Elastic shells with high-contrast material properties as acoustic metamaterial components

We analyze the acoustic multiple-scattering properties of fluid-filled, elastic cylindrical shells with highly contrasting material properties, and we find for a water background that air-filled shells homogenize into high-bulk modulus, low-density effective fluids. With the exception of a few local resonances spanning very narrow band windows, we find that for common elastic materials the shells are indistinguishable from their effective fluid counterparts for wavelengths larger than the shell's outer diameter. Furthermore, we find that when the elastic shell is composed of a material with impedance larger than water, there will be a specific shell thickness for which the effective fluid properties become impedance-matched. Finally, we demonstrate that the shells can be used as constituent components in regular lattices to create homogenized acoustic metamaterial devices.