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Navigation by Sound |
Down through the ages, fishermen and seafarers had been taking advantage of the way sound travels through water and using rudimentary techniques of echolocation . In the days of the ancient Phoenicians, for example, fishermen gauged the distance to a headland concealed by fog by making a loud noise such as ringing bells and listening for the echoes. By 1902, ships passing along the American coast were warned of hidden shoals by underwater bells placed on stationary lightships. Ten years later, the Titanic tragedy motivated the Submarine Signal Company of Boston (now part of Raytheon Company) and others to begin developing more active devices that would warn of icebergs and other navigational hazards. Within a week of the tragedy, L. R. Richardson filed a patent with the British patent office for echo ranging with airborne sound, following a month later with a patent application for the underwater equivalent. The first functioning echo ranger, however, was patented in the United States in 1914 by Reginald A. Fessenden, who worked for the Submarine Signal Company. Fessenden's device was an electric oscillator that emitted a low-frequency noise and then switched to a receiver to listen for echoes; it was able to detect an iceberg underwater from 2 miles away, although it could not precisely determine its direction.
More sophisticated echo sounders were developed during World War I by the allies, but they were no match for the German U-boat menace because they could not locate and track a moving object. Shortly after the war, H. Lichte, a German scientist looking into using acoustics to clear German harbors of mines offered a theory on the bending, or refracting, of sound waves in seawater that would provide clues to solving the difficulty. Building on work by Lord Rayleigh and an earlier Dutch astronomer named Willebrord Snell, Lichte theorized in 1919 that, just as light is refracted when it passes from one medium to another, sound waves would be refracted when they encountered slight changes in temperature, salinity, and pressure. He also suggested that ocean currents and changes in seasons would affect sound propagation. Unfortunately, Lichte was so far ahead of his time that his insights went unrecognized for almost six decades.
In the United States, efforts to develop more sophisticated echolocating devices continued between the wars under the guidance of Harvey C. Hayes of the Naval Engineering Experimental Station at Annapolis, Maryland. Hayes encouraged the U.S. Navy to play a role in civilian oceanography during peacetime, a collaboration that continues today. Thus, by the years just prior to the outbreak of World War II, U.S. naval ships were equipped with sonic depth finders as well as improved echo rangers called sonar (for sound navigation and ranging) that could pick up the noise of a submarine's propeller or an echo off a sub's hull from several thousand yards away. However, the devices were mysteriously unreliable. In the summer of 1937, officers aboard the U.S.S. Semmes were at a loss to explain or correct the ship's sonar problems during exercises in the waters off Guantánamo Bay, Cuba. For some reason, the performance of the devices consistently deteriorated in the afternoon; they sometimes failed to return echoes at all. The captain of the Semmes sought help from the Woods Hole Oceanographic Institution (WHOI) in Woods Hole, Massachusetts. Columbus Iselin, then associate director of WHOI, joined the Semmes with his laboratory's research ship Atlantis to investigate this puzzling "afternoon effect."
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