DOUBLE-TUNING OF CO-AXIAL INLET-OUTLET ELLIPTICAL CHAMBER MUFFLER

Abstract

The simplest improvisation on the simple expansion chamber (SEC) of length L to get the wide band transmission loss (TL) spectrum is to extend one of the inlet and outlet pipes acoustic length by L/2 and other by L/4 inside the chamber. Evanescent higher modes at the junctions of the extended inlet/outlet pipes and muffler chamber are generated to satisfy the compatibility condition of acoustic particle velocity at junctions. The effect of the evanescent higher modes can be approximated as an inline inertance of the extended pipes which makes the acoustic lengths of the extended pipes more than their geometric lengths. Therefore, the lengths of the quarter-wave (Q-W) resonators surrounding the extended inlet/outlet pipes increase. The difference between the acoustic length and the geometric length of an extended pipe inside the chamber is known as the end-correction of the respective pipe. The 3-D finite element analysis of an extended inlet/outlet muffler with extended pipes of L/2 and L/4 inside the chamber results in the leftward shift of the frequency in the TL spectrum due to the Q-W resonators. These shifts in QW resonance frequencies are used for the computation of end-corrections. For an elliptical chamber, the cut-on frequencies of higher modes depend primarily on the major axis length D1 and aspect ratio. Thus, the end-corrections of the extended inlet/outlet pipes of the same diameters would be different of a circular chamber and elliptical chamber for the same cross-sectional area. In the present study, a multivariate regression analysis is performed to develop parametric expressions for the end-corrections of the extended pipes in terms of the aspect ratio, L/D1 ratio, the cross-sectional area ratio of the chamber to extended pipes, and the pipe wall thickness to diameter ratio. These expressions can be used to design a double-tuned co-axial inlet-outlet elliptical chamber muffler. © 2023 Proceedings of the International Congress on Sound and Vibration. All rights reserved.