  Pattern formation of sound is predicted in a driven resonator where
subharmonic generation takes place. A model allowing for diffraction of the
fields (large-aspect ratio limit) is derived by means of the multiple scale
expansions technique. An analysis of the solutions and its stability against
space-dependent perturbations is performed in detail considering the
distinctive peculiarities of the acoustical system. Numerical integration
confirm the analytical predictions, and shows the possibility of patterns in
the form of stripes and squares.
