Low‐frequency propagation measurements in sound channels often show a frequency‐independent excess loss that is evidently caused by internal scatter from large‐scale inhomogeneities. Using the Garrett–Munk internal wave spectrum (GM 75) as the scattering model, we have obtained an approximate solution of the ray‐diffusion equation on the deep sound‐channel axis. The estimated attenuation coefficient, ∠5×10−4dB/km, is consistent with the smaller experimental values reported.Subject Classification: [43]30.20; [43]20.20; [43]30.40.