Abstract: Pressure field of finite amplitude sources is examined in the paper. Detailed measurements of nonlinear distortion within beams radiated by plane and focused circular pistons are presented. Comparison of experimental results are made with numerical calculations based on the nonlinear parabolic wave equation (KZK) with the equivalent boundary condition adequate to the actual boundary condition. Special attention is paid to impact that the pressure distribution at the radiating surface has on the phenomena occurring in the vicinity of the source. Focused beam is examined over a distance extending up to post local region. Investigation is carried out with various pressure amplitude at the source in pseudo continuous wave conditions.