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.