40 Years of Nonlinear Underwater Acoustics

The development in nonlinear underwater acoustics over the past 40 years has in particular involved research, development and exploitation of the parametric acoustic array. Based on studies of "scattering of sound by sound", the concept of the parametric acoustic array was established in 1963. The potential exploitation of the advanced beam qualities of the parametric acoustic array led to a substantial research activity over nearly two decades, where the fundamental aspects of the array were studied in depth. Four distinct operating regimes for the parametric sources were identified. The attenuation of parametric sources, governing the effective length of the array, may be determined by small-signal absorption or nonlinear absorption in either the near-field or the far-field of the primary beams. In particular the transmission qualities of the parametric array have formed subject for investigations, but also a considerable research effort has been put into the parametric receiving array. Exploitation of advances in signal processing has been done in order to improve the beam qualities of the parametric array, and data have been obtained in and outside the interaction region of the primary beams. A substantial experimental research effort has been carried out and a few parametric transmitting arrays have been manufactured and marketed by companies over the recent two decades. However, the wide-spread use of the parametric array as expected from its beam qualities has never been realized. In spite of several more or less successful attempts to improve the conversion efficiency of the parametric array, the conversion efficiency is still low (<1–2%). This is the main disadvantage of the parametric array and probably the main limitation for its use.