Scientists have been interested in the properties of sound waves for a long time. Early 19th century investigators realized that the speed of sound in water is different from its speed in air (see Good Vibrations). Building on fishermen’s and other seafarers’ knowledge of the way sound travels through water, researchers developed devices that used sound and echoes to measure distances. These devices were adapted for military use by the United States Navy, but naval officers encountered problems they couldn’t explain with their limited understanding of how sound traveled in water (see Navigation by Sound). Using a new device called a bathythermograph, scientists mapped out the thermal layers in the ocean and studied how the speed of sound varied with the temperature and pressure of the water. They realized that there were certain “shadow zones” in the water where objects were invisible to detection using sound waves (see A Sound-free Shadow Zone). Several factors including salinity
, temperature, and pressure influence how far sound travels underwater. Research on these issues has yielded great insight into the structure of the ocean (see Propagation of Sound in the Ocean).
Scientists then conducted experiments to test earlier proposed theories about the bending of sound waves in seawater. In analyzing the results of these tests, they discovered a channel across which sound waves could travel thousands of miles with minimal loss of signal. The U.S. Navy exploited this to devise a system called SOSUS which allowed them to detect and get information about submarines (see A Sound Pipeline). After the cold war ended, the Navy permitted civilian use of this device and scientists were able to learn a great deal about the geology and biology of the ocean, including the movement of whales around the world (see Listening to the Ocean). SOSUS also has enabled scientists to make ocean temperature measurements on a global scale, which has provided some evidence for global warming (see Probing the Ocean Interior with Sound).
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