The effect of a rough moving surface upon transmission between an omnidirectional source and receiver in an isovelocity medium is described for the case of a “slightly rough” surface using the method of physical optics. This is done by relating the spectral density of the received pressure to the spectral density of the source and the wave spectrum of the surface and also the geometry. The dependence of the received spectrum upon the interference between the direct and reradiated waves and the characteristics of the “frequency smearing” caused by scattering are described. A favorable comparison of the theory with the results of an experiment performed under nearly ideal conditions in a model tank is made.