Sonar

The invention:



A device that detects soundwaves 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 Langévin (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 Langévin,

who built the first working active sonar system by 1917. Langévin’s

original reason for developing sonar was to locate icebergs, but the

horrors of German submarine warfare inWorldWar I led to the new

goal of submarine detection. Both Langévin’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 WorldWar 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 × 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.

Asimple 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

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 beammaking

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.



Paul Langévin

If he had not published the Special Theory of Relativity in

1905, Albert Einstein once said, Paul Langévin would have

done so not long afterward. Born in Paris in 1872, Langévin 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.

Langévin’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, Langévin

worked on the implications of the space-time continuum.

DuringWorldWar II, Langévin, 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.







See also :  Aqualung ; Bathyscaphe ; Bathysphere ; Geiger counter ;

Gyrocompass ; Radar ; Richter scale ;  Paul Langévin .



 Further Reading