Second Harmonic Component in Focused Gaussian Beam Propagating through Nonuniform Sound-Speed Layer

Abstract

The fundamental and second harmonic components in a focused sound transmitting through a layer of an inhomogeneous medium with a nonuniform sound speed are analyzed. The nonlinear wave equation including the perturbation of the sound speed is solved with a successive approximation. The influence of the nonuniformity on the measurement of the acoustic nonlinearity parameter B/A in biological samples inserted in the focal region is also investigated. The axial sound-speed variation causes the relative uncertainty of the measured B/A to be the same as the rate of the sound speed within the sample. On the other hand, since the sound beam is refracted owing to the lateral variation of the sound speed, the second harmonic amplitude of the sound detected with the receiver noticeably decreases. Thus, the B/A is underestimated by half the rate of change of the sound speed within a lateral range equivalent to the sample thickness.