Effect of acoustically-induced elastic softening on sound absorption coefficient of hollow glass beads with inner closed cavities

This study investigated the effects of sound wave magnitude on the sound absorption characteristics of granular materials. The normal incidence sound absorption coefficients of the hollow glass bead packings were measured with different incident sound pressure levels. The measurement results exhibited spectral peaks of sound absorption coefficients at several frequencies, implying that the first peak was caused by the resonance of the hollow glass beads as a layer. Although the first peak in the sound absorption coefficients did not vary when an incident sound pressure was low, the first peak shifted to a lower frequency when the sound pressure exceeded a certain magnitude. These results indicate that the hollow glass beads are softened at high incident sound pressures. Furthermore, by modelling the velocity-dependent elasticity of the hollow glass beads, the velocity and elasticity in the hollow glass beads were simulated in the direction of the sound propagation. The simulation results indicate that the velocity and elasticity profiles depend on the sound wave magnitude. Finally, the velocities of the hollow glass beads under acoustic excitation were measured by inserting an accelerometer into the beads. The measurement results demonstrated that the velocity profile depends on the sound wave magnitude, which parallels the simulation results.