Second harmonic component of a nonlinearly distorted wave in a focused sound field

Abstract

The second harmonic in a focused nonlinear field is examined with a numerical computation and with experiments done in water. The computation is accomplished by rewriting an expression derived by Lucas and Muir [J. Acoust. Soc. Am. 74, 1522–1528 (1983)] into a form appropriate for numerical computation at any point. The primary velocity distribution at the source is experimentally defined and utilized in the computation. The experiment is conducted with a 2-MHz focusing sound source consisting of a circular transducer coupled to an acoustic lens. The experimental values for amplitude and phase of the second harmonic component are in reasonable agreement with the theory. The second harmonic component focuses more sharply than the fundamental component, and the amplitude ratio of the second harmonic component to the fundamental component attains the maximum value at the focus. The results lead to an interpretation that the component generated in the focal region is the dominant contributor to the propagating second harmonic component.