The invention: A device that detects sound waves transmitted through water, sonar was originally developed to detect enemy submarines but is also used in navigation, fish location, and ocean mapping.
The people behind the invention:
Jacques Curie (1855-1941), a French physicist Pierre Curie (1859-1906), a French physicist Paul Langevin (1872-1946), a French physicist
Active Sonar, Submarines, and Piezoelectricity
Sonar, which stands for sound navigation and ranging, is the American name for a device that the British call “asdic.” There are two types of sonar. Active sonar, the more widely used of the two types, detects and locates underwater objects when those objects reflect sound pulses sent out by the sonar. Passive sonar merely listens for sounds made by underwater objects. Passive sonar is used mostly when the loud signals produced by active sonar cannot be used (for example, in submarines).
The invention of active sonar was the result of American, British, and French efforts, although it is often credited to Paul Langevin, who built the first working active sonar system by 1917. Langevin’s original reason for developing sonar was to locate icebergs, but the horrors of German submarine warfare in World War I led to the new goal of submarine detection. Both Langevin’s short-range system and long-range modern sonar depend on the phenomenon of “piezoelectricity,” which was discovered by Pierre and Jacques Curie in 1880. (Piezoelectricity is electricity that is produced by certain materials, such as certain crystals, when they are subjected to pressure.) Since its invention, active sonar has been improved and its capabilities have been increased. Active sonar systems are used to detect submarines, to navigate safely, to locate schools of fish, and to map the oceans.
Sonar Theory, Development, and Use
Although active sonar had been developed by 1917, it was not available for military use until World War II. An interesting major use of sonar before that time was measuring the depth of the ocean. That use began when the 1922 German Meteor Oceanographic Expedition was equipped with an active sonar system. The system was to be used to help pay German World War I debts by aiding in the recovery of gold from wrecked vessels. It was not used successfully to recover treasure, but the expedition’s use of sonar to determine ocean depth led to the discovery of the Mid-Atlantic Ridge. This development revolutionized underwater geology.
Active sonar operates by sending out sound pulses, often called “pings,” that travel through water and are reflected as echoes when they strike large objects. Echoes from these targets are received by the system, amplified, and interpreted. Sound is used instead of light or radar because its absorption by water is much lower. The time that passes between ping transmission and the return of an echo is used to identify the distance of a target from the system by means of a method called “echo ranging.” The basis for echo ranging is the normal speed of sound in seawater (5,000 feet per second). The distance of the target from the radar system is calculated by means of a simple equation: range = speed of sound x 0.5 elapsed time. The time is divided in half because it is made up of the time taken to reach the target and the time taken to return.
The ability of active sonar to show detail increases as the energy of transmitted sound pulses is raised by decreasing the sound wavelength. Figuring out active sonar data is complicated by many factors. These include the roughness of the ocean, which scatters sound and causes the strength of echoes to vary, making it hard to estimate the size and identity of a target; the speed of the sound wave, which changes in accordance with variations in water temperature, pressure, and saltiness; and noise caused by waves, sea animals, and ships, which limits the range of active sonar systems.
A simple active pulse sonar system produces a piezoelectric signal of a given frequency and time duration. Then, the signal is amplified and turned into sound, which enters the water. Any echo that is produced returns to the system to be amplified and used to determine the identity and distance of the target.
Most active sonar systems are mounted near surface vessel keels or on submarine hulls in one of three ways. The first and most popular mounting method permits vertical rotation and scanning of a section of the ocean whose center is the system’s location. The second method, which is most often used in depth sounders, directs the beam downward in order to measure ocean depth. The third method, called wide scanning, involves the use of two sonar systems, one mounted on each side of the vessel, in such a way that the two beams that are produced scan the whole ocean at right angles to the direction of the vessel’s movement.
Active single-beam sonar operation applies an alternating voltage to a piezoelectric crystal, making it part of an underwater loudspeaker (transducer) that creates a sound beam of a particular frequency. When an echo returns, the system becomes an underwater microphone (receiver) that identifies the target and determines its range. The sound frequency that is used is determined by the sonar’s
Active sonar detects and locates underwater objects that reflect sound pulses sent out by the sonar.
Paul Langevin
If he had not published the Special Theory of Relativity in 1905, Albert Einstein once said, Paul Langevin would have done so not long afterward. Born in Paris in 1872, Langevin was among the foremost physicists of his generation. He studied in the best French schools of science—and with such teachers as Pierre Curie and Jean Perrin—and became a professor of physics at the College de France in 1904. He moved to the Sorbonne in 1909.
Langevin’s research was always widely influential. In addition to his invention of active sonar, he was especially noted for his studies of the molecular structure of gases, analysis of secondary X rays from irradiated metals, his theory of magnetism, and work on piezoelectricity and piezoceramics. His suggestion that magnetic properties are linked to the valence electrons of atoms inspired Niels Bohr’s classic model of the atom. In his later career, a champion of Einstein’s theories of relativity, Langevin worked on the implications of the space-time continuum.
During World War II, Langevin, a pacifist, publicly denounced the Nazis and their occupation of France. They jailed him for it. He escaped to Switzerland in 1944, returning as soon as France was liberated. He died in late 1946.
purpose and the fact that the absorption of sound by water increases with frequency. For example, long-range submarine-seeking sonar systems (whose detection range is about ten miles) operate at 3 to 40 kilohertz. In contrast, short-range systems that work at about 500 feet (in mine sweepers, for example) use 150 kilohertz to 2 megahertz.
Impact
Modern active sonar has affected military and nonmilitary activities ranging from submarine location to undersea mapping and fish location. In all these uses, two very important goals have been to increase the ability of sonar to identify a target and to increase the effective range of sonar. Much work related to these two goals has involved the development of new piezoelectric materials and the replacement of natural minerals (such as quartz) with synthetic piezoelectric ceramics.
Efforts have also been made to redesign the organization of sonar systems. One very useful development has been changing beam-making transducers from one-beam units to multibeam modules made of many small piezoelectric elements. Systems that incorporate these developments have many advantages, particularly the ability to search simultaneously in many directions. In addition, systems have been redesigned to be able to scan many echo beams simultaneously with electronic scanners that feed into a central receiver.
These changes, along with computer-aided tracking and target classification, have led to the development of greatly improved active sonar systems. It is expected that sonar systems will become even more powerful in the future, finding uses that have not yet been imagined.
See also Aqualung; Bathyscaphe; Bathysphere; Geiger counter; Gyrocompass; Radar; Richter scale; Ultrasound.
