Effect of casing on sound absorption characteristics of fine spherical granular material

To determine sound absorption coefficients of fine granular materials, the materials must be cased to suppress their flowability. In this study, effects of casing on the sound absorption coefficients of fine granular materials were investigated. The normal incidence sound absorption coefficients of cased hollow glass beads were measured using cylindrical impedance tubes. The measurement results demonstrated that the hollow glass beads present a sound absorption peak, which was attributed to the vibration of a particle frame, in the frequency range of 180 to 700 Hz for sample thickness in the range of 20 to 550 mm. With an increase in the material thickness or diameter of the casing, the first peak of the sound absorption coefficients shifted to a lower frequency. The sound absorption coefficients were calculated using an elastic frame model for porous materials. The effects of friction between the lateral wall of the case and the particle frame were incorporated in the bulk modulus and bulk density of the particle frame in the model. The model predicts correctly the frequency at which the first sound absorption peak occurs with an uncertainty of 10%.