Acoustic metamaterial with negative density

We fabricated a one-dimensional acoustic metamaterial with negative effective density using an array of very thin elastic membranes. We observed acoustic equivalence of the plasma oscillation at ωc=735 Hz. The metamaterial was opaque in the frequency range from 0 to 735 Hz, and was transparent above 735 Hz. We report direct observation of negative acceleration in this acoustic medium below 735 Hz. The frequency characteristics of the metamaterial have the same form as that of metals with negative permittivity. We also provide a simple theory to explain the experimental results.