Résumé:
We describe a method, which uses a high frequency (MHz range) ultrasonic beam to probe lower frequency motions of a solid surface immersed in water. The analysis shows that two physical phenomena are involved in the measurement. On one hand a Doppler effect is induced by the acoustic path variations, and on the other hand a nonlinear interaction occurs between the reflected probe wave and the low frequency acoustic wave transmitted in the liquid volume by the moving surface. We have studied the relative importance of these two effects in various experimental configurations: the low frequency wave was radiated successively by a small piezoelectric crystal (0.4-mm2 square surface), a large planar transducer and a focused one. The problem is to know the physical quantity measured by the probe: a displacement if the Doppler effect dominates, a particle velocity if the nonlinear interaction is predominant or a mix of these two quantities. It is shown that the relative influence of the two effects is closely related to the directivity of the LF wave radiated in the liquid. Experiments were carried out with a 30-MHz ultrasonic probe to characterize transducers in the MHz range and to examine industrial pieces tested by ultrasounds. A comparison with results given by an optical heterodyne interferometer demonstrates the interest of our active ultrasonic probe to investigate acoustic fields. This active ultrasonic probe was also applied to characterize high intensity focus ultrasound (200 bar), produced by a lithotripter.

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