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Analysis of the difference-frequency wave generated by the interaction of two axisymmetric and co-focused ultrasound beams

We investigate the interaction of two co-focused ultrasound beams at different frequencies in the presence of an inclusion inside the focal region. We solved the boundary-value problem based on the Aanonsen's nonlinear wave equation in the parabolic approximation. A comparison between the theory and experimental data for the scattering by a small sphere is presented. Results are in good agreement with the presented theory.