Compact acoustic double negative metamaterial based on coexisting local resonances

Acoustic metamaterials generally exploit resonances to assume negative properties. While many types of resonances can be used for achieving a negative bulk modulus, the number of types of resonances for generating a negative mass density is limited. In this work, a double negative airborne acoustic metamaterial is proposed, whose negative density is achieved from Fabry-Pérot resonance. More specifically, each unit cell of the metamaterial comprises only a single element, allowing coexistence of local Helmholtz resonance and Fabry-Pérot resonance to simultaneously generate a negative modulus and negative density. The metamaterial exhibits a reversed phase velocity and negative refraction, even if the constitutional elements are randomly distributed. This is a pioneering work that an airborne acoustic double negative metamaterial derives negative density from Fabry-Pérot resonance and maintains negative refraction with its constitutional elements randomly distributed.