In the parabolic and quasi-linear approximations, a theoretical description for the acoustical parametric field from a focusing source after inserting a sample is derived by using the superposition method of Gaussian beams. This model takes into consideration the attenuation, diffraction and nonlinear effects. The namerical computations of the axial sound pressure amplitude curves for the difference frequency sounds after the insertion of a sample are presented. The results indicate that this new model can be extended to measure or image the nonlinear parameter in the focused field.