Kirby R.

Pod silencers are dissipative silencers used in HVAC applications where the ductwork has a circular cross-section. Normally, 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. This central pod may be shown to increase sound attenuation over and above that derived from the outer lining. Pod silencers are, to some extent, similar in their design to splitter silencers used in rectangular ductwork. Moreover, like splitter silencers, they are used in a wide range of ductwork, from the relatively small to very large applications with duct diameters of the order of several meters. This means that, for medium to high frequencies, the performance of the silencer depends heavily on the propagation and attenuation of higher order modes. Accordingly, when modelling pod silencer performance, it is important to account for higher order mode propagation, both in the inlet/outlet duct section as well as in the silencer itself. This paper adopts a numerical mode matching approach (point collocation) to examine sound propagation in large pod silencers and compares results to those previously found assuming plane wave propagation only. Included in the model is the effect of a perforated sheet separating the absorbent material from the airway, and a “bull-nose” fairing on the inlet/outlet plane of the silencer. Transmission loss predictions are presented for representative silencer dimensions in the absence of mean flow.