Flute-Model Acoustic Metamaterials with Simultaneously Negative Bulk   Modulus and Mass Density
  We experimentally constructed a three-dimensional flute-model "molecular" structure acoustic metamaterial(AM)from a periodic array of perforated hollow steel tubes (PHSTs) and investigated its transmission and reflection behaviors in impedance tube system. The AM exhibited a transmission peak and an inverse phase, thus exhibiting the local resonance of the PHSTs. Based on the homogeneous media theory, the effective bulk modulus and mass density of the AM were calculated to be simultaneously negative; the refractive index was also negative. PHST AM slab focusing experiments showed that the medium with a resonant structure exhibited a distinct metamaterial property. 
H. C. Zeng, C. R. Luo, H. J. Chen, S. L. Zhai and X. P. Zhao
physics.class-ph cond-mat.mtrl-sci