Sonar is an acronym for
"Sound Navigation and Ranging." It is a tool that uses underwater
acoustics to determine water depth, the location of mines, and the presence of
submarines. It serves as the Navy's front line of defense from underwater
attack, and is the only reliable way to identify, track, and target submarines.
There are two major classes of sonar: passive and active.
Passive Sonar
Passive sonar is a listening
device that uses hydrophones (underwater microphones) that receive, amplify, and
process underwater sounds. It is primarily used to detect the presence of
submarines. Determination of the type of submarine can be made by matching the
frequency of sound detected with the known frequencies of various submarine
engine plant sounds and the unique sounds emitted by the different types of
propellers in water (cavitation). The advantage of passive sonar is that it
places no sound in the water, and thus does not reveal the location of the
listening vessel. Passive sonar can indicate the presence, character, and
movement of submarines, but it is ineffective at detecting modern, quiet
submarines running submerged on batteries, as well as submarines that are
motionless (lying in wait) or running at very low speeds. Active sonar is needed
for precise location, classification, and rapid targeting.
Active Sonar
Active sonar emits pulses of
sound that travel through the water, reflect off objects, and return as an echo
to an underwater acoustic receiver. It uses an underwater speaker to generate
sound waves. The sound waves travel until they strike an object and are
reflected in various directions. Some of the reflected waves return to the
receiver, where they are converted back into electric signals, amplified and
displayed. A careful interpretation of the sound can provide the direction and
distance of the object, as well as its size and speed. This is accomplished
through "echo ranging," which measures the time it takes for a sound
wave to travel from the transducer, reflect off the object, and return to the
receiver. Active sonar is critical for locating and tracking submarines. It is
used sparingly, however, because it also allows an enemy submarine to pinpoint
the position of the ship emitting the sound. To put Navy active sonar use in
perspective, it is important to note that, of the U.S. Navy's approximately 280
surface ships, only about 58% are equipped with mid-frequency active sonar.
About half of these ships are underway at any given time, and for each ship,
active sonar is turned on only a small percentage of the time (during certain
types of training and maintenance activities).
United
States Navy graphic
Types of
Active Sonar
Different types of active sonars
operate at different frequencies, according to their purpose.
High Frequency:
High frequency sonar (>10 kHz) is primarily used for determining water depth
(fathometers), hunting mines, and guiding torpedoes. At higher frequencies, the
sound energy is greatly attenuated (weakened due to scattering and absorption)
as it travels through the water. This results in shorter ranges, typically less
than five nautical miles.
Mid Frequency: Mid frequency sonar, which includes the
AN/SQS-53 system, has been in use since World War II, and is the primary tool
for identifying and prosecuting submarines. Mid frequency sonar (1 kHz - 10 kHz)
suffers moderate attenuation and has typical ranges of 1-10 nautical miles.
Low Frequency: Low frequency sonar (< 1 kHz) produces sound
that suffers less attenuation as it travels through the water, providing greater
range than other sonars. Achieving ranges up to 100 nautical miles, low
frequency sonars are primarily used for long-range search and surveillance of
submarines. Surveillance Towed Array Sensor System Low Frequency Active (SURTASS
LFA) is the U.S. Navy's low-frequency sonar system.
Navy
Sonar Operator
Sonar Configurations
Sonar systems can be mounted to
the hulls of various combat ships, towed behind ships in an array, dipped into
the water from helicopters, or attached to free-floating buoys (sonobuoys).
Marine Mammals and Sound
All pinnipeds and cetaceans depend on sound to some degree to sense the ocean
environment, and some are known to use echolocation. Echolocation allows these
animals to determine the distance of objects (food, predators) and features of
the underwater environment (seafloor depth, topography) for navigation purposes.
They accomplish this by projecting sounds, called sonar clicks, that are
reflected back when the sounds strike an object. The farther away an object is,
the longer it takes for the echo to return, allowing the echlocating animal to
tell the distance. Echolocation makes it possible to navigate and feed at night
and in deep or murky water, or at great distances where visual sensing would be
ineffective. For example, a dolphin can detect a target the size of a golf ball
almost a football field away, much farther than the dolphin can see underwater.
Marine mammals also use
underwater vocalizations to communicate with each other. Because sound waves
travel efficiently in water, some ocean-dwelling animals are able to communicate
over great distances through sound. Based on the few marine mammal species for
which hearing has been tested to date, it appears that a given species' hearing
is tuned to a broad range of frequencies with the greatest sensitivity typically
encompassing the range of vocalizations and echolocation (for echolocating
species).
There are 119 species of marine
mammals, ranging from one to 40 meters in length and from 45 to 95,000 kg in
mass. These species have widely varying sensitivities to sound based on
frequency. This is a reflection of how different species have evolved to cope
with life in the marine environment, including differences in size, prey,
habitats, and the predators they try to avoid. Species that live in the same
habitat may have overlapping, but not necessarily identical, hearing ranges. For
example, smaller odontocetes have a hearing range of up to 150 kHz, while
mysticetes (including gray whales) have an estimated hearing range from 20 Hz to
at least 3 kHz.
Sonar
simulation
High-powered sonar transmitters
may harm marine animals, although the precise mechanisms for this are not well
understood. Some marine animals, such as whales and dolphins, use echolocation
systems similar to active sonar to locate predators and prey. It is conjectured
that sonar transmitters could confuse these animals and cause them to lose their
way, perhaps preventing them from feeding and mating.
It has been suggested that
military sonar may induce whales to panic and surface too rapidly leading to a
form of decompression sickness.
SONAR-LINKED
STRANDINGS
Numerous mass stranding
events and whale deaths across the globe have been linked to military
sonar use.
October 1989: At least 20 whales of three species strand
during naval exercises near the Canary Islands.
December 1991: Two Cuvier's beaked whales strand during
naval exercises near the Canary Islands.
May 1996: Twelve Cuvier's beaked whales strand on the
west coast of Greece as NATO ships sweep the area with low- and
mid-frequency active sonar.
October 1999: Four beaked whales strand in the U.S.
Virgin Islands during Navy maneuvers offshore.
May 2000: A beaked whale strands in Vieques as naval
exercises are about to begin offshore.
May 2000: Three beaked whales strand on the beaches of
Madeira during NATO naval exercises near shore.
April 2002: A beaked whale and a humpback whale strand
near Vieques during an offshore battle group training exercise.
September 2002: At least 14 beaked whales from three
different species strand in the Canary Islands during an anti-submarine
warfare exercise in the area. Four additional beaked whales strand over
the next several days.
May 2003: As many as 11 harbor porpoises beach along the
shores of the Haro Strait, Washington State, as the USS Shoup tests its
mid-frequency sonar system.
June 2004: As many as six beaked whales strand during a
Navy sonar training exercise off Alaska.
July 2004: Approximately 200 melon-headed whales crowd
into the shallow waters of Hanalei Bay in Hawaii as a large Navy sonar
exercise takes place nearby. Rescuers succeed in directing all but one of
the whales back out to sea.
July 2004: Four beaked whales strand during naval
exercises near the Canary Islands.
January 2005: At least 34 whales of three species strand
along the Outer Banks of North Carolina as Navy sonar training goes on
offshore.
Navy Granted Authority to Use
Sonar in Training off California-Click Here
Data
compiled from The British Antarctic Study, NASA, Environment Canada,
UNEP, EPA and
other sources as stated and credited Researched by Charles
Welch-Updated dailyThis
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