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 (in the form of force chains) is
visible. We utilize two complementary methods, high-speed imaging and
piezoelectric transduction, to provide particle-scale measurements of both the
amplitude and speed of an acoustic wave in the near-field regime. We observe
that the wave amplitude is on average largest within particles experiencing the
largest forces, particularly in those chains radiating away from the source,
with the force-dependence of this amplitude in qualitative agreement with a
simple Hertzian-like model of particle contact area. In addition, we are able
to directly observe rare transient force chains formed by the opening and
closing of contacts during propagation. The speed of the leading edge of the
pulse is in quantitative agreement with predictions for one-dimensional chains,
while the slower speed of the peak response suggests that it contains waves
which have travelled over multiple paths even within just this near-field
region. These effects highlight the importance of particle-scale behaviors in
determining the acoustical properties of granular materials