Abstract: The stationary acoustic field in a two pIane parallel medium, sea surface and sea bottom, when a source near to the sea surface radiates, is characterized by the thickness of the medium and the reflectivity of the boundary surfaces. It is known that the maxima and minima of pressure are produced at same given depths depending on the radiated frequency and distance. Considering the ship noise as an acoustical source, the frequency spectrum of the acoustic pressure at certain depth, is not only the spectrum of the acoustic ship noise but it is also modified by the wave field originated by consecutive reflections at the boundary surfaces. As a consequence, over the free field spectrum of (he ship noise, a series of minima appears. The corresponding frequencies are those for which the receiver is located at the nodes of the stationary wave field.