This special issue focuses on compelling three-dimensional (3D) volumetric and boundary effects on underwater sound propagation and scattering in complex and time-varying (thus four-dimensional) underwater environments. It consists of 24 papers covering analytical, numerical, and experimental studies and presents a collection of up-to-date research on this active and relevant topic.

Underwater 3D acoustic effects caused by reflection, refraction, diffraction, and/or scattering of sound energy in both vertical and horizontal planes have been observed and investigated extensively for many decades. A textbook by Lee and Schultz (1995) and a review article by Tolstoy (1996) provided an excellent summary of 3D acoustic research and different modeling techniques up to the 1990s. The research on 3D effects continued in the following years, and many efficient and effective numerical models were developed, e.g., Smith (1999), Sturm and Fawcett (2003), and Heaney and Campbell (2016). In addition, experimental evidence of 3D propagation...

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