Amplitude measurements of acoustic fields by light reflection at the free surface of a liquid

A novel technique for amplitude measurements of acoustic field by light reflection at the free surface of a liquid is presented. The motion of the boundary layer of the liquid induced by the acoustic pressure, modulates the phase of a collimated laser beam. This phase modulation is observed using dark-ground techniques, and it gives a measurement of the acoustic power as function of the position. Experiments with piezocomposite 1-3 transducers were carried out. The technique presented is able to give quantitative information quickly about the motion of individual ceramic rods of the piezocomposite, and allows fast quality control for piezocomposite 1-3 transducers. We achieved a lateral resolution of 0.2/spl times/0.4 mm. The theoretical limit for the axial resolution is of the order of a few Angstroms.