Logan Schwan

Complex dispersion relation of surface acoustic waves at a lossy metasurface

The complex dispersion relation of surface acoustic waves (SAWs) at a lossy resonant metasurface is theoretically and experimentally reported. The metasurface consists of the periodic arrangement of borehole resonators in a rigid substrate. The theoretical model relies on a boundary layer approach that provides the effective metasurface admittance governing the complex dispersion relation in the presence of viscous and thermal losses. The model is experimentally validated by measurements in the semi-anechoic chamber. The complex SAW dispersion relation is experimentally retrieved from the analysis of the spatial Laplace transform of the pressure scanned along a line at the metasurface. The geometrical spreading of the energy from the speaker is accounted for, and both the real and imaginary parts of the SAW wavenumber are obtained. The results show that the strong reduction of the SAW group velocity occurs jointly with a drastic attenuation of the wave, leading to the confinement of the field close to the...

Coupling FEM, bloch waves and TMM in meta poroelastic laminates

The propagation of airborne plane waves in the presence of a meta poroelastic laminate, that is a poroelastic matrix coated with thin elastic layers at its facings and periodically-embedded with in ...

Sound propagation in the presence of a resonant surface

The interactions between acoustic waves and an array of resonators are studied. The resonators are arranged periodically on an impedance surface so that the scale separation between sound wavelength and the array period is achieved. An asymptotic multi-scale model which accounts for viscous and thermal losses in the resonators is developed and is used to derive an effective surface admittance. It is shown that the boundary conditions at the surface are substantially modified around the resonance frequency. The pressure field on the surface is nearly canceled leading to a phase shift between the reflected and the incident waves. The array can also behave as an absorbing layer. The predictions of the homogenized model are compared with multiple scattering theory (MST) applied to a finite size array and the limitations of the former are identified. The influence of the surface roughness and local scattering on the reflected wave is discussed.