06THRESHOLDS OF ACOUSTIC CAVITATION.
09Technical memo. no. 57
HARVARD UNIV CAMBRIDGE MA ACOUSTICS RESEARCH LAB
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27Cavitation thresholds in water were measured as a function of frequency, dissolved gas, ambient pressure, and suspended particle size. The apparatus used comprises a two-liter volume of water enclosed by a spherical glass shell driven at its radially symmetric resonance frequencies by eight multiresonant piezoelectric transducers. Large acoustic pressures can be produced, ranging from 10 bars at 27 kcs to 200 bars at 1.16 Mcs. The threshold data can be divided into three regions. In region A, defined by f 200 kcs, acoustic pressure, P sub a 3 bars, and air saturation pressure P sub s 600 mm Hg, small air bubbles grow by rectified diffusion and stabilize at the pressure nodes. In region B, defined by f 200 kcs, P sub a 3 bars, and P sub s 500 mm Hg, transient cavities are formed that can be detected visually and aurally. In region C, f 200 kcs, P sub a 3 bars, for any value of P sub s, transient cavities are formed, but their presence can be detected only acoustically. 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 repeat cavitation.