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Science by Laurisa

The Science of Echolocation

by Laurisa White Reyes

When I was a kid, a favorite evening pastime of mine was to stand outside in the clearing in front of our house and toss pebbles into the air. The bats would swoop down after them, then shoot suddenly skyward in hot pursuit of some legitimate prey. I didnít understand then how bats were able to distinguish a rock from a bug in the dark, but today echolocation, the ability to detect objects with sound waves, isnít so much of a mystery.

Ears vs. Eyes

Most animals, including humans, rely primarily on their eyes to observe their surroundings. Through sight we can determine the quantity, size, distance and texture of objects. If I were to tie a blindfold around your eyes so that you could not see and ask you to walk through your house, chances are you would bump into more than a few chairs and walls.

Not all animals are so dependent on the sense of sight to get around, however. Bats and dolphins are two kinds of animals that actually use their ears more than their eyes. Other animals that use echolocation are shrews, seals, and some kinds of birds. Using echolocation, these animals can actually navigate the dark nighttime skies or deep oceans just as well, if not better, than we can navigate around our homes with our eyes.

The word echolocation is made up of two smaller words: Echo ó the way sound reflects off of solid objects, and location Ė determining an objectís position relative to another object.

Bats ĎSeeí With Their Ears

Have you ever stood on a bridge just before sunset? There is a bridge not far from my home that is the home to thousands of bats. During the day the bats are asleep, but just before the sun sets, they all wake up and begin "talking" to each other with high pitched squeaks. This communication gets louder and louder until at last they take flight into the night sky in search of food. These squeaks are actually not used for echolocation.

The sounds bats use in order to locate insects and to navigate without colliding with other bats or inanimate objects like trees or houses, are such a high frequency humans canít even hear them. Humans can hear sounds up to 20 kilohertz, but bats can emit sounds as high as 200 kilohertz.

They emit a consistent series of squeaks at regular intervals. The sound waves travel away from them until they reach a solid object. The sound waves then bounce off the object and travel back the way they came. By measuring the time between when the sound leaves its mouth to when it reaches its ears as well as the pitch and frequency, the bat is able to determine the exact location, the size, and texture of the object. Insects fly using high-speed wing flapping which gives off a different "echo" than a rock, so the bat can tell the difference. This unique echo is called a "signature."

Dolphins Use Echolocation

Sound waves travel 4.5 times faster in water than in air. Bottlenose dolphins and some types of toothed whales use echolocation to navigate and hunt in murky ocean waters. Dolphins emit clicks, which are focused into a beam directly in front of them. The sound waves travel through the water and are reflected back by solid objects. Just as humans can recognize objects by the way they look, dolphins can recognize different kinds of prey by their unique signatures. Since high frequency sounds do not travel very far in water, dolphins use sounds humans can hear. To hear these sounds, visit www.seaworld.org/infobooks/Bottlenose/echodol.html.

Humans And Echolocation

Believe it or not, some humans use echolocation. Try this little experiment. Hold a Styrofoam plate, magazine, or other flat object in front of your face at armís length. Make a loud sound like a whistle or hoot. Bring the object a little closer and make the sound again. Finally, bring the object to within an inch or two of your face and make the sound again. Can you hear the difference? When the plate is held at armís length you probably did not notice any "echo." But the closer the plate is to your face, the louder the sound becomes. That difference in volume can tell you how close or how far the object is.

Echolocation is used in submarines and is the basis of SONAR (sound navigation and ranging) technology. Electronic pulses of sound travel through the water, bounce off objects, and return to the ship. Specially trained technicians listen to these echoes to determine the distance between the ship and the target of the sonar.

Some blind people have learned to use echolocation just the way bats and dolphins do. Studies have shown that humans can develop a sensitivity to sound that can help them locate the distance and size of objects in front of them. Several different types of sounds were used in these studies: Cane taps, finger snaps, clapping, whistling, and talking. Even hand-held sound-making devices were used.

The difficulty with some of these sounds is that when sound waves bounce off an object, they return to their original source. So the sound of a cane tap will return to foot level, snapping returns to hand level, etc. This made it difficult for the person making the sound to translate the echoes properly. What works best, it seems, are sounds emitted from the head, such as whistling, so that the echoes return as close to the ears as possible. The person emitting the sound can make more accurate judgments regarding the location of objects in front of them.

Websites on Echolocation

http://www.cccoe.k12.ca.us/bats/echo.html

http://people.howstuffworks.com/submarine4.htm

http://hubel.sfasu.edu/courseinfo/SL02/jb2echolocation.htm

 

L.W.R.