The ultrasound transmission in a PMMA spherical bead packing is investigated during the sintering process under stress. Velocity and amplitude measurements of the coherent longitudinal wave are performed to monitor the evolution of the elastic properties of the solid frame from noncohesive packing to sintered granular material. Comparison between experimental velocity data and the prediction by the Digby contact model reveals the crucial role of the bonding effect on the mechanical behavior of the compacts. By using the correlation technique of acoustic speckles, we also observe the important rearrangements in the granular compacts before the onset of sintering. (C) 2010 Elsevier B.V. All rights reserved.