Dynamat is the name of the Greek god of car audio. Legend has it, if you leave a burnt voice coil under your pillow, he'll bless you with a 2.3db gain at your next competition.
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Seriously, dynamat is a name brand of sound dampening material. it's meant to cover bare metal surfaces in your vehicle that rattle because of your subwoofer. It also is helpful in eliminating road and engine noise, and is pretty much a necessity for a professional sounding system. Think of dynamat as acoustic insulation. It keeps sound in your car the same way the insulation in your house keeps heat in.
There are dozens of brands similar to dynamat for a lot cheaper, so if you plan to get dynamat for your ride, shop around and you'll find something just as effective for cheaper. I got my "dynamat" for free: A huge roll of 3/4" foam rubber padding I found in the dumpster at my workplace.