Sound reflection at a duct open end induces resonant amplification of acoustic energy in the duct and causes high intensity sound radiation. This phenomenon sometime causes low frequency noise pollution radiation from pipe and some self-sustained acoustic oscillation in machines. In order to reduce such kind of acoustic resonance, acoustic damping should be added to the duct. Sound reflection coefficient at a duct open end is determined by its outlet shape and wave length. Sometimes a non-reflecting horn with sound absorbing materials is used at the open end in order to realize non-reflecting condition. But, as for low frequency sound, it becomes too huge to be set on real application. In this paper two different outlet shapes; unflanged straight duct and horn that are made of Micro-Perforated Panel (MPP), which can provide wide-band absorption without fibrous and porous materials will be presented. Analytical and 3D Finite element solutions for both cases are presented and used throughout this analysis. The numerical and analytical solutions are compared with measurements and it has been shown that sound reflection and radiation at that open end can be remarkably reduced. The duct side wall next to open end that is made of appropriate MPP plate can reduce the radiated sound up to 5 dB(A) when L/D =10 (pipe length to its diameter). It can be used as an efficient noise control element in automotive intake and exhaust systems.

sound reduction, reflection coefficient, insertion loss, acoustic damping, duct outlet, fem, exhaust system, tail pipe