Finite-amplitude propagation in lossless and absorptive media

Abstract

The propagation of finite-amplitude waves in distilled water and an absorptive fluid media with acoustic characteristics similar to tissue was investigated. Axial and focal beampatterns of linear and quasilinear waves were obtained from lens focused sources of 1.75, 2.25, 2.94, and 3.38 MHz using a PYDF needle-type hydrophone. Experimental beampatterns were compared to spherically converging and focused Gaussian beam theories. The shock parameter at the focus predicted from Gaussian theory was in general agreement with the estimate made from the relative strengths of the first four harmonics to the fundamental. Apodization provided by the lens reduced, but did not eliminate, the nearfield maxima, minima, and sidelobes associated with a piston source. Additional sidelobes predicted from the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation were evident in the harmonic beampatterns. Harmonic focal beamwidths decreased as n−1/2 for the lower source frequencies, in agreement with theory. Beamwidths were generally more narrow in the absorptive media than in the water. The second harmonic peak location on-axis was distal to the fundamental, although the difference was small for higher source frequencies and propagation in the absorptive media. Higher harmonics had sharper axial focal dimensions, but the position of the axial peak was dependent on the trade-off between propagation growth and decay caused by attenuation. The extension of the results to propagation in tissue is noted.