Experimental study on sound absorption of hollow glass beads with inner closed cavities under low-frequency vertical vibration.

Normal incidence sound absorption coefficients α of hollow glass beads were measured under a sinusoidal vertical vibration of 10 Hz. It was found that α depends on the vibrational displacement amplitude. As the displacement amplitude increased, the frequency width of the peak extended toward low frequencies. In addition, different evolutions of α were observed for the time periods during which the displacement of the sinusoidal vibration was positive or negative. It seems that the variation in packing state, which is caused by the inertial forces due to vibration, causes the different evolutions of α.

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.

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%.