  Using numerical simulations it is shown that a jammed, random pack of soft
frictional grains can store an arbitrary waveform that is applied as a small
time-dependent shear while the system is slowly compressed. When the system is
decompressed at a later time, an approximation of the input waveform is
recalled in time-reversed order as shear stresses on the system boundaries.
This effect depends on friction between the grains, and is independent of some
aspects of the friction model. This type of memory could potentially be
observable in other types of random media that form internal contacts when
compressed.
