Applications of nonlinear acoustics are getting popular in the field of nondestructive characterization of material microstructure and damage. Before the nonlinear acoustic technique is fully utilized, it is important to understand the behavior of nonlinear acoustic fields generated by finite amplitude excitation. A two wave mixing technique can be considered and the difference frequency component can be used to determine the nonlinear material properties. One important advantage of using the difference frequency component is known to minimize the source nonlinearity and to acquire low attenuating nonlinear signals. In this paper, the expression for the difference frequency sound beam is derived from the quasilinear theory of the Westervelt equation which includes the material nonlinearity, beam diffraction and material attenuation. Simulation results are presented and the beam properies are discussed in comparison with the behavior of the second harmonic beam fields.