Modelling of low frequency muffler based on acoustic black hole effect

Sharma, N

Abstract

This thesis investigates the influence of acoustic metamaterials, or linings with graded properties, on the performance of open termination ducts and mufflers working within low frequency range. The functioning of such linings has been associated to the Acoustic Black Hole effect. These structures work on the principle of impedance matching, and in theory, have been realized to achieve total absorption of sound in closed terminating channels. This theoretical understanding, backed up with numerical analyses, has been reviewed to develop the concepts underlying this research work.

Firstly, a semi analytical model has been formulated for an open expansion chamber, embodied by the muffling section. Plane wave radiation incident through inlet end of the muffler, is made to undergo impedance matching while traversing through the metamaterial lined flare of varying wall admittance. The impact of lining, in absence and presence of losses due to visco-inertial and thermal energy exchanges has been analyzed. Parametric variations of flare shape and muffler dimensions have been compared. Finally, considering particular cases, comparative analysis with finite element method based model has been made. The semi-analytical models have been validated against numerical results for acoustical properties such as transmission loss and reflection and absorption coefficients in the transmission regime, while observing a good agreement between them.

It has been concluded that the structured lined flare has higher performance capability than the simple expansion chamber. Power law function based flare shape can be considered to be steady in operation over the low frequency range. The findings of this thesis show that the application of acoustic metamaterials for sound absorbing or muffling devices has a promising future. Thus, industrial ducts, vents and mufflers that form a major part of such systems, when augmented with the metamaterials can hopefully yield quieter machineries in times to come.

Citation

Sharma, N. (in press). Modelling of low frequency muffler based on acoustic black hole effect. (Dissertation). University of Salford

• Thesis Type: Dissertation
• Acceptance Date: Mar 19, 2023
• Deposit Date: Feb 26, 2018
• Publicly Available Date: Feb 26, 2018