Abstract: The paper presents the results of theoretical investigations of the finite amplitude waves interaction in water. The mathematical model and some results of numerical investigations are presented. The acoustic pressure amplitude changes for different frequency waves (especially difference frequency wave) along the sound beam were investigated. The mathematical model was built on the basis of the KZK equation. It was assumed that a circular piston was the source of two different frequency finite amplitude waves. The problem was considered as an axial symmetric one. The solution of the problem was looked for inside a truncated cone. To solve the problem the finite-difference method was applied.