Scattering of Ultrasonic Waves in Water by Solid Cylinders

Abstract

An x-cut quartz crystal with natural frequency about 1075 kc, placed at the center of one end of a water-filled steel tank (3′×3′×20′), was driven off resonance at 1145 kc. One side of the crystal vibrated in air. The driving circuit consisted of a Colpitts oscillator, buffer amplifier, and power amplifier. The crystal was matched to the output of the power amplifier by link coupling. The radiation in the water was received by a pick-up system consisting of an ammonium dihydrogen phosphate microphone connected to a tuned radiofrequency amplifier. The detected output was measured by means of an electronic voltmeter; readings were proportional to the maximum excess pressure. The obstacle used was a 14″ steel rod, placed in the beam where the latter's half-width was slightly greater than the diameter of the obstacle. Three types of measurements were made: (1) intensity distributions across the beam at different distances along the axis of the beam, (2) wave-front determinations at different distances along the axis, and (3) observations of the effect of moving the obstacle across the beam. The following results have been obtained: (1) The obstacle scatters the original beam into three major peaks flanked by a considerable number of subsidiary peaks. Any asymmetries in the original beam are magnified in the scattered beam. The peaks diverge along rays which appear to originate from a point somewhere between the crystal and obstacle. (2) The undiffracted wave front consists of two sides and a central portion, the three having roughly the same curvature. The obstacle flattens the central portion and steepens the sides. This steepening is to be expected since the velocity of sound is greater in steel than in water. (3) Moving the obstacle causes a continuous change in the scattering pattern. Actual splitting of the beam into three parts occurs only as the center of the beam is approached. Similar experiments are planned for rods of different sizes and for hollow rods of varying thicknesses and acoustic impedance