipe we use is equal parts of siliconized acrylic calk mixed with pure silicone calk. Your added mass can be sand, we use 30 grit from the masonary store. You can also just use pebbles, washed gravel or rocks, lead / fishing weights or even a tape dispenser. Other items include Amy Riordan's old retainer , broken speaker magnets or my favorite, all the lead droppings found at the shooting range. All the materials can be obtained locally for most users. For users that come see us locally, that don't want to do this and refuse to go to the hardware store to buy their own Peel & Seal; we sell de-badged Peel & Seal for $30 a roll and we still sell about 2 rolls a week. Go figure. You can also order the Peel & Seal under the Mule brand from many roofing wholesalers. Alibaba is also a great site to check out if you feel like importing a pallet or two for you, your friends or your shop.
Remember that with the use of any damping material in your ride, you're adding weight. The typical application for a loud system user, the amount needed for complete damping of a car is several hundreds of pounds, several layers of material. Then add in the weight of the equipment and you'll need to beef up the suspension to keep up with all of that mass. You of course compromise fuel efficiency and handling for a quiet ride. Hopefully you didn't ignore the door jams and firewall after spending loads of cash for something you could have bought locally for much less and your car is pretty awesome in the first place right? If this is not the case, this good sir, makes you a Douche aka a Bro, bro.
You can however redeem yourself if this is just a case of ignorance. Correct your course and the musk will fade from your ride over time. However, if you continue to use machismo and cash to solve problems, you are on a crash course to hanging out with Ant, (4:35), doing totally non-gay things like showing off your muscles with other mostly naked dudes. You can totally smell the macho nachos!
To wrap it up, educate yourself. The most valuable real estate is the gray matter between your ears. Hopefully what I've shown today will save you money and most importantly, time. Remember that branding can be blinding. Take commodities for example. In the grocery store, do you care what brand of sugar you buy? It's all the same stuff and even comes from the same place, but some people do care and are willing to pay more for for it. As a consumer this is a rip off. As a vendor this is the art of branding and marketing. She's a lovely devil that will always have you wanting more. However when followed blindly, she will screw you, bleed you and roll you. Use your head, pick your compromises and don't be afraid to change your mind when everyone else is doing it.
Co-Founder of Robot Underground