echolocation (èk´o-lo-kâ´shen) noun
1. A sensory system in certain animals, such as bats and dolphins, in which usually high-pitched sounds are emitted and their echoes interpreted to determine the direction and distance of objects.
2. Electronics. A process for determining the location of objects by emitting sound waves and analyzing the waves reflected back to the sender by the object. Also called echo ranging.
- ech´olocate´ verb

Echolocation

Echolocation, high-pitched sounds emitted by certain animals to locate their prey or to avoid obstacles. The sound waves are reflected back to the animal indicating such specialized information as distance to an object or the size and direction of prey.

Learning From Bats
by Karlin Lillington

9:02 a.m.  10.Nov.98.PST

New software developed by Brown University researchers can emulate the echolocation capabilities of brown bats, and may eventually help the United States and British Royal navies detect submarines and underwater explosive mines.

Brown University professor James Simmons built the program with the help of new research that has revealed the extraordinary sensitivity and sophistication of bat sonar.

The researchers found that trained brown bats form highly detailed, three-dimensional sound images of objects, even when the targets are  very close together.

"This capability extends well beyond that of current active sonar systems, and the Navy would very much like to replicate it for use in detecting and classifying man-made objects such as submarines, mines, and unexploded ordinance," said Dr. Harold Hawkins, a researcher in the Cognitive and Neural Science and Technology Division of the Office of Naval Research in Arlington, Virginia.

Hawkins co-manages the biosonar program at the Office of Naval Research, which in turn helps fund the Brown University Bat Laboratory.

Simmons says he and his team at the so-called "Bat Lab" were interested in how bats pursue their prey -- particular kinds of moths -- as they fly at night in an obstacle-filled environment.

"Bats of the kind used by Professor Simmons use their biosonar system to discriminate between moths they prefer to eat and those they find distasteful, while pursuing the moths through the trees and shrubs... [their] branches and leaves reflect the bat's signal many times over," said  Hawkins.

"Amazingly, the bat is able to ignore all this reverberation and successfully track down the preferred prey."

Bats emit constant, high-pitched cries that are deflected back to them as the sounds hit an object. Two echoes returning to the bat from a single cry tell the animal that its sonar is bouncing off of two objects. The delay between the returning echoes indicates the distance between the objects.

In this way, the bat builds up a 3D image of an object by the direct echoes which return from the surfaces of an object, and by the ricocheting patterns of many echoes as they overlap and interfere with each other.

To measure the bats' capabilities, Simmons devised an experiment in which the animals were trained to respond when they heard two distinct echoes played back to them from a loudspeaker system.

To his surprise, the mammals could discern two echoes even when they were only two microseconds apart. Until now, scientists had assumed an animal brain would be incapable of detecting a two microsecond gap,  because measurements had indicated that nerve cells take 300 microseconds to process a stimulus.

Simmons concluded that since it is not likely the bats need such precise echolocation to simply track prey, the animals were creating a 3D world built out of sound.

The Navy is keenly interested in "range mapping" echolocation software capable of seeing in such exquisite detail with sound alone.

Range maps "are powerful sources of information for discriminating  between mines and natural objects or man-made objects that are not mines," Hawkins said.
 

Currently, the Navy uses divers and trained dolphins to detect mines in shallow waters. Hawkins claimed that no dolphins have been injured or killed in such work.

"Nonetheless, this is potentially dangerous work, and the Navy would like for it to be carried out by machines rather than people and dolphins.  Machines are expendable, but people and marine mammals are not."

But humans have a ways to go before they catch up to the bats.

Though Simmons now has a working computer model of the bat's echolocation abilities, mimicking the bats is not good enough, he said.

"The question is, can you make a computer do it fast enough to be useful? It takes 20 to 30 seconds to process an echo, and... a bat doesn't take more than tenths of a millisecond."

Echolocation - the location of objects by their echoes - is a highly specialized faculty that enables dolphins to explore their environment and search out their prey in a watery world where sight is often of little use.

As sound travels four and a half times faster in water than in air, the dolphin's brain must be extremely well adapted in order to make rapid analysis of the complicated information provided by the echoes.

Although the ability to echolocate has only been proved experimentally for a few odontocete species, the anatomical evidence - the presence of the melon, nasal sacs and specialized skull structures - suggests that all dolphins have this ability.

The dolphin is able to generate sound in the form of clicks, within its nasal sacs, situated behind the melon. the frequency of this click is higher than that of the sounds used for communication and differs between species. The melon acts as a lens which focuses the sound into a narrow beam that is projected in front of the animal.

When the sound strikes an object, some of the energy of the soundwave is reflected back towards the dolphin. It would appear that the panbone in the dolphin's lower jaw receives the echo, and the fatty tissue behind it transmits the sound to the middle ear and then to the brain. It has been recently been suggested that the teeth of the dolphin, and the mandibular nerve that runs through the jawbone, may transmit additional information to the dolphin's brain.

As soon as an echo is received, the dolphin generates another click. The time lapse between click and echo enables the dolphin to evaluate the distance between it and the object; the varying strength of the signal as it is received on the two side's of the dolphin's head enable it to evaluate direction. By continuously emitting clicks and receiving echoes in this way, the dolphin can track objects and home in on them.

The echolocation system of the dolphin is extremely sensitive and complex. Using only its acoustic senses, a bottlemose dolphin can discriminate between practically identical objects which differ by ten per cent or less in volume and surface area. It can do this in a noisy environment, can whistle and ecolocate at the same time, and echolocate on near and distant targets simultaneously -feats which leave human sonar experts gasping.

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