  Avalanches of granular media are mainly characterized by the observation and
the measurement of the main angles of avalanche corresponding first to the
movement of a group beads and to the whole movement of a great part of the
grains. These characteristics do not give any information about the
rearrangements of the grains inside the granular beads layer. As any movement
of a grain produces a deformation of the structure, it is quite normal to
expect a sound that will propagate inside the granular medium. We present an
acoustical experimental study of the of avalanches precursors signals in
spherical granular beads glass and silica aerogels powder (size of grains less
than 80 micrometers). Movements inside the material produce sound pulses, these
acoustic emissions have been recorded with two piezoelectric transducers placed
on the lower part of the material layer. Our results show clearly signals
before any movement on the upper part of the bead's layer: recursive precursors
in glass beads and non-recursive precursors in aerogels, for angles less than
the first avalanche angle. Our methodology allow us to obtaining information of
all internal movement in the granular material including avalanches,
precursors, and small arrangements.

Materials Science