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Cutting Down on the Din
| Article
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10827 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
3 / 1993 |
2,475 Words |
| Author
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Gail Dutton
Gail Dutton is an independent writer specializing in science
and technology. She lives in Southern California. |
At Saddleback MRI Medical Group in Laguna Hills, California, the din of magnetic resonance imaging (MRI) fell from earsplitting jackhammer-like levels to a comfortable hum comparable to the drone of a room air conditioner. This almost 70 percent reduction in noise--about 20 decibels--was brought about by a noise cancellation device.
CSX Transportation in Baltimore experienced similar results when they fitted noise cancellation (antinoise) devices to their loading equipment. Noise was reduced from 123 decibels--the equivalent of a jet during take off--to about 80 decibels--the equivalent of the inside of a bus. The antinoise devices also improved fuel efficiency by 24 percent, according to General Manager Roger Posey.
Noise cancellation technology eliminates intrusive noise rather than all sound. In a car, for example, while it would eliminate engine and road noises, it would not interfere with the conversation of occupants, the radio, or emergency horns or sirens. Currently the technology is being developed for use in mufflers, fans, engine mounts, personal headsets, communications devices, cabin-quieting systems for autos and planes, and panels that provide quiet zones in a cacophonous environment.
Unlike the passive methods of sound reduction--such as baffles, lead sheeting, fiberglass, or acoustic foams--that absorb sound in order to lessen its impact, noise cancellation devices actively reproduce and eliminate specific sounds before they can be noticed, much like the Patriot missiles eliminated SCUD missiles during the Persian Gulf War.
"Effectiveness really depends upon application," according to Irene Lebovics, vice president of Noise Cancellation Technologies (NCT) in Stamford, Connecticut. In the lab, noise cancellation completely eliminates noise. In practice, noise tends to be reduced by 15-35 decibels. This is nevertheless significant, because lowering sound by even 10 decibels cuts the perceived loudness by half and reduces the actual sound energy 10 times.
Rather than merely muffling the noise, antinoise eradicates it by duplicating the noise at an equal, but opposite wave pattern. When the noise wave peaks, the antinoise wave is at its valley, thus canceling the sound in the same way that two plus negative two equals zero (2+-2=0). "The key to this is coupling--how closely and how perfectly the waves match," Lebovics says.
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