| Granular materials are inherently heterogeneous, leading to challenges in formulating accurate models of sound propagation. In order to quantify acoustic responses in space and time, we perform experiments in a photoelastic granular material in which the internal stress pattern (force chains) is visible. We utilize two complementary methods, high-speed imaging and piezoelectric transduction, to provide particle-scale measurements of both the speed and amplitude of an acoustic wave. Hertzian contact theory accurately predicts the observed increased wave amplitude along stronger force chains, but fails to describe the increased sound speed of the leading edge of the signal compared to the peak. We interpret this behavior in the context of direct measurements of the propagation of sound along the force chain network. The importance of local forces, combined with a nonlinearity resulting from transient contacts formed during propagation, highlights the importance of particle-scale behavior in determining the acoustical properties of granular materials. |
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