Second harmonic component of nonlinearly distorted wave in a 20-MHz focused sound field.

Abstract

The nonlinear distortion of the 20-MHz ultrasound focused by a solid acoustic lens is theoretically and experimentally examined. The nonlinearity which may exist in the solid lens is neglected in the present theory as well as the linear absorption within the lens. The focused beam is detected with a planar receiver as large as the beam width, after passing through the similar receiving lens. In another way, the beam is also detected with different planar receiver, whose aperture is larger than the beam width, without using a receiving lens. At lower source level, the experimental result agrees well with the successive approximation solution for the Khokhlov-Zabolotskaya-Kuznetsov equation both in the amplitudes and phase difference of the fundamental and second harmonic components. When the source is intense within the upper limit of 10-W peak input-power, the harmonic amplitudes of the fundamental and second harmonic components seriously decrease while passing through the focus, and tend to saturate due to nonlinear absorption.