  Propagation of sound beam diffracted from a circular aperture in far-field
region has been studied in this paper by the method of angular spectrum
representation and stationary phase method. This nonparaxial theory is useful
when beam angle is not very small and the wavelength $\lambda $ is comparable
to aperture diameter $a$ unlike the situation in paraxial approximation. Here
we have studied two cases, one for a Gaussian source and other for a plane
piston source.
