A finite difference formulation is presented which is useful in the study of acoustically treated inlet and exhaust ducts used in turbofan engines. The difference formulation can readily handle acoustic flow field complications, such as axial variations in wall impedance and cross-sectional area, that would occur in a sonic inlet. In formulating the difference solutions, the continuous acoustic field is lumped into a series of grid points spread uniformly throughout the field. At each point, the pressure is separated into its real and imaginary terms. Example solutions are presented for sound propagation in a one-dimensional straight hard-wall duct and in a two-dimensional straight soft-wall duct without steady flow.