TEMPORAL EVOLUTION OF TRANSIENT TRANSVERSE BEAM PROFILES IN NEAR-FIELD ZONES

The transmission through transversal planes of short ultrasonic pulses, generated in water by circular planar and focussed broadband piezoelectric transducers, is analysed in detail by means of a thin temporal chopping of the transverse beam profiles. This is carried out with a quasi-punctual microprobe in the MHz frequency range using a detection procedure based on shifting narrow temporal gates (widths of the order of 1/[10f(o)]). In this way, the resulting series of temporal chops enables certain sequences of acoustic events, normally hidden in conventional plots, to be visualized. This fact facilitates the analysis of some significant near-field transient effects which must be taken into account for high resolution pulse-echo applications. The influence of some kinds of radiation irregularities from the transducer vibrating surface is also analysed in field-planes parallel to the emitting aperture. Theoretical results, obtained through approaches based on the impulse response method for particular near-field zones, are compared with experimental data revealing some non-ideal effects.