B-Quiet Sound Deadening Dampeners are by far the most common material in the industry.  They work by adhering the dampening material, B-Quiet Ultimate or B-Quiet Extreme to the structure you are trying to quiet down.  By mass loading the panel it reduces the resonance and absorbs structural vibrations, converting them into low-level heat.  They can be used on their own to improve the sound from your car stereo, reduce the noise from the road and reduce engine noise or they can also be used in conjunction with a composite to reduce road/engine/exhaust noise.