Abstract
A new examination has been made of the diffraction pattern in the vicinity of the focus, for large aperture concave bowl ultrasonic transducers. The study gives results which show a departure from those derived from optical theory, hitherto accepted. This departure increases progressively with increase in bowl aperture and leads to the expectation that large aperture ultrasonic microscopes will provide a resolution somewhat greater than indicated by classical theory. The study has also shown that the major part of the wave energy radiated by the transducer is confined near the focus within a cylindrical region, in which it is propagated as a plane wave. By a suitable choice of bowl dimensions and ultrasonic frequency the radius of this region may be made a fraction of a milimetre, while its length is several milimetres. From the realisation of the plane wave nature of propagation near the focus a technique has been developed for the determination of the ultrasonic velocity in samples of solids of very small volume of the order of 50 mm 3.
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