This book describes the phase and amplitude fluctuations of acoustic signals that travel in the ocean acoustic waveguide with little interaction with the ocean boundaries. The book is a vastly expanded update of the 1979 text Sound Transmission through the Fluctuating Ocean, edited by Stan Flatté with contributions from Roger Dashen, Walter Munk, Ken Watson, and Fred Zachariasen. The focus here is on physical concepts and methodologies that over the last three decades have proven to be most useful.

The original text was written when a basic grasp of the subject was just emerging from three important lines of research. First, a few state-of-the-art field efforts utilizing controlled electronic sources (mostly in the kilohertz range) and a few hydrophone receivers on stable or navigated platforms were providing reliable observations of phase and amplitude statistics for single-frequency (CW) transmissions and most importantly for fixed acoustic paths: Fadeouts or scintillations and rapid phase jumps were seen to be all too common. Interestingly at this stage, classified military systems vastly exceeded the capabilities available to civilian investigators, one example being the remarkable SOund SUrveillance System (SOSUS). Second, from work in the late 1960s and early 1970s, oceanographers and acousticians had come to the realization that sound-speed fluctuations in the ocean were both anisotropic and inhomogeneous and that the ocean internal-wave field, described in some approximation by the recently developed Garrett-Munk (GM) internal-wave spectrum, was a significant contributor to this variability. Lastly, a new theoretical tool, the Feynman path integral method, allowed one to calculate several acoustic field statistics for both CW and fixed path and account for the anisotropic and inhomogeneous internal-wave field as well as the important fact that sound interacting with this random field is deterministically refracted by the ocean waveguide. These breakthroughs well justified the writing of the classic 1979 text; however, in the last thirty-five years the field has come a long way.

Using technology that was developed for Ocean Acoustic Tomography, experiments are now being conducted using wide-aperture vertical receiving arrays that are precisely navigated and time-synchronized. The analogy to astrophysics would be the difference between gazing at the heavens using the 200-in.