A two-dimensional, moving, incompressible and electrically conducting fluid having trapped magnetic field (i.e. field threading the fluid) is confined by a uniform vacuum magnetic field. The fluid-vacuum field interface is horizontal and assumed to be free from instabilities. Dissipation by viscosity and resistivity is neglected, hence, for steady motion, the trapped field may be regarded as frozen into the fluid, in the sense that both move together. A pressure disturbance is introduced into the system, and it is found that a harmonic stationary wave is formed upstream provided certain criteria hold.