How is sound used to find submarines?
Submarines have a unique advantage over other types of ships because they are able to stay below the sea surface. One of their primary jobs is to stay hidden. Since they are out of sight most of the time they are at sea, submarines must be found using other senses. One of the few ways of finding submarines is by using sound.
Submarines can be found using passive acoustics or active acoustics. In passive acoustics, submarines can be detected by the sounds that they make. These sounds travel through the water for great distances. In order to hear the submarine, there must be receivers that are listening for them. The U.S. Navy has placed hydrophones on the seafloor in strategic areas of the continental shelf in the North Pacific Ocean and North Atlantic Ocean. These fixed hydrophones are called the Sound Surveillance System (SOSUS). The hydrophones are connected to shore stations where the acoustic data are analyzed. SOSUS is capable of locating a submarine within a radius of 50 nautical miles or less (Wit, 1981). Another system of receivers used by the U.S. Navy is called Surveillance Towed Array System (SURTASS). As the name implies, SURTASS is an array, or line, of hydrophones towed behind a moving ship. Because they are not in a fixed position like SOSUS, SURTASS greatly increases the area where submarines can be found. Submarines themselves are equipped with passive sonar systems that are used to detect and determine the relative position of underwater acoustic sources.
The Navy can also use active acoustics to find submarines much the same way people use active acoustics to find fish (See "How is sound used to locate fish?"). By transmitting a sound pulse, they can determine the direction of the echoes that return from objects hit by the sound. They can also measure the time it takes for echoes to return and calculate the distance to the object causing the echo. Much research has been done on classifying the kinds of echoes that different objects make.
The tactics of antisubmarine warfare are quite complex. The first step is to detect the sounds of a submarine among the background, or ambient, noise of the ocean. This is even more complicated when a modern diesel submarine is operating on batteries. It is then difficult for passive sonar to pick up the sub because of its minimal noise. Rotating propellers and other machinery, such as engines and pumps produce noise that can be identified by experienced sonar operators. Also the sub's own active acoustical systems create noise while they are transmitting sound, or "pinging." Different ships and submarines have unique sounds or noises associated with them. These sounds are used to determine the "signature" of a particular vessel.
Another acoustics system for assisting with antisubmarine warfare is the U.S. Navy's sonobuoy system. Sonobuoys are dropped into the ocean (from either an aircraft or ship) to record underwater sounds. These instruments include a hydrophone and a radio transmitter to send sound signals back to the aircraft or ship. The U.S. Navy uses these instruments to listen for enemy submarines. They can be dropped by airplanes into the water from altitudes as high as 30,000 feet. The sonobuoys are approximately three feet long and 5 inches in diameter. Passive acoustic sonobuoys detect acoustic signals generated by submarines and provide information on their location. The exact location of a target can be determined by deploying a pattern of sonobuoys. Sonobuoys may also be used to record marine mammal calls and listen for earthquake activity.
An example of a passive acoustic sonobuoy is the Navy's Directional Frequency Analysis and Recording DIFAR} device. This sonobuoy deploys four directional hydrophones and one omnidirectional hydrophone at depths up to 1000 feet. It detects acoustic energy from 5 to 2,400 Hz. It can operate for up to eight hours. The Navy has also provided these sonobuoys for research to track whale populations and monitor underwater volcanic activity.
Another passive acoustic sonobuoy is the Navy's LOFAR buoy system. This is used primarily for intelligence gathering. It deploys an omnidirectional hydrophone and can detect acoustic energy from 5 Hz to 40 kHz. This buoy can work at depths up to 400 feet and stay in the water for up to eight hours. It provides data on ambient noise levels. It has also been used during research for fishery and marine mammal projects. There are two types of active acoustic sonobuoys. One is used for searching and one is used for the final fixing of a target submarine position. The searches are accomplished by sending out an acoustic signal over large areas.