A technique is presented for transforming certain acoustical boundary value problems in subsonic moving medium to a problem involving a stationary medium. The method is described by discussing the Sommerfeld problem of diffraction of a plane wave by a half plane immersed in a subsonically moving medium. It is shown that additional sources are induced by the flow at a leading edge impinged by a plane wave, while a trailing edge only gives rise to the usual diffracted wave. The flow also convects the geometrical acoustics part of the field. A transformation applied to the supersonic diffraction problems yields a unique reference problem, which is solved by Fourier transform methods. The pressure field is decomposed in a sum of a geometric acoustics field and a supersonic diffracted wave showing some remarkable similarities with the subsonic solution.