عنوان مقاله [English]
Sound travels to long distances in the ocean by various forms of ductedpropagation. When sound travels in a duct, it is prevented from spreading indepth and remains confined between the boundaries of the duct. The surfaceduct is a zone bounded above by the sea surface and below by the SLD.Within the surface duct, sound rays are alternatively refracted and reflected.A surface duct exists when the negative temperature gradient within it doesnot exceed a value determined by the effect of pressure on sound speed. Specifically, the surface duct is characterized by a positivesound-speed gradient.At very low frequencies, sound ceases to be trapped in the surface duct. So calculating themaximum wavelength for duct transmission is very important. In this paper we used a profile as a pattern of sound velocity distribution to calculate related parameters. Finally an optimum frequency for minimum scattering of sound channel propagation presented.