### Sound Channel Convergence Zone

 A sound channel is a region in the water column where sound speed first decreases with depth to a minimum value, then increases. The depth of minimum velocity resulting from this gradient is called the Sound Channel Axis, as shown in the figure at right. The lower boundary of the channel is the depth of maximum sound speed below the sound channel axis. The upper boundary is the depth above the sound channel axis where the sound speed is equal to the maximum sound speed of the lower boundary. Sound waves produced in the channel are trapped because sound traveling above the sound channel axis is refracted downward, while below the axis, sound is refracted upward.  Generally, the thicker the sound channel, the more sound will be trapped. As with a surface duct, frequencies above the cutoff frequency will be trapped, while lower frequencies have wavelengths that are too long and will not be trapped. The figure to the left is useful for determining the cut-off frequency in a sound channel, which depends on duct width, the frequency of interest, and the gradient of the sound speed. For example, if the duct is 500 feet wide, and the rate of change in sound speed is 2.5 feet per second (upper curve), we can read down to the x-axis and see that the cut-off frequency is 200 Hz. So anything at 200 Hz or higher will be trapped in this sound channel.
 Sound channels are classified as either shallow sound channels or deep sound channels. Shallow sound channels occur in the main thermocline. They are usually associated with fronts and eddies, and are therefore, transitory.  The deep sound channel (DSC) is a permanent feature of the deep ocean. The axis of the DSC is at the base of the thermocline, where the sound speed begins to increase. The upper boundary is the SLD, and the lower boundary is where the sound speed below the axis is equal to the sound speed at the top of the channel (the SLD). While the Deep Sound Channel may be too deep to access with listening devices, whales use this channel to communicate over very long distances.
 Convergence Zones (CZ) are surface or near-surface regions where "focusing" of sound rays occurs, resulting in very high sound levels. In a CZ, sound rays leaving the near surface region are refracted downward by the negative sound speed gradient of the main thermocline, then refracted back toward the surface from the deep layer where the sound speed gradient becomes positive. These rays intersect at or near the surface through the combined effects of downward and upward refractions, resulting in an infinite sound pressure level for that specific point. This effect, known as "caustics", is shown in the figure to the right. Multiple convergence zones are possible. For example,if the refracted sound reaches the sea surface, the sound waves reflect at the air-sea boundary, back into the water column to undergo further propagation toward the second CZ as depicted in the figure to the left. Primarily because fo its effectiveness, and ease of prediction (all one needs to know is the SVP and the water depth), the presence of convergence zones (CZ) is of major importance in USW operations. Many of the search and detection tactics are based on whether or not Convergence Zone sound paths exist. ```

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