16 Silencers

Pagina 1

Silencers Practical silencers are complex devices, which operate reducing pressure oscillations before they reach the atmosphere, producing the minimum possible loss of engine performance. However they are composed of many simple elements based on only two basic principles: absorption and/or reflection of sound waves. The most common simple elements of silencers are: a) sound absorbing element: a chamber filled with sound absorbing material, through which a perforated exhaust pipe passes. Then part of the acoustic energy, carried by the flowing waves, is absorbed and dissipated as heat by the elastic material. b) expansion box: is an enlargement of the exhaust pipe, which modifies the system impedance, in order to mismatch the box inlet duct from the outlet pipe. The result is that only a part of the acoustic power, carried by the incident pressure wave, is transmitted toward the exhaust system exit. c) side resonator: is a separate closed chamber communicating with the main exhaust pipe through holes or short side branches. Then intense attenuations of the pressure levels are obtained for noise components with the same frequency of the natural resonance of the lateral chamber. d) perforated element: is formed by ducts, with many small holes on their lateral surface, which go through cavities. When the pressure inside the perforated duct is larger than in the surrounding volume, exhaust gases pass from the duct to the cavity and vice versa, when the pressure difference is reversed. Since flows through the holes dissipate acoustic energy, this element produces reduction of sound oscillations both by viscous dissipation and by ware reflection. In the first element the absorption and dissipation of the sound energy is clearly dominant, while the other three are called…