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In this paper we explain how acoustical wave propagates from far sources under the sea. Normal mode method is one of the fittest methods for calculating the patch of propagating acoustical energy in low frequency; it is for layered area in which layers have the same characteristics and all of them are parallel to the horizon. Pressure and density of acoustical energy can be determined everywhere, using normal mode method together with boundary conditions in calculated propagating modes and summing these modes with coefficient which is dependent on the
depth of source. At the end, for some velocity profiles and actual and virtual density, application of this method is described.