The paper is a contribution to the theory of the parametric acoustic array formed by the nonlinear interaction of sound waves in a thermoviscous fluid. The source of the primary waves is a circular piston, and emphasis is put on an analysis of the farfield properties of the generated difference frequency sound field from two sound beams. Effects from interaction in the divergence regions are studied and compared with the effects from a model composed of a plane collimated nearfield region of interaction, and a farfield divergence region. Previous models are analysed with emphasis on a discussion of the range of validity. Theoretical results are compared with experimental observations.