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Recommended for: Grades 6-12

Resource: Sound Waves Underwater: The Loch Ness Monster

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Media Type:
QuickTime Video

Length: 5m 26s
Size: 7.5 MB

Because sound waves travel longer distances through liquids than do light waves, measuring devices use sound to determine depth and locate underwater objects. In this video segment, adapted from NOVA sonar's strengths and limitations are on display as a team of enthusiasts and scientists attempt to find a mythical monster.

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Teachers' Domain, Sound Waves Underwater: The Loch Ness Monster, published January 22, 2004, retrieved on ,
http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.lochness/

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Taking a cue from bats and dolphins, which use echoes to determine the position, size, and shape of objects and to navigate through their surroundings, humans have developed sonar, a technology that uses sound to "see" underwater, where light travels poorly. (Sonar is short for sound navigation and ranging.) A sonar device emits a pulse of sound waves in a given direction. A computer then measures the time it takes for the waves to travel outward, bounce off a solid "target" object, and return. Knowing the speed of sound in water (about 1,500 m/sec), a sonar operator calculates the distance of the object or the depth of the water using this formula: distance = 1/2 speed of sound x measured time. ("One-half" is used because the time measured is actually the time it took for sound to travel both out and back.)

A typical sonar image displays various colors, according to the strength of the return signal received by the device. As the sound wave encounters an object or the seafloor itself, its energy is scattered in all directions. Depending on the density, texture, and reflectivity of the object's surface, the amount of energy returned to the device varies. The stronger the return and the greater the number of sound pulses bounced off the object, the clearer the sound image and the more detailed the picture of the object will be.

Though sound can travel farther and faster underwater than through air, it rarely travels straight and true. Just as light waves refract when the matter they travel through changes, sound waves also change speed and direction when they encounter variations in water temperature, pressure, and turbulence. The conditions in Scotland's Loch Ness, where research teams have tried to find evidence of the famed "monster" are particularly challenging. The lake -- whose underwater profile reveals a deep chasm with steep-sloped walls -- also contains pockets of warm and cold water, making readings even more difficult to analyze.