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A Sound-Free Shadow Zone |
The scientists had at their disposal a new device called a bathythermograph, or BT, invented in 1937 by Athelstan Spilhaus of WHOI and the Massachusetts Institute of Technology (MIT). The BT was a small torpedo-shaped device that held a temperature sensor and an element to detect changes in water pressure. Lowered overboard from a ship, the BT recorded pressure and temperature changes as it dropped through the water. Because the pressure in decibars is approximately equal to the depth in meters, technicians could correlate depth with temperature. Spilhaus thought his BT would have wide applications in learning many fundamentals about the ocean--the effect of temperature and depth on marine life, for instance, and the structure of ocean currents, especially the eddies along the sides of grand currents such as the Gulf Stream. But Iselin and the U.S. Navy used the BT to make a different, and more immediately useful, discovery.
The BT readings demonstrated that by early afternoon, the sun had warmed a layer of surface water 5 to 9 meters (16 to 30 feet) thick until it was about 1 to 2 degrees Celsius (2 to 4 degrees Fahrenheit) warmer than the water beneath it. Below the surface layer the water rapidly grew colder with depth. Knowing that the speed of sound increases with temperature, the scientists realized that signals from the ship's sonar would travel quickly through the warm layer and then slow dramatically when they hit the cooler layer below. They discovered that sound waves passing between layers with different properties underwent refraction  bending away from the region where sound travels faster and toward the region where its speed slows. This bending creates an acoustics "shadow zone," allowing any submarine positioned just beneath the dividing line between the warmer and cooler layers of water, to become invisible to sonar signals.
Columbus Iselin immediately recognized the significance of the acoustic shadow zone and the BT to submarine warfare. A submarine equipped with a BT could use it to determine where the shadow zone lay in relation to the pursuing ship, thus becoming nearly invisible to an enemy sonar. A sub chaser, for its part, could use a BT to opposite effect, adjusting the direction of its sonar to take into account the expected refraction.
During World War II, the BT became standard equipment on all U.S. Navy subs and vessels involved in antisubmarine warfare. Naval officers went to WHOI to learn how to use the BT, and oceanographers traveled to naval bases around the country to train battle-bound sailors. Submariners were directed to send all of their BT records to WHOI or to the University of California Division of War Research at Point Loma, where sonar charts were prepared and issued to the fleet.
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