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The present research work investigates the influence of internal 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 (ABH) effect. These structures work on the principle of impedance matching, and in theory, realized to achieve total absorption of sound in closed terminating channels. Sub-wavelength absorbers based on the ABH effect are being currently researched upon both numerically (O. Guasch, et. al., 2017) and experimentally (A. El-Ouahabi, et. al., 2015). Theoretically, they have been derived to achieve total absorption of sound (M. A. Mironov and V. V. Pislyakov, 2002), thus representing an ideal ABH. Based on similar principles, theoretical basis has been established to develop semi analytical and numerical models for an open expansion chamber, embodied by the muffling section (N. Sharma, et. al., 2017). 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. This study considers the impact of lining, essentially comprising of nearly periodic internal ring like structures, in absence and presence of losses due to visco-inertial and thermal energy exchanges. Parameters such as the transmission loss, absorption and reflection coefficients and the likes have been discussed for understanding the practicalities associated with sound absorption through open terminating muffling systems. The results show that there is good agreement in the models developed and the numerical validation. The findings of this work 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 have a high potential in yielding quieter machineries in times to come.
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