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Sound From the Underwater World
Although man's direct observation of the ocean depths is still extremely limited, he has been ingenious in devising methods of indirect observation. To the older methods of dredging, netting and water-sampling was added the use of sonar, an echo-location apparatus developed during World War II. Now rapid progress is being made in mapping ocean floors by echo location, and details of the mountains and troughs of the Atlantic and Pacific oceans have been worked out.
Submarine work with sonar devices first made man aware of the world of underwater sound. When, during the war, submarine listeners started systematic observations, they came across a great variety of sounds that had nothing to do with enemy ships. To identify these mysterious beeps, groans and croaks, scientists started underwater sound studies that are still continuing.
Whales and porpoises, as might be expected of social mammals, are very noisy. But so are many fish and many different kinds of invertebrates also make sounds. Sound travels faster in water than in air: At 0°C, the speed of sound in sea water is 1543 m per second; in air of the same temperature, it is only 332 m per second. Thus it is not surprising that animals in the sea have come to use sound for communication, and perhaps, like bats in the air, even for navigation.