In non-cohesive granular materials the elementary excitations are the vibrations. Due to the nature of the contacts between individual grains, the sound propagation will be along meandering tortuous paths. We have studied the high-frequency vibrations in such a material. The vibrations experienced by a single grain were exceedingly noisy even when the amplitude was low. At low vibration amplitudes, thermal fluctuations dominate the response, so a local heater can probe the extent of the vibration.
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Liu et al. (1994) studied this question.
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