  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.
