Piezoelectric transducer surface vibration characterization using acoustic holography and laser vibrometry

The acoustic pressure radiated by a piezoelectric source can be predicted based on the normal velocity distribution along the transducer surface. However, up to now, there have been no reliable direct methods of surface vibration measurement in liquids. The transducer vibration is frequently considered as being uniform (thickness mode), which may be incorrect due to the transducer structure (e.g., phased arrays) or due to excitation of Lamb waves in the piezoelectric plate in addition to the thickness vibration mode. A holographic method has been developed and experimentally used that enables reconstruction of the radiator vibration pattern. The method includes: measurement of wave amplitude and phase at different points of a plane and perpendicular to the acoustic axis at some distance from the source; theoretical time reversal of the waveform in each gridpoint; backpropagation of the field to the source using the Rayleigh integral. Vibration of piezoelectric transducers was also studied using a laser vibrometer. A special experimental and theoretical study was performed in order to show that this well-developed method gives wrong results when the transducer is in contact with the liquid, because of strong acousto-optic interaction in a condensed media. It is shown that, although the acoustic holographic approach has limited spatial resolution (of the order of a wavelength), it allows fairly exact prediction of the radiated field pattern.