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Propagation of Sound in the Ocean |
Once the war was over, the BT database provided a foundation for the kinds of basic ocean research Athelstan Spilhaus originally had in mind. In 1946, the Navy created the Office of Naval Research, which went on to become the premier funder of research in ocean acoustics . Scientists now resumed their investigations of the conditions that affect the propagation of an underwater sound signal.
A number of factors influence how far sound travels underwater and how long it lasts. For one, particles in seawater can reflect, scatter, and absorb certain frequencies of sound just as certain wavelengths of light may be reflected, scattered, and absorbed by specific types of particles in the atmosphere. Seawater absorbs 30 times the amount of sound absorbed by distilled water, with specific chemicals (such as magnesium sulfate and boric acid) damping out certain frequencies of sound. Researchers also learned that low-frequency sounds, whose long wavelengths generally pass over tiny particles, tend to travel farther without loss through absorption or scattering.
Further work on the effects of salinity, temperature, and pressure on the speed of sound underwater has yielded fascinating insights into the structure of the ocean. Speaking generally, the ocean is divided into horizontal layers in which sound speed is influenced more greatly by temperature in the upper regions and by pressure in the lower depths. At the surface is a sun-warmed upper layer, the actual temperature and thickness of which varies with the season. At midlatitudes, this layer tends to be isothermal, that is, the temperature tends to be uniform throughout the layer because the water is well mixed by the action of waves, winds, and convection currents; a sound signal moving down through this layer tends to travel at an almost constant speed. Next comes a transitional layer called the thermocline, in which temperature drops steadily with depth; as temperature falls, so does the speed of sound. However, at a point roughly 600 meters to 1 kilometer (0.4 to 0.6 miles) below the surface, further changes in temperature are slight (the water the rest of the way to the bottom is effectively isothermal). Now the dominant factor influencing the speed of sound is the increasing pressure, which causes sound to speed up.
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