Silencers are installed within the exhaust system of most engines. It is engineered as an acoustic sound-proofing tool made to reduce the loudness of the sound pressure orginited in the engine by the process known as acoustic quieting.
There are a series of passages and chambers lined with roving fibreglass insulation in the silencer system. There are also some resonating chambers harmonically tuned to create the effect of destructive interference wherein opposite and similar sound waves cancel each other out.
Some people remove or install an aftermarket silencer to increase power output or reduce fuel consumption, for recreational reasons such as motorsport and hypermiling and/or for personal sound preferences. In many countries, modification of a vehicle's silencer is usually highly regulated if not strictly prohibited.