No abstract

The field of a circular ultrasonic transducer emitting a single-cycle pulse into water has been observed using a specially constructed small (150 μm) wide-band receiving probe and a compact stroboscopic schlieren system. The theoretically predicted plane-wave and diffracted edge-wave components of the field have been resolved. Good agreement with the theory for a pistonlike source is obtained, except in a region less than 1.5 transducer radii from the transducer. The output of the transducer used in the transmit–receive mode to detect small targets has been measured and the results are in accord with a time-domain principle of reciprocity between transmission and reception. Implications of the results for field plotting and for the location and characterization of small targets are considered.

A simple model has been introduced which allows calculations to be made of the particle displacement and velocity waveforms which arise when short pulses of ultrasound are radiated into solid media from transducers typical of those used in nondestructive evaluation. The model makes use of the plane- and edge-wave nature of the pulses radiated and, for simplicity, incorporates an empirical extension of the impulse response method for propagation in a fluid medium. Calculations made using the model are compared with other theoretical results obtained previously by harmonic synthesis, as well as with experimental measurements of pulse waveforms made using a miniature piezoelectric receiving probe. The results obtained are of particular relevance to the characterization of small targets using ultrasound.

Field patterns of short-pulse ultrasonic transducers are quite different from those of continuousor quasicontinuous-wave sources andsdepend critically on the actual waveform emitted. The observed patterns, including those provided by using a small target to probe the field in the transmit-receive mode, are also dependent on the method of detection used--whether full-or half-wave. The structure of the field and the marked difference between the pulsed pressure beam pattern and the corresponding transmit-receive mode beam pattern are explained in terms of the plane and edge waves radiated by the source. The field radiated by a transducer is compared with measurements of the field structure of a circular aperture insonified by a plane wave. Consequences of the pulsed beam structure for ultrasonic pulse-echo evaluation are pointed out.