Convergence Zone. This sound transmission path is based on the principle that sound energy from a shallow source travels downward in the deep ocean and is refracted at depth. The refracted rays travel upward and reflect off the surface about 30 miles from the sound source. The reflected rays travel downward, and the pattern repeats itself. The sound rays reappear in the surface layer at successive intervals of about 30 miles out to several hundred miles.

There are two conditions necessary for convergence zone transmission: (1) The sound velocity at depth must be equal to or greater than the sound velocity at the surface and (2) the water depth below the deeper sound velocity maximum must be great enough to permit the refracted sound rays to converge in a small area at the surface.

The three transmission paths just discussed depend upon the restrictive conditions of the velocity profile and the depth of the sound source and receiver. Thus, if velocity gradients are ignored, path predictions are not possible. The fourth path can be predicted roughly without con-sidering gradients. This path is the bottom reflected path, commonly termed bottom bounce.

Bottom Bounce. Bottom bounce transmis-sion uses angled ray paths to overcome velocity gradient changes. The sound energy is directed downward at an angle. With steeply inclined rays, transmission is relatively free from thermal effects at the surface, and the major part of the sound path is in nearly stable water. The sound energy is affected to a lesser degree by velocity changes

Figure 2-2-4.-Deep-water sound-transmission paths.

than the more nearly horizontal ray paths of other transmission modes.

Long ranges can occur in water deeper than 1,000 fathoms, depending on the bottom slope. It is estimated that 85% of the ocean is deeper than 1,000 fathoms, and bottom slopes are generally less than or equal to 1 degree. On this basis, relatively steep angles can be used for single bottom reflection to a range of approximately 20,000 yards. At shallower depths, multiple bounce paths develop which produce scattering and its high intensity energy loss.

Learning Objective: Define and differen-tiate between the elements used in the active and passive sonar equations.

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