By far the most common type of material. It uses mass loading to lower panel resonance and absorb structural vibrations, converting them into low level heat. Most commonly used throughout a vehicle to control structural related noises. They can also be used in conjunction with a composite to reduce road/engine/exhaust noise.
As the name suggests, these products literally absorb airborne sound waves. The sound waves have to propagate through the absorber and the open/closed cell foam will slow it’s progress reducing the overall level in the process. Most commonly used on floors and firewalls to reduce road/engine/exhaust noise.
Best results are achieved by using more than one of sound deadening, so a composite is quite simply a combination of two of the above types into a single easy to install product.