Hybrid silencer transmission loss above a duct’s plane wave region

Abstract

For large ducts, the removal of low frequency and tonal noise is normally achieved through the use of inefficient dissipative silencers; however, a combination of dissipative and reactive solutions could be more effective. But reactive noise control solutions are rarely applied to large diameter duct systems since it is commonly assumed that the low cut-on frequency of higher order modes severely restricts their efficiency. However, it is possible for a reactive silencer to remain operational outside of the plane wave region, provided the reactive elements are distributed across the cross-section of the duct. Of course, at higher frequencies, the sound field within a duct will have nonplane wave modal content, and the transmission loss is expected to differ compared to the plane wave condition. This effect is investigated here using numerical (FEM) predictions for hybrid dissipative-reactive parallel baffle silencers and the performance of the reactive elements is explored under different excitations. The effects of non-planar fields and individually excited modes are analyzed, and it is found that the frequency range over which quarter wave resonators contribute to transmission loss can be extended above the cut-on frequency of the duct by increasing the number of baffles.