Multi-mode sound propagation in pod silencers

Abstract

Pod silencers are dissipative silencers used in HVAC applications in which the absorptive material is used to line the outer wall of the duct and a central cylindrical pod is inserted to form a silencer with an annular flow passage. Pod silencers are used in a wide range of ductwork, from the relatively small to very large applications with duct diameters of the order of several metres, and so in general silencer performance is likely to be influenced by higher order mode propagation. Accordingly, a numerical mode matching technique is used here to model higher order mode propagation in a pod silencer. A comparison of predictions with those previously found assuming plane wave propagation is presented and it is demonstrated that, even for relatively small silencers, higher order modes play an important role in the performance of pod silencers.