Mode conversion of Rayleigh and Lamb waves to compression waves at a metal-liquid interface

A theoretical treatment is given of a potentially useful ultrasonic mode-conversion process, from Rayleigh or Lamb waves (SAW's) to angled compression waves in an adjacent liquid. Since the compression waves can form a well-defined beam, with typical angular width of only 1 degree or 2 degrees, it is possible to use this technique in under-liquid viewing applications. Another use, in liquid level measurement, has advantages over conventional ultrasonic methods. Attractive features are the high power efficiency of the mode-conversion, in both directions, and its inherent separation of electromechanical transducer from the liquid, which may be vital in certain hazardous environments. With such applications in mind, the emphasis here is on ultrasonic processes in the liquid. After a descriptive account of pressure wave generation in the liquid, the main part of the paper is a general treatment of far-field beam profiles and includes transverse beam-width and discussion of pulsed operation. Finally we examine inverse mode conversion (compression wave to SAW), to identify the main physical processes occurring and arrive at a figure for power efficiency. This turns out to be 80% (max) against nearly 100% for transmission.