Influence of acoustic intensity on the second-harmonic beam-profile

A method for fast numerical simulation of nonlinear wave propagation based on a quasilinear approximation, has previously been presented by H. Torp et al. (2002). The aim of the current study was to validate this method by comparing the simulation results to the results from a conventional simulation model, and to experimental measurements. The reference simulation model was based on solving the KZK-equation using an operator splitting approach. Experimental measurements were obtained in a water tank. An annular array probe was used to generate a focused transmit pulse. Hydrophone measurements of the ultrasonic field were used for experimental verification of the simulation. Beam-profiles from the two simulation methods and the hydrophone measurements were compared for pulses with a mechanical index (MI) ranging from 0.1 to 1.9. The results showed an almost perfect match between the two simulation models for MI less than 0.5. For higher MI values, the KZK-based method showed a gradual increase in the side-lobe level, whereas the main-lobe width and shape were preserved. These effects were also demonstrated by the hydrophone measurements.