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 expansion 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 confirms the analytical predictions and shows the possibility of patterns in the form of stripes and squares.