Sound VelocityLearning Objective: | Identify the properties that control sound velocity in the ocean and recognize how each controls the speed and direction of a sound wave. |
If you remember from an earlier lesson, velocity takes into account both speed and direction.The speed of sound in seawater is governed by temperature, pressure, and salinity. An increase in temperature increases the speed of sound in water, while a decrease in temperature decreases the speed of sound. The same relationship applies to pressure and salinity. An increase in pressure causes an increase in sound speed, as does an increase in salinity, and vice versa. Since pressure is a function of depth in the sea, if we were to discount the effect of temperature and salinity, sound would travel faster at the ocean bottom than it does at the surface. However, we cannot discount either of these other two variables, especially temperature. It is the most important property controlling the speed of sound in water.As far as direction is concerned, sound waves travel in straight lines only in a medium in which the speed is everywhere constant. For this to occur in seawater, the temperature, pressure, and salinity values would have to be unchanging. Changes in any or all of these variables do occur, which in turn affects the speed of sound waves. The change in speeds along the sound wave causes the wave to change direction. Sound waves are bent (refracted) in the direction of the slower sound velocities. The degree of refraction is proportional to the velocity gradient. If the velocity gradient is such that there is a rapid increase in the speed of sound with depth, a sound wave will be sharply refracted toward the surface, the direction of the slower sound velocities.On the other hand, a rapid decrease in the speed of sound with depth causes a sound wave to be sharply refracted toward the ocean bottom. Sound in the ocean, especially as it relates to anti-submarine warfare, will be covered in greater detail in Unit 2.
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