Propagation of acoustic waves in a metamaterial with a refractive index of near zero

A metamaterial system with a refractive index of near zero for acoustic wave is realized. The designed system, which uses Helmholtz resonators, is analyzed using the equivalent circuit theory. Through the simulation and the experiment, we observed a large phase velocity in the metamaterial and a concentration of the energy density in the narrow normal waveguide directly coupled to the metamaterial. This is different to the metamaterial for the electromagnetic wave, because the concentration of energy density of the electromagnetic wave occurs in the metamaterial inside. The acoustic metamaterial system can be applied to the development of device for the acoustic energy concentrator and the acoustic filter.