Quasi-two-dimensional acoustic metamaterial with negative bulk modulus

We present the experimental realization and characterization of an acoustic metamaterial with negative bulk modulus. The metamaterial consists of a two-dimensional array of cylindrical cavities, and the bulk modulus is controlled by their radius size and length. Experiments are performed in a two-dimensional waveguide where a slab of seven layers is used to extract the parameters of the metamaterial. A complete characterization of the constructed structure is reported, including the dispersion relation of the acoustic bands and the skin depth effect, which both have been measured, and the data are well supported by semianalytical models and by finite-element simulations.