ISSN 1063-7710, Acoustical Physics, 2007, Vol. 53, No. 2, pp. 119–122. © Pleiades Publishing, Ltd., 2007. Original Russian Text © G.A. Volkov, A.A. Gruzdkov, Yu.V. Petrov, 2007, published in Akusticheskiœ Zhurnal, 2007, Vol. 53, No. 2, pp. 149–152. 119 INTRODUCTION Cavitation is understood as the breaking of continu- ity of a liquid by an external action. Depending on the nature of this action, it is customary to distinguish between hydrodynamic and acoustic cavitations. Acoustic cavitation occurs under the effect of a sound field that produces a negative pressure in the liquid and, hence, leads to the growth of vapor-gas bubbles. These bubbles, which are called cavitation nuclei, originate at solid microparticles or molecules of a dissolved gas. Conventionally, it is agreed that the cavitation strength of a liquid is characterized by the cavitation threshold, which is understood as the negative pressure beyond which a fast growth of cavitation nuclei is observed. The results of measuring the cavitation threshold are reported by many authors, and the analy- sis of these data makes it possible to reveal a number of regularities. In particular, the experiments on acoustic cavitation show that, at relatively low radiation fre- quencies, the cavitation threshold value exhibits no fre- quency dependence. This value is called the static cav- itation threshold. At the same time, at higher frequen- cies, the experimental data are characterized by a considerable spread and the value of the cavitation threshold grows with increasing frequency [1–4]. A sharp rise of the threshold pressure amplitude is explained as follows. The physical nature of cavitation consists in the development of a structure of cavitation nuclei in the region of reduced pressure. Since the cav- itation is a process developing in time, for its onset not only the critical pressure value should be reached, but also a certain time interval is necessary within which this pressure is maintained. This statement is supported by the experimental data on cavitation-caused fracture under a short pulsed loading [5]. With an increase in the sound wave frequency, the duration of time intervals within which the pressure is tensile decreases. The above-mentioned facts suggest that, at high fre- quencies of sound waves, the cavitation threshold can- not adequately characterize the cavitation strength and must be complemented with some other parameters. It should also be noted that prediction of the conditions for the development of cavitation is of practical interest, because cavitation imposes a natural limit on the radia- tion power of hydroacoustic arrays [6]. THE INCUBATION TIME CRITERION Since the strength largely depends on the time parameter, it is reasonable to describe the strength properties of a liquid by the characteristic time of the development of the structure of cavitation nuclei, which should determine the scale on the time axis. We will call this quantity the incubation time. For determining the acoustic cavitation threshold, we take the criterion that was used in [5] for the case of pulsed loading. This cri- terion has the form (1) 1 τ -- Pt ' ( ) ( ) sgn Pt ' ( ) P s ----------- α t ' 1, ≥ d t τ – t ∫ PHYSICAL ACOUSTICS The Incubation Time Criterion and the Acoustic Strength of Sea Water G. A. Volkov, A. A. Gruzdkov, and Yu. V. Petrov St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199164 Russia Institute of Problems of Machine Science, Russian Academy of Sciences, Vasil’evskii Ostrov, Bol’shoi pr. 61, St. Petersburg, 199178 Russia e-mail: gruzdkov@mail.ru Received December 8, 2005 Abstract—The approach based on the incubation time concept is used to analyze experimental data on acoustic cavitation in degassed water and in sea water. Earlier, a similar approach proved to be effective in analyzing the cavitation due to pulse loading and in studying the dynamic strength of solids. The proposed criterion takes into account the existence of the static cavitation threshold for low-frequency loading and makes it possible to explain the growth of the cavitation threshold for high-frequency loading, as well as the appreciable spread in experimental data. PACS numbers: 43.35.Ei, 62.60.+v DOI: 10.1134/S1063771007020017