Experimental Investigation of Ultrasonic Intensity Gain in Water Due to Concave Reflectors

Abstract

This paper discusses a method of producing high intensity sound waves in liquids. A beam of ultrasonic waves (4.25 mc, 15 × 12 mm cross section, acoustic power≃2 watts) was focused with an ordinary watch glass (6.8 cm radius of curvature). The intensity in the focal region is large enough to raise an ultrasonic fountain 10 cm high accompanied by a spray of fog droplets. The distribution of intensity in the focal region was determined by measuring the screening effect of properly placed obstacles. The sound intensity in the focal region and in the plane wave was measured by the radiation pressure on beads of convenient size. The absolute intensity in the plane wave was also calculated from the driving potential and the measured mechanical Q of the crystal, and reasonable agreement was found with the direct measurement. A gain in intensity by a factor of about 70 was measured where simple diffraction theory predicts 74. For the highest voltages used the extrapolated negative peak pressure was 41 atmospheres. No cavitation was observed.