Negative Effective Density in An Acoustic Metamaterial
  We report theoretical and experimental results for a new type of homogenized acoustic metamaterials with negative effective mass density. We constructed one-dimensional metamaterial, which is a tube with an array of very thin elastic membranes placed inside. This structure exhibited negative effective density in the frequency range from 0 to 735 Hz. The experimental result is in excellent agreement with our theoretical model that predicts negative effective density below a cut-off frequency. The frequency characteristics of this effective density is analogous to that of the permittivity of the electromagnetic plasma oscillation. 
Sam Hyeon Lee, Choon Mahn Park, Yong Mun Seo, Zhi Guo Wang, Chul Koo   Kim