The outflow of combustion gases creates noise. The purpose of the silencers in an exhaust system is to reduce or dampen this noise.
A distinction is made between the following silencers depending on their position in the exhaust system:
As catalytic converters and particulate filters already dampen a good deal of the noise, the use of front silencers is becoming less and less common with diesel engines.
The design of the silencers is influenced by the following factors:
There are two ways of reducing or preventing the propagation of sound:
Sound insulation or absorption
With absorption, sound waves are converted into heat by friction in a sound absorption material. These are usually porous materials in the form of fibreglass, which does not represent a health hazard.
Sound dampening or reflection
With sound dampening, reflective barriers are arranged so as to prevent the propagation of sound. Such barriers can take the form of changes in cross-section or pipe baffles which reflect the sound.
Variations on this type of sound dampening are interference and restriction systems. With interference dampening, the flow of exhaust gas is split up and routed into pipes of different lengths. This means that the sound waves have to cover different distances. When they meet up againthey collide and cancel one another out. With a restriction system, perforations and pipe constrictions ensure that the flow of exhaust gas is broken up and thus dampened.
Most modern silencers employ a combination of different dampening mechanisms - either in the form of separate silencers (centre and end silencers) or a single combined silencer.