Andrew Tarantola
At the end of World War II, just as relations between the US and the Soviet Union were beginning to freeze, America found itself dangerously vulnerable to Soviet ballistic missile submarine attack. To counter this potential nuclear threat to the Eastern seaboard, the US Navy set about devising an ingenious system of underwater listening stations that would eventually become one of the most effective oceanic monitoring systems ever created.
Development on the Sound Surveillance System (SOSUS) began in 1949 as a tactical, long-range means of countering the emerging Soviet submarine threat, most of which were still running on easily trackable diesel engines. The idea was to monitor the SOFAR channel, a horizontal layer of ocean water through which sound travels at its slowest speed. This allows low frequency sounds, like the knocking of a diesel engine to travel hundreds of miles before dissipating. As Edward C Whitman of Undersea Warfare explains:
In general, in warmer waters near the ocean surface, the sound speed is relatively high. At greater depths, where the water is increasingly cooler, the sound velocity decreases toward a minimum. At that point, pressure effects take over, and the sound speed begins to rise again as depth continues to increase. The deep sound channel is found at the depth where the sound velocity is a minimum. Because sound "rays" always tend to bend away from regions of higher sound velocity, a wave directed upwards from the sound channel axis will be refracted back down again – and a wave directed downwards will be bent upwards. Thus, sound paths from sources in the deep sound channel weave back and forth across the channel axis and – because they become "trapped" in a deep ocean layer away from the surface or bottom – can travel long distances with minimum attenuation. Moreover, if there exist propagation mechanisms available to bring near-surface sound down to the depth of the sound channel, those signals will also become trapped and traverse long distances with minimal loss. The sound channel axis is normally found at a depth of several thousands of feet, depending on thermal conditions, and because of the unusually warm waters of the Gulf Stream and Sargasso Sea, it lies more deeply in the Atlantic than in the Pacific
https://gizmodo.com/sosus-the-us-navys-long-range-undersea-ears-for-spotti-1588077646
