Tunable acoustic metamaterial elements, neutral inclusions, transparency and causality

Acoustic metamaterial elements based on cylindrical elastic shells with an internal mass supported by axisymmetric stiffeners are described. The shell´s effective acoustic properties are tuned by varying the mass and stiffness. The elements can be impedance and index matched to water, resulting in near-zero scattering cross section over a broad range of low frequencies. The physical constraints necessary for low frequency transparency imply that the scattering is non-causal which means there must be scattering at high frequency.