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Echolocation

Echolocation

Echolocation is the process of using sound waves to locate objects that may be invisible or at a distance. Some bats use sound to locate their insect prey. Bats have vocal chords modified to emit the high-frequency sounds needed for good resolution and specially adapted ears to receive the sound. Animals also use echolocation for orientation, avoiding obstacles, finding food, and for social interactions. The animal produces sounds and listens for the echoes reflected from surfaces and objects in its environment. By analyzing the information contained in these echoes, the animal can perceive the objects.

In all species that use echolocation, the sound pulses are short bursts at relatively high frequencies, ranging from about 1,000 Hz in birds to at least 200,000 Hz in whales. Bats use frequencies from about 30,000 Hz to about 120,000 Hz. The pulses are repeated at varying rates depending on what the animal is doing. A flying bat will emit about one pulse per second. In a hunting bat close to its target, the rate may increase to several hundred pulses per second.

Most bats, including all small bats (suborder Microchiroptera ) and one genus of large bats (Megachiroptera ) use echolocation. Other animals thought to use echolocation are a few species of shrews and two kinds of birds. Echolocation is also used by most toothed whales and porpoises (Odontoceti ). Baleen whales (those that exist primarily on krill and similar organisms) do not use echolocation.

Echolocation in Bats

There are two groups of bats, large bats and small bats. Large bats eat fruit and find their way around using their excellent eyesight. Small bats are mostly insectivores that find their flying prey at dusk using echolocation. Bats produce sounds with their larynx, or voice box, which has adapted to produce loud, high-frequency sounds. The quality, frequency, duration, and repetition rate of the sounds produced varies with the species of bat and with the situation. For example, as a bat closes in on its prey, it will repeat the pulses of sound more frequently.

Although the frequency of bat cries varies among species, the cries usually occur in a range between 30,000 and 80,000 Hz. The use of such high frequencies is an essential feature of the bat's sonar system. Because the target object (a moth or other small insect) is so small, a high-frequency, very short-wavelength sound must be used. A sound wave with a frequency of 80,000 Hz has a wavelength of around 4 millimeters (1/8 inch), which is suitable for locating a small moth.

Bat ears are well adapted to receive high-frequency sounds. In most bat species, the size of the outer ear is large relative to the size of the head. In some species that use relatively faint sounds, the outer ear is twice the size of the rest of the head. The large surface of the outer ear acts as an efficient collector of sounds. The outer ear is tuned to receive the frequencies emitted by the bat larynx, which helps the bat to hear the sounds it produces and tune out other sounds. The outer ear is also very mobile and can be rotated and tilted in various ways. The bat ear canal also contains a special organ that allows the canal to be closed to reduce the entrance of excessively loud sounds.

Neurophysiological and behavioral studies of bat hearing have revealed some curious features. One such feature is that bats do not respond behaviorally to frequencies below 10,000 Hz, although studies demonstrate that they can hear these frequencies. This lack of a response is probably due to the bat's dependency on hearing for echolocation. Below 10,000 Hz, the wavelength is too long to be of any use in finding prey. It is also a frequency range where environmental noise is likely to occur, so bats have evolved the ability to selectively ignore sounds that are distracting and are not useful in finding prey. Researchers have also observed that bats are not easily disturbed by extraneous sounds of low frequencies, even very loud sounds. This peculiarity of hearing in bats may account for their resistance to distracting sounds.

Echolocation in Other Mammals

Dolphins and toothed whales use echolocation to orient themselves and locate objects in the water. These animals probably rely on sound production and reception to navigate, communicate, and hunt in dark or murky waters where sight is of little use. They produce sounds with their larynx and a complex system of cavities connected to their blowhole. The sounds used in echolocation are a rapid series of clicks. The clicks contain a wide range of frequencies, but most of the sound energy is in the 50,000 to 200,000 Hz range. These high frequencies are necessary for echolocation in water. Because the speed of sound in water is five times greater than in air, the wavelength of a sound of a given frequency is five times longer in water than in air. To achieve the same resolution, the frequency must be five times higher.

All toothed whales, including dolphins, have a fat-filled organ in the front part of the head called a melon. The melon acts like a lens for sound waves, focusing the sound waves into a narrow beam. Dolphins and other toothed whales generate a wide variety of clicks, whistles, and other noises used in communication and echolocation. The clicks they use for echolocation are of a higher frequency than those used for other forms of communication. This improves resolution and allows smaller prey to be located. The clicks are generated in a series of interconnected passages behind the melon. When the sound strikes an object such as a prey fish, some of the sound is reflected back toward the dolphin. Another fat-filled cavity in the dolphin's lower jaw acts as a receptor for this sound. The sound is carried from the fat-filled cavity to the middle ear and perceived by the animal's brain.

As soon as an echo from one click is received, the dolphin generates another click. The time lapse between click and echo enables the dolphin to determine the distance between it and the object. The difference in sound intensity received by each ear allows the animal to determine the direction. By emitting a series of clicks and listening to the echoes, the dolphin is able to locate and follow its prey.

see also Acoustic Signals.

Elliot Richmond

Bibliography

Allen, Glover M. Bats. Cambridge, MA: Harvard University Press, 1939. Reprint, New York: Dover Publications Inc., 1962.

Au, Whitlow W. L. The Sonar of Dolphins. New York: Springer-Verlag, 1993.

Bonner, Nigel. Whales. London: Butter and Tanner Ltd., 1980.

Fenton, M. Brock. Bats. New York: Facts on File Inc., 1992.

Gaskin, David Edward. The Ecology of Whales and Dolphins. London: Heinemann Educational Books Ltd., 1982.

Graham, Gary Lynn. Bats of the World: A Golden Guide. New York: Golden Press, 1994.

Harrison, Richard John, and Michael M. Bryden, eds. Whales, Dolphins, and Porpoises. New York: Facts on File, 1988.

Tuttle, Merlin D. America's Neighborhood Bats, rev. ed. Austin: University of Texas Press, 1997.

Wilson, Don E. Bats in Question: The Smithsonian Answer Book. Washington, D.C.: Smithsonian Institution Press, 1997.

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SONAR

SONAR

K. LEE LERNER

SONAR, an acronym for Sound Navigation and Ranging, is a technique based on echolocation used for the detection of objects underwater.

Historical development of SONAR. Ancient drawings depict the use of long tubes as non-mechanical underwater listening devices to detect and transmit sound in water. In the late nineteeth century, scientists began to explore the physical properties associated with sound transmission in water. In 1882, a Swiss physicist, Daviel Colladen, attempted to calculate the speed of sound in the known depths of Lake Geneva. Based upon the physics of sound transmission articulated by English physicist Lord Rayleigh (18421914), and the piezoelectric effect discovered by French scientist Pierre Curie (15091906), in 1915, French physicist Paul Langevin (18721946) invented the first system designed to utilize sound waves and acoustical echoes in an underwater detection device.

In the wake of the Titanic disaster, Langevin and his colleague Constantin Chilowsky, a Russian engineer then living in Switzerland, developed what they termed a "hydrophone" as a mechanism for ships to more readily detect icebergs (the vast majority of any iceberg remains below the ocean surface). Similar systems were put to immediate use as an aid to underwater navigation by submarines.

Improved electronics and technology allowed the production of greatly improved listening and recording devices. Because passive SONAR is essentially nothing more than an elaborate recording and sound amplification device, these systems suffered because they were dependent upon the strength of the sound signal coming from the target. The signals or waves received could be typed (i.e. related to specific targets) for identifying characteristics. Although skilled and experienced operators could provide reasonably accurate estimates of range, bearing, and relative motion of targets, these estimates were far less precise and accurate than results obtained from active systems unless the targets were very closeor were very noisy.

The threat of submarine warfare during World War I made urgent the development of SONAR and other means of echo detection. The development of the acoustic transducer that converted electrical energy to sound waves enabled the rapid advances in SONAR design and technology during the last years of the war. Although active SONAR was developed too late to be widely used during World War I the push for its development produced enormous technological dividends. Early into World War II, the British Anti-Submarine Detection and Investigation Committee (its acronym, ASDIC, became a name commonly applied to British SONAR systems) made efforts to outfit every ship in the British fleet with advanced detection devices. The use of ASDIC proved pivotal in the British effort to repel damaging attacks by German submarines.

SONAR and RADAR. Although they rely on two fundamentally different types of wave transmission, SONAR and Radio Detection And Ranging (RADAR) and both are remote sensing systems. While active SONAR transmits acoustic (i.e., sound) waves, RADAR sends out and measures the return of electromagnetic waves.

In both systems these waves return echoes from certain features or targets that allow the determination of important properties or attributes of the target (e.g., shape, size, speed, distance to target, etc.). Because electromagnetic waves are strongly attenuated (diminished) in water, RADAR signals are mostly used for ground or atmospheric observations. Because SONAR signals easily penetrate water, they are ideal for navigation and measurement under water. Within the ocean, the speed of sound varies with changes in temperature and pressure, and these conditions can also be determined by alterations in SONAR signals.

SONAR usually operates at frequencies in the 10,00050,000 Hz range. Higher higher frequencies provide more accurate location data, but propagation losses (i.e. loss of signal strength over distance) also increase with frequency.

FURTHER READING:

BOOKS:

Van Trees, Harry L. Radar-Sonar Signal Processing and Gaussian Signals in Noise. Indianapolis, IN: John Wiley & Sons, 2001.

Waite, A. D. Sonar for Practising Engineers. Indianapolis, IN: John Wiley & Sons, 2001.

ELECTRONIC:

Canadian Center for Remote Sensing, "History of Remote Sensing." 2001. <http://www.ccrs.nrcan.gc.ca/ccrs/org/history/morleye.htm> (February 1, 2003).

SEE ALSO

P-3 Orion Anti-Submarine Maritime Reconnaissance Aircraft
Remote Sensing
SOSUS (Sound Surveillance System)
Undersea Espionage: Nuclear vs. Fast Attack Subs

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Echolocation

Echolocation

In the animal kingdom, certain animals determine the location of an object by producing sounds, then interpreting the echoes that are created when those sounds bounce off the object. This process is called echolocation. The only animals that use this unique sense ability are certain mammalsbats, dolphins, porpoises, and toothed whales. It now is believed that these animals use sound to "see" objects in equal or greater detail than humans.

Mammals developed echolocation as an evolutionary response to night life or to life in dark, cloudy waters. Long ago, bats that ate insects during the day might have been defeated in the struggle for survival by birds, which are better flyers and extremely sharp-sighted. Similarly, toothed whales, porpoises, and dolphins might have been quickly driven to extinction by sharks, which have a very keen sense of smell. These marine mammals not only compete with sharks for food sources, but have themselves been preyed upon by sharks. Echolocation helps them find food and escape from predators.

Words to Know

Decibel: A unit used to measure the loudness of sounds.

Frequency: For a sound wave, the number of waves that pass a given point in a particular amount of time.

Mammal: Warm-blooded animals that have a backbone and hair or fur. The female mammal has mammary glands that produce milk to feed her young.

Predator: An animal that hunts, kills, and eats other animals.

Bats

Echolocation in bats was first clearly described by scientists in 1945. Bats that eat frogs, fish, and insects use echolocation to find their prey in total or near-total darkness. After emitting a sound, these bats can tell the distance, direction, size, surface texture, and material of an object from information in the returning echo. Although the sounds emitted by bats are at high frequencies that are out of the range of human hearing, these sounds are very loudas high as 100 decibels, which is as loud as a chainsaw or jackhammer. People may hear the calls as clicks or chirps. The fruit-eating and nectar-loving bats do not use echolocation. These daytime and early evening bats have strong eyes and noses for finding food.

One question that had puzzled scientists is how a bat can hear the echo of one sound while it is emitting another sound; why is the bat not deafened or distracted by its own sounds? The answer is that the bat is deafened, but only for a moment. Every time a bat lets out a call, part of its middle ear moves, preventing sounds from being heard. Once the bat's call is made, this structure moves back, allowing the bat to hear the echo from the previous call.

Marine mammals

Echolocation may work better underwater than it does on land because it is easier for sound waves to travel through water than through air. Echolocation may even be more effective for detecting objects underwater than light-based vision is on land. Sound with a broad frequency range interacts in a more complex manner with an object it strikes than does light. For this reason, sound can convey more information than light.

Like bats, marine mammals such as whales, porpoises, and dolphins emit pulses of sounds and listen for the echo. Also like bats, these sea mammals use sounds of many frequencies and a highly direction-sensitive sense of hearing to navigate and feed. Echolocation provides all of these mammals with a highly detailed, three-dimensional image of their environment.

Whales, dolphins, and porpoises all have a weak sense of vision and of smell, and all use echolocation in a similar way. The mammal first emits a sound pulse. A large fatty deposit found in its head, sometimes called a melon, helps the mammal to focus the sound. An echo is received at a part of the lower jaw sometimes called the acoustic window. The echo's vibration is then transmitted through a fatty organ in the middle ear where it is converted into a nerve impulse that delivers the information to the brain. The brains of these sea mammals are at least as large relative to their body size as is a human brain to the size of the human body.

Captive porpoises have shown that they can locate tiny objects and thin wires and distinguish between objects made of different metals and of different sizes. This is because an object's material, structure, and texture all affect the nature of the echo returning to the porpoise.

[See also Acoustics; Cetaceans; Radar; Sonar ]

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echolocation

echolocation A method used by some animals (such as bats, dolphins, and certain birds) to detect objects in the dark. The animal emits a series of high-pitched sounds that echo back from the object and are detected by the ear or some other sensory receptor. From the direction of the echo and from the time between emission and reception of the sounds the object is located, often very accurately.

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echolocation

echolocation In animals, system of orientation used principally by whales and bats. The animal emits a series of short, high-frequency sounds, and from the returning echo it gauges its environment. Bats also use the system for hunting.

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