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
