Abstract: We start from the cubic KZK equation for ultrasonics beam that accounts first, second and third powers of density in pressure Taylor series expansion. In a condition of moderate amplitude and nearfield one can use approximate solutions provided by perturbation method including terms resonant to the multiple frequencies on a transducer. We consider three resonant harmonics within Rayleigh distance range. The second and third harmonics averaged over the beam cross-section are expressed in terms of some standard integrals and nonlinear constants. Fourier transforms of a signal on a receiver are equalized to the results of the evaluation that give equations for the non-linear constants determination. This in tum allows to compute constants B/A and C/A of the equation of state (virial expansion).