Acoustic cavitation and stealth in active sonar

This article follows on from the [RAD 6 714] article on active sonars and the treatment of the problem of range estimation under "ideal" conditions. Even if such conditions do exist, as illustrated by the real-life cases dealt with in the above-mentioned article, there are still situations where certain physical phenomena can lead to a substantial reduction in performance. Two of the most significant of these are acoustic cavitation and target stealth. The general aim of this article is to give the specialized engineer, or those interested in a particular sector, an overview of the sonar field and its constraints.

In radar, in an ideal infinite medium, it is possible to increase range by increasing transmitted power, all other things being equal. This is not the case with active sonar, where the emission level is limited by acoustic cavitation, i.e. the vaporization of water under the effect of the negative pressure caused by the emitted wave. This is the subject of 1 , where we first review the two possible physical origins of water vaporization: boiling and cavitation, which are sometimes difficult to distinguish. The dynamics of acoustic cavitation produced when a transducer emits water are described, as are its manifestations (bubble sheaves, sonoluminescence). Its effects can be harmful (limitation of emitted power, erosion of materials) but also desirable. Analytical formulas for calculating the cavitation threshold, i.e. the power at which a given transducer will cavitate, are given. This threshold depends mainly on hydrostatic pressure (i.e. immersion) and transducer dimensions. Two methods of preventing cavitation are analyzed, along with their operational consequences.

The 2