Observations of the propagation of very short ultrasonic pulses and their reflection by small targets

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.

Theoretical Predictions and Experimental Measurements of Echo-Responses from Angled Targets

“…The experimental results were obtained using flat-ended brass cylinders as the targets, the transducer being tilted to achieve the above angles of alignment. A 19mm-diameter heavily-damped lead metaniobate (PMN) transducer was used (Panametrics V3289), earlier work having shown that its behavior was close to that predicted for an ideal-piston radiator [5,8] (as assumed in the present model).…”

“…Considering a point-target of elementary surface se as a rigid point at r T in the fluid, Weight and Hayman [5] have shown it was possible, by reciprocity, to write the reception in the same way as the radiation. The instantaneous pressure at the surface of the receiver <p>(t), defined as, <p>ltl =ffP1r",tl dS, For complex target geometries the principle used to derive the modelling is, starting from Kirchhoff's integral, to widen the reciprocity by making apparent in the integrals a term independent of the reception (i.e.…”

Ultrasonic Transducer Standards

“…The edge wave is generated at the circumference of the transducer, and is due to the relative motion of the transducer and the baffle. The edge wave follows the plane wave in the nearfield and is superimposed in the farfield for a transmitter (Weight and Hayman, 1978), as illustrated in Fig. 9a.…”

A Geometrical Interpretation of the Echo Formation of Short Pulses on Simple Shaped Targets

“…As explained in [3] the radiation by an ideal plane transducer can be described as the sum of a plane wave coming from the surface of the transducer and a toroldal wave coming from its periphery. On the axis the transient field is formed by two Dirac's pulses of opposite polarity.…”

“…Here we displaya geometrical interpretation of the scattering of pulses from broad-band transducers by simple shaped on-axis rigid targets like balls, cones and small disks. This new interpretation is based on two existing theories : the decomposition of the incident field in term of geometrical and edge waves [3], and an adaptation of Dory's "Theorie du Profil Reflecteur" [4]. At first, the underlying theories and approximations used for this interpretation are recalled.…”