Unfortunately, restriction is always accompanied with decreased flow, resulting in back pressure.
Back pressure is the increase in exhaust system pressure that negatively affects exhaust flow. Many restrictive stock exhaust systems can create excessively high back pressures that can exceed 5-6 psi. This generally results in a pronounced hissing sound coming from the exhaust system under WOT (wide open throttle). Merely adding a performance glasspack will result in a dramatic performance improvement.
Absorption Method – One of the most popular ways to muffle sound. The glasspack muffler is a good example. The Glasspack muffler consists of a tube (usually the same size or slightly bigger than the exhaust pipe) is punched with louvers and then wrapped with an absorbing, heat resistant material such as fiberglass matting. It is then covered with the outside casing. As the sound waves travel through the muffler the louvers allow the sound waves to enter the fiberglass never to return.