A Low Frequency Muffler based on the Acoustic Black Hole Effect, by Neha Sharma (University of Salford)

DENORMS Action’s Workshop “Modelling of high performance acoustic structures Porous media, metamaterials and sonic crystals”, Rome, 24-25th January 2017

Website of DENORMS Action

Programme of the Workshop

Acoustic metamaterials (i.e. subwavelength absorbers), metasurfaces and sonic crystals for audible sound manipulation

Speaker: Neha Sharma (University of Salford)

Abstract: 

A significant amount of research has been done during the last few decades, with the aim of achieving better absorption of sound through systems, serving a broad range of applications. Besides improving the efficacy of the existing technologies, new solutions are being sought after that can enhance the quality of sound and in turn, the overall experience of the device. The application of meta materials as sound absorbers offers a new approach to this purpose. Sub-wavelength absorbers that work on the principles of Acoustic Black Hole effect have a promising impact on achieving total absorption, at least, in theory. However, there have been a few numerical and experimental studies backing up one such research idea [Mironov and Pislyakov, Sov. Phys. Acoust., 2002]. In this work, a theoretical basis has been established for acoustic wave propagation through a terminating muffling structure that completely absorbs the wave by the black hole effect.

In the present study, a similar principle has been applied to design an open expansion chamber for effective absorption of low frequency sound. A comparison between closed and open structures has been made to understand the applicable differences. The muffling section, embodying the open expansion chamber, comprises of a narrow tube of axi-symmetric waveguide with constant (or weakly varying) cross section. To achieve impedance matching, the plane wave radiation incident through the inlet end of this muffler, is subjected to a protruding flair of varying wall admittance. The design concept has been analyzed both semi-analytically and numerically, targeting low frequency applications. Exact solutions have been obtained for particular geometries which also account for the absence and presence of losses.The impact has been studied based on the reflection and absorption coefficients. A good agreement between numerical and analytical results is demonstrated. This on-going work specifically targets applications with silencers and mufflers, including automotive and aviation sound attenuation, as also vents and ducts, where generally, due to the customer driven constraints associated with the muffling section, there still persist issues in achieving the desired sound attenuation.