We present theoretical and experimental results for a new type of acoustic metamaterials. An analysis of the theoretical model predicts negative modulus for all frequencies below a cut-off frequency. This frequency characteristics is significantly different from that of local-resonant type metamaterials which exhibits a stop band for the negative modulus. Our theoretical model gives the frequency characteristics of negative modulus analogous to that of the permittivity in a metallic plasma. We constructed one-dimensional version of the theoretical model, which is a tube with an array of side holes on the tube wall. Experiment shows that the effective modulus is negative in the frequency range from 0 to 440 Hz.