Abstract: T. G. Muir et al. observed experimentally that the maximum of the received sound pressure in the sediment insonified by a parametric array departed significantly from the prediction of Snell's law. The wave fronts penetrated more steeply into the sediment and the attenuation with depth was less than that predicted by plane wave theory at the postcritical incidence.
It was found that due to the variation of the length of parametric array and the variation of the attenuation in the sediment, the maximum of the received sound pressure occurs at the line-of-sight between projector and hydrophone. It has been proved both theoretically and experimentally that Snell's law is still valid when the length of parametric array and the attenuation in the sediment are kept constant. Due to the contribution of the secondary sound close to the boundary, the postcritical penetration of a parmetric array can be deeper. The lateral wave should be considered at the postcritical incidence .