Parametric acoustic arrays: A Bergen view.

At the University of Bergen (UoB), Norway, research activity in physical acoustics started in the mid‐1960s with investigations on the parametric acoustic array (PAA). The newly appointed professor in applied mathematics, Sigve Tjo/tta, had some years earlier been at Brown University and was inspired by the concept at a fundamental level, but also wanted experimental confirmation. No previous acoustical activity existed at UoB. The PAA project was started as a master project at Department of Physics, where the main activity was in nuclear, high‐energy, and ionospheric physics. Bellin and Beyer’s experiment served as a model. The results provided new information on the axial and directional properties of the difference frequency wave field. Inspired by this, theoretical modeling continued along with further measurements. Other nonlinear effects like acoustic streaming (boundary layer, density gradient) were also investigated. In 1975, a project together with SIMRAD and Norwegian Technical University resulted in a bottom penetrating PAA, later commercialized as “TOPAS.” Numerical modeling based on the KZK equation resulted in the “Bergen Code,” still in use for computing nonlinear acoustic propagation problems. In later years activity at UoB has expanded to encompass linear physical acoustics of various sorts, occasionally using PAA as a tool.