Metamaterial characterization for acoustic cloaking applications

Acoustic metamaterials rely on sub-acoustic wavelength materials to achieve their unique material properties. An application of interest requires acoustic metamaterials that are homogeneous but anisotropic materials. Previous work has demonstrated at least three types of unit cells exhibit homogeneous anisotropic mass density and homogeneous isotropic bulk modulus. An analysis into the methods used to characterize the effective material parameters of different unit cells with water as the background fluid was performed. Nondestructive acoustic excitation techniques were used to extract the material parameters of different grades of foam to identify use in an acoustic metamaterial to determine if they exhibited the desired homogeneous anisotropic behavior. Different grades of foam glass exhibited varying degrees of anisotropic stiffness. The metamaterial unit cell behaved as expected, demonstrating anisotropic mass density and isotropic bulk modulus.