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

Takumasa Tsuruha,Yasushi Takano,Makoto Otani

doi:10.1121/10.0009531

Takumasa Tsuruha, Yasushi Takano + Show 1 more

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https://doi.org/10.1121/10.0009531

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Abstract

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.

Highlights

Granular materials comprise an ensemble of randomly packed solid grains with pores
This sound absorption at low frequencies is likely caused by sound energy dissipation due to collisions and sliding between the grains as sound waves pass through the solid grains
The sample number of the signals used for the calculation of the sound absorption coefficient was 5 Â fs, and the values other than that at 0.4 Â fs points, having the value of the sound pressure cut out by the Hanning window, were set to zero. These results suggest that the mechanical properties of the granular material, which are related to the interaction forces between the grains, vary sufficiently to cause a change in the sound absorption coefficient when they are subjected to vertical vibration with a displacement amplitude greater than a certain level

Summary

Introduction

Granular materials comprise an ensemble of randomly packed solid grains with pores. A number of studies on granular materials have been conducted in various industrial fields. Vermiculites and glass beads, whose grain sizes are in millimeters, exhibit high sound absorption coefficients at high frequencies such as 1 kHz when the material thickness is approximately 50 mm Such sound absorption at high frequencies is caused by sound energy dissipation due to thermal conduction and viscous friction in the air layers in the vicinity of the solid grain surfaces as sound waves pass through the pores.. The sound absorption characteristics of granular materials composed of rigid spheres with ordered arrangements have been investigated.7 As another example, silica powder and hollow glass beads, whose grain sizes are in micrometers, are characterized by a spectral peak of the sound absorption coefficients in the low-frequency range of 250–350 Hz when the material thickness is 40 mm. This sound absorption at low frequencies is likely caused by sound energy dissipation due to collisions and sliding between the grains as sound waves pass through the solid grains.

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