Controlling Sound with Metamaterial Waveguides and Acoustic Surface Waves

Abstract

This thesis will present four experimental studies into controlling sound in air using 1D arrays of resonant elements or ‘meta-atoms’. This work will explore multiple approaches to tailoring dispersion, starting with an initial metasurface design (a 1D periodic array of blind holes) and developing the complexity of the dispersion spectra through first, symmetry and translations conditions and later, additional coupling mechanics. In the first experimental chapter, the bound acoustic modes of a periodic array of resonant cavities exhibiting both mirror and glide-symmetry are explored. This study investigates the use of symmetry and periodic translation conditions to support additional modes. The samples, each comprised of a single array of cavities, are arranged to allow the coupling between the acoustic modes supported by each surface to be controlled. The investigated system supports a pair of modes, which can either form a band gap or be degenerate at the 1st Brillouin zone boundary, as a result of the symmetry condition imposed. The second experimental chapter builds upon the effects of the symmetry and periodic translation conditions detailed in the previous chapter by exploring Frieze symmetry groups. A Frieze group is a set of symmetry and translation operators which can be used to tile a 1D strip using an initial geometry. Desirable dispersion effects, such as band gaps and degeneracies can be achieved without the symmetry and translation conditions used in the previous chapter. The dispersion of seven samples, each demonstrating a unique Frieze group were experimentally characterised. The third experimental chapter expands upon the internal connections required to explore Frieze groups by extending their length to connect next-nearest neighbouring unit-cells. This chapter investigates the effects of distant interactions, in the form of next-nearest neighbour coupling on the dispersion of ASWs. These distant connections, themselves resonant cavities, support a standing wave at a wavenumber within the 1st Brillouin zone and exhibit regions of negative group velocity. The final experimental chapter introduces membranes to an array of resonant cavities to support a pair of ASWs. Where the previous studies used resonant cavities which require a greater volume to achieve a lower resonant frequency, a membrane derives its resonance from its elastic properties, as well as its dimensions. A method for producing membrane samples using additive manufacturing is explored. This coupled system supports a mode which can be reduced in frequency by a half, when compared to cavities without membranes, by tuning the resonant frequencies of the membrane and cavity.