Human uses of ultrasound: ancient and modern

For untold millennia certain animals have used ultrasound to probe places w here light is unavailable, echo-locating bats being among the most adept. W ith ultrasonics, bats can quickly and safely 'see' at night in pursuing ins ects or flying in dark caves. Unable to hear ultrasound, humans have nevertheless made use of it. They di d this anciently by taming wolves, with their keen ultrasonic hearing, for aiding in the hunt. Currently, they are doing this by developing technology to detect, generate and process ultrasound for searching in air or other g ases, in water or other liquids, and in solids. The story of these technological developments is a large and fascinating mi rror of human history involving the advent of such discoveries and inventio ns as magnetostriction, piezoelectricity, sonar, ultrasonic microscopy, etc . - the list is long. By now we are skilled in probing for underwater objec ts, the internal structure in materials, organs inside the human body, etc. - again the list is long. A number of different ultrasonic systems can be categorized into one of thr ee key generic approaches: pulse-echo exploration, intensity mapping, and p hase-amplitude measurement. In addition, each of these categories can be co mbined with the others to produce hybrid systems for which an unambiguous c ategorization is difficult or impossible. Challenging problems remain but solutions are being found. New principles a nd techniques are being discovered that will improve the use of ultrasound. Employing tomo-holographic techniques to reduce ambiguity in probing three -dimensional objects, near-field techniques to boost resolution and using l imited-diffraction beams to provide image construction with ultra high fram e rates are cases in point. (C) 2000 Published by Elsevier Science B.V. All rights reserved.