Thresholds of Acoustic Cavitation in Water

Abstract

Cavitation thresholds in water have been measured as a function of frequency, dissolved gas, ambient pressure, and suspended-particle size. The apparatus used comprises a 2-1 volume of water enclosed by a spherical glass shell driven at its radially symmetric resonance frequencies by 8 multiresonant piezoelectric transducers. Large acoustic pressures can be produced, ranging from 10 bar at 27 kc/sec to 200 bar at 1.16 Mc/sec. The threshold data can be divided into three regions: In region A—defined by f<200 kc/sec, acoustic pressure Pa<3 bar, and air saturation presure Ps>600 mm Hg—small air bubbles grow by rectified diffusion and stabilize at the pressure nodes. In region B—defined by f<200 kc/sec, Pa>3 bar, and Ps<500 mm Hg—transient cavities are formed that can be detected visually and aurally. In region C—where f>200 kc/sec, Pa>5 bar, for any value of Ps—transient cavities are formed, but their presence can be detected acoustically only. The threshold tends to a slope of 12 dB/octave for frequencies above 1 Mc/sec. Experiments on cavitation at pressures larger than the threshold indicate that only a finite number of cavitation events can be produced in a given sample of water when it is isolated from contamination by airborne motes. In this way, water can be “strengthened” by a factor of at least 8 by repeated cavitation.