Abstract: The derivation of the next nonlinear term of the KZK equation is done within original perturbation scheme on the base of the virial expansion for thermic equation of state. We also derived equations for second and third harmonic components in the nearfield of the sound wave generated by a piston transducer. Calculation scheme for a numerical estimation of the integrals for the fundamental and second harmonics is proposed as well as for the averaged third harmonic component. Some results of performed calculations are given for the illustration of the method possibility and comparison with direct finite difference solutions of the KZK equation.