Generalizing the Concept of negative medium to acoustic waves

Electromagnetic metamaterials are artificial materials exhibiting simultaneously negative permeability and permittivity, and the “double negativity” gives rise to many interesting phenomena such as negative refraction, backward waves and superlensing effects. We will see that the concept can be extended to acoustic waves. We will show the existence of acoustic metamaterial, in which both the effective density and bulk modulus are simultaneously negative at some particular frequency range, in the sense of an effective medium. Such a double negative acoustic system is an acoustic analog of Veselogo’s medium in electromagnetism, and shares many novel consequences such as negative refractive index, flat slab focusing and super-resolution. The double negativity in acoustics is derived from low frequency resonances, as in the case of electromagnetism, but the negative density and modulus can come from a single resonance structure, as distinct from electromagnetism in which the negative perm eability and negative permittivity originates from differentresonance mechanisms.