Three-dimensional acoustic analysis of circular expansion chambers with a side inlet and a side outlet

Abstract

The effects of higher-order acoustic modes upon the performance of a circular expansion chamber with its inlet and outlet located on the cylinder wall are investigated. The sound distribution in the expansion chamber is obtained, theoretically, considering the influences of the diffracted sound pressure generated by the presence of end plates in the chamber. A theoretical method for the estimation of the transmission loss is suggested by using the derived four pole parameters, assuming that the chamber can be modeled as a piston-driven circular rigid cylinder. A series of experimental observations are taken for verification, and it is found that they are in good agreement with the theoretical results. The influences of those modes are studied for a variety of chamber lengths and combined inlet/outlet locations. As a result of this study, it is concluded that the diffracted sound pressure affects the performance of the expansion chamber considerably in the range above the cutoff frequency of the n=1, m=0 asymmetric mode, and that the plane-wave theory can hardly be adopted even in the low-frequency range where the fundamental mode prevails.