Investigation of nonlinear wave distortion in a focal and post-focal region in water

The subject of interest is sources of finite amplitude. Focusing sources belonging to both of the two main differentiated groups of such sources have been examined. The first one consists of sources with additional lens, and the second one of the single-element PZT sources. Previous investigations concerned mainly phenomena occurring close to the source—in a prefocal and a focal region. The phenomena appearing at the longer distances in the post-focal region, where the nonlinear wave distortion is obviously considerably greater than in a pre-focal region, is the main topic of this paper. Experiments are conducted in measurements of spatial pressure distribution in a beam radiated by: (1) a single PZT element focused source with a fundamental frequency of 1.5 MHz, (2) a PZT plane circular source coupled with the plano-concave lens with a frequency of 1.0 MHz. A PVdF hydrophone with diameter of 1 mm was used as a pressure probe. The results of the experiments are compared to the theoretically predicted ones. The sound field was modeled by the KZK equation with the equivalent boundary condition adequate to the actual boundary condition.