  Ultrasound transmission in PMMA spherical bead packings is investigated
during the sintering process under stress. Velocity and amplitude measurements
of coherent longitudinal waves are performed to monitor the evolution of the
elastic properties of the solid frame from noncohesive packing to sintered
granular material. Comparison between the experimental velocity data and the
prediction by a contact model [Digby, J. Appl. Mech. 48, 803, (1981)] reveals
the crucial role of the bonding effect on the mechanical behavior of granular
compacts. By using the correlation technique of acoustic speckles, we also
observe the important rearrangements in granular packings before the onset of
sintering.
