  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.
