The fundamental and second harmonic signals beyond the 1.0 mm thick polystyrene plate with a slit of 0.1 mm or 0.2 mm width inserted at the focal plane of a focused Gaussian beam of 1.9 MHz frequency and 85 mm focal length, which are detected with a concave receiver set confocal with the source, are investigated. In the theory based on a model that the sound transmitting through the slit uniformly delays across the slit in its phase compared to the sound passing through the solid plate, the calculation is simplified by using analytical solution for the spatial distribution of the concave receiver sensitivity and by integrating the virtual source induced by the nonlinear propagation weighted with the receiving sensitivity distribution. Furthermore the amount of phase delay is experimentally estimated. The calculated result well explains the experimental observation that only the second harmonic amplitude is significantly decreased by the insertion of the slit into the beam, and shows that this phenomenon is primarily due to the difference in frequency.
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