Exploration of Acoustic Metasurfaces

Abstract

The exploration of acoustic metasurfaces presented in this Thesis involves the characterisation and verification through finite element method (FEM) modelling and experimentation of a range of different acoustic metasurfaces. In airborne acoustics, the patterning of sub-wavelength structures on acoustically-rigid material provides the boundary condition that enables acoustic surface waves to exist. These surface waves exist purely in the fluid layer above the rigid material and propagate parallel to the surface, and are evanescent perpendicular to the surface. The first study explores the radiative and bound acoustic modes supported by a rigid grating formed of three same-depth, narrow grooves per unit cell. One of the grooves is twice the width of the other two, forming a ‘compound’ grating. The structure supports so-called ‘phase’ resonances where the phase difference of the pressure field between the grooves on resonance varies by multiples of π. The dispersion of these modes has been measured experimentally by monitoring the specularly reflected signal as a function of the angle of incidence. In addition, by near-field excitation, the dispersion of the non-radiative surface modes has been characterised. The results are compared with the predictions of a finite element method model. The acoustic surface waves supported by hard surfaces patterned with repeat period, meandering grooves are next explored. The single, continuous groove forms a glide-symmetric surface, inhibiting the formation of a bandgap at the first Brillouin zone boundary. Consequently, the acoustic surface waves exhibit an almost constant, sub-speed-of-sound, group velocity over a broad frequency band. The dispersion of these surface waves has been experimentally measured by a near-field scanning technique and compared to finite element modelling. In addition, the influence of covering the straight sections of the channels has been explored. Covering the channel reduces the coupling strength to free radiation which has been shown to significantly alter the standing wave condition at the first Brillouin zone for small channel depths. For such structures, the standing wave condition now comes from an open-ended cavity resonance. In the final results chapter, underwater acrylic plates are investigated. Due to the change in the fluid, the solid may no longer be regarded as being acoustically-rigid, and acoustic energy propagates into the solid. Because of this, even on a flat surface, surface acoustic waves, Scholte waves are found at the interface of the solid and fluid. Here in particular, so-called soft solid (acrylic) plates are investigated where the shear velocity of the solid is less than the speed of sound in the fluid. The effect of adding structure to thin acrylic plates has been thoroughly explored through FEM modelling. By adding periodic grooves to the plate, unusual dispersion characteristics have been shown. The hybridisation of the modified Scholte-like modes with the cantilever resonances of the solid pillars has been described. The effect of varying grating parameters on the band diagram has been explored, showing that the dispersion of the modes is highly dependent on the structure. Experimental verification was performed on a simple grating (one groove per period) showing good agreement with the FEM model.