Nonlinear propagation of focused ultrasound beams from a wide-angle aperture

Sound pressure amplitudes of the first three harmonics in finite-amplitude CW ultrasound beams are measured along and across the beam axis in fresh wa ter. The sound source is a 1.6 MHz focused transducer with a circular apert ure of 73 mm in diameter and a focal length of 75 mm. The source pressure a mplitude is supposed to be distributed uniformly. The aperture half-opening angle of 29 degrees exceeds the upper limit of the applicability of the pa raxial approximation, so the present authors propose a beam model equation using the oblate spheroidal coordinate system for theoretical prediction [J .Acoust.Soc.Am. 107, 3035-3046 (2000)]. Numerical solutions of the model eq uation are overall in good agreement with the experiment. Thus, the support ed validity of the model equation allows us to further study nonlinear prop agation of ultrasonic pulses from a strongly focusing source. It has been d etermined numerically that the direct wave from the center of the sourer ha s a crucial effect on forming a shock-like spike in the post-focal region.