  Elastic properties of a granular packing show nonlinear behavior determined
by its discrete structure and nonlinear inter-grain force laws. Acoustic waves
show a transition from constant, pressure-dependent sound speed to a shock-wave
like behavior with amplitude-determined propagation speed. This becomes
increasingly visible at low static confinement pressure as the transient regime
shifts to lower wave amplitudes for lower static pressure. In microgravity,
confinement pressure can be orders of magnitude lower than in a ground based
experiment. Also, the absence of hydrostatic gradients allows for much more
homogeneous and isotropic pressure distribution. We present a novel apparatus
for acoustic wave transmission measurements at such low packing pressures. A
pressure control loop is implemented by a microcontroller that monitors static
force sensor readings and adjusts the position of a movable wall with a linear
motor until the desired pressure is reached. Measurements of acoustic waves are
possible using accelerometers embedded in the granular packing as well as
piezos. For excitation we use a voice coil-driven wall, with a large variety of
signal shapes, frequencies and amplitudes. This enables experiments both in the
linear and strongly nonlinear regime.
