The nonlinear sound pressure field from finite amplitude, plane, rectangular apertures, is considered. Experimental measurements on a 2.25-MHz transducer are presented for various source geometries and excitation levels. These results are compared with simulations based on the transformed beam equation, using a new algorithm which generalizes the Bergen code to treat the case of nonaxisymmetric on-source boundary conditions.

Results of a systematic investigation of a high intensity sound beam from a plane circular transducer are presented for a large range of excitation levels. The ratio of the shock formation distance to the Rayleigh distance ranges from 1.7 to 0.06. A detailed description of the experimental setup is given. Problems encountered when calibrating highly intense and highly directive sound sources are discussed, and a novel method of calibration is proposed. Experimental results for the case of cw excitation are presented and compared with a simulation using the Bergen Code, thus allowing to assert the usefulness of the model (Khokhlov–Zabolotskaya–Kuznetsov equation) and of the numerical algorithm (spectral decomposition and finite difference method of the transformed beam equation) for high intensity sound beams. A new effect, nonlinear defocusing of the beam, is uncovered.

The experimental investigation reported by Nachef et al. (see preceding abstract) represents a good opportunity to assert the validity of the transformed beam equation and its numerical implementation [Naze Tjo/tta et al., J. Acoust. Soc. Am. 88, 2859–2870 (1990)], and their usefulness to simulate highly intense sound beams. Comparison between theoretical and experimental results shows that in spite of the fact that it does not account for higher-order nonlinear effects, the model reveals a remarkable ability to accurately simulate the fine structure of the sound field, at any range and level of excitation under consideration. The present theoretical investigation confirms the nonlinearly induced shift in the near-field structure of the beam (defocusing) experimentally discovered by Nachef et al. [Work supported by The Norwegian Research Council for Science and Humanities (NAVF), and Fondation Scientifique de Lyon et du Sud-Est, France.]