Advances in the acoustics of flow ducts and mufflers

Abstract

This review paper deals with advances made in the last two decades in the acoustics of flow ducts for heating, ventilation and airconditioning (HVAC) systems and engine mufflers. The context, concepts, methods used and results have been highlighted. Frequency-domain one-dimensional analysis of reflective mufflers has been emphasized because of its basic importance and wide application. Finally, problems needing further research have been identified.

Muffler modeling by transfer matrix method and experimental verification

“…In the late fifties and sixties, equivalent electrical circuits based on the four-pole transmission matrices were widely used to predict the muffler performance through the use of analogue computers (Igarashi and Toyama, 1958;Davies, 1964). In the seventies and eighties Alfredson (1970), Munjal (1970 and1987), Thawani and Noreen (1988), Sullivan and Crocker (1978) and Jayaraman and Yam (1981) presented approaches to the partial and full modeling of mufflers. In addition, Craggs (1989) reported a technique that combines the use of transfer matrix approach and finite elements in the study of duct acoustics.…”

Modal filters for mitigation of in-duct sound

“…For shorter filters, oscillations in the Transmission Loss were detected, resulting from the superposition of reflections from the filter inlet and outlet. This finding corresponded to the behavior known from established silencers [3]. For the subsequent computations in this paper, filter lengths of 4D were chosen to guarantee linear filter behavior.…”

“…Common characteristics of a silencer are the Insertion Loss IL and the Transmission Loss TL [3]. The Insertion Loss relates the acoustic power at the inlet and the outlet side of the silencer under working conditions, i.e.…”

“…For both these extreme cases a large number of publications exist and a reference work on the subject is Ref. [3]. The intermediate and mid-frequency range, however, where the plane wave and a low number of acoustic modes propagate, is much less treated in the literature on silencers.…”

“…For model validation, a filter prototype consisting of four plates is built and its acoustic properties are measured. The performance of silencers is commonly described by the Transmission Loss or Insertion Loss [3]; those values, however, do not represent the actual absorption inside the damper. Therefore the Power Loss is computed, which requires the decomposition of measured and computed acoustic pressure fields into their complex modal amplitudes.…”