Observation of nonlinear interaction of acoustic waves in granular materials: demodulation process

Experimental study of the demodulation effect of ultrasonic waves in glass beads are reported. Both impulsive (burst) and continuous regime have been studied for amplitude modulated signals with its carrier frequency of 100 k Hz. It is shown that two low frequency acoustical modes corresponding to th e modulation frequency (in the range of few kHz) are excited due to the non linear processes in the granular medium. The excitation of a first mode pro pagating mainly through the beads (solid-based mode) and the excitation of a second mode propagating mainly through the air saturating the granular ma terial (fluid-based mode) are observed. Possible physical mechanisms of gen eration and regimes of propagation in the granular assemblages are discusse d. (C) 2001 Elsevier Science B.V. All rights reserved.