Diffraction in finite-amplitude focusing sound field and its application to acoustical imaging.

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A sinusoidal sound wave emitted by a spherically focusing source distorts in waveforms with propagation toward the focus mainly due to the elastic nonlinearity of the medium, which can be described with the nonlinearity parameter. It was theoretically and experimentally proved that the largest part of the second harmonic component seen at the post focal region was generated in the focal region, because the secound harmonic components generated in the preand post-focal region cancel each other affected by the phase shift of the sound wave passing through the focus. Inserting a sample whose nonlinearity parameter differs from that of surrounding medium (water) into the focal region, the magnitude of the second harmonic component with the phase parameter of <pi>/2 radians seriously changes. This leads to the possibility to detect and visualize how the nonlinearity parameter of samples immersed in the focus differs from water. On the other hand, since the wavf propagation of a cylindrically focusing source does not suffer a similar phase shift the second harmonic component generated at eh focal region cannot be dominant in the post focal region. Therefore only the spherically focused sound may be appropriate for the present research project. Assuming a general case where the linear parameters (the ambient density, the sound speed, and the absorption coefficient) of the sample differed from those of surrounding medium as well as the nonlinearity parameter, the influence of the sample on the sound wave was theoretically and experimentally discussed. The result shows that the second harmonic component observed in the post focal region strongly depends on the linear acoustic parameters as well as on the nonlinerity parameter. However the method to estimate the nonlinearity parameter when both the density and sound speed are known has been shown. The realization of a gaussian focusing source will hoprfully make possible to visualize the nonlinerity parameter of the sample.