Transmission LossAnalysis of Expansion Chamber Mufflers with Mean Flow

Abstract

Based on a variational formulation, a generalized three-dimensional acoustic finite element model is first developed to analyze the acoustic characteristics of expansion chamber mufflers with mean flow. The convective effects induced by mean flow are taken into account in the mass, damping and stiffness matrix of the finite element model. In addition, the boundary conditions, including non-reflex terminations and absorptive materials, are derived for transmission loss (TL) analysis. By doing so, the TL with the factors of mean flow, area expansion ratio and absorptive materials is then calculated. Calculation results correlate well with the plane wave theory and those in previous literature for expansion chambers. The effect of mean flow on TL analysis of expansion chambers is obviously found at high frequency. The proposed method is also employed to assess the performance of ladder mufflers. Finally, a novel expansion chamber muffler with inside perforated plates is designed and its effectiveness demonstrated herein.