Simulating nonlinear acoustic field propagation with application to fisheries research

Abstract

Sound traveling through water was traditionally thought to be a linear or close to linear process. With the recent improvement of echosounders and their accompanying equipment, one has become aware that excess sound attenuation may occur due to nonlinear effects. Echosounders used today do not correct for nonlinear effects, but disregarding them altogether may very well lead to erroneous results when interpreting measurements made with the echosounder. This possibility is studied in the thesis. A simulator for the linear and nonlinear propagation of sound is developed, and verified for several different propagation problems. The field for a source appropriate for use in fisheries research is calculated by means of the simulator. The size of the nonlinear loss is estimated for several power settings and ranges, and is found in some cases to reach several decibels. The thesis also discusses the possibility of utilizing the second harmonic directly, as is common in Tissue Harmonic Imaging.