Magazine article E Magazine

The Unquiet Oceans: Undersea Noise Experiments Threaten the Sensitive Hearing - and Communications - of Marine Mammals

Magazine article E Magazine

The Unquiet Oceans: Undersea Noise Experiments Threaten the Sensitive Hearing - and Communications - of Marine Mammals

Article excerpt

Anyone who has ducked beneath the surface of the ocean is familiar with the sensation. The screech of gulls, the laughter of children in the surf, even the distant whine of pleasure craft are all shut out for the moment.

But, in fact, the oceans are full of sounds, most of which are inaudible to human ears. Because water is a very efficient conductor of sound waves, moving them at five times their airborne speed, it is no surprise that some sea creatures have capitalized on the use of sound for navigation and communication.

While flippered animals such as seals, sea lions and walruses have shown some proficiency utilizing underwater sounds, the real stars of underwater acoustics are the tinned marine mammals, including whales, porpoises and dolphins. These animals not only use sound to locate prey (and each other), but some evidence suggests that they also communicate in a code of clicks, whines and squawks that scientists have yet to crack completely.

Producing these sounds is, of course, only half of the picture. These marine mammals must also have hearing sensitive enough to discern these signals, sometimes over great distances. Bottlenose dolphins, for example, have been tested to have a normal hearing range of frequencies from 200 to 150,000 hertz (Hz). Humans, by comparison, hear only between 20 to 20,000 Hz.

Such sensitivity to sound has left marine mammals vulnerable to the man-made hazard of underwater noise pollution. Military, industrial, and scientific activities all do their part in contributing to the racket beneath the sea.

The U.S. Navy, for example, is preparing an environmental impact statement on the sub-surface noise generated by a new submarine detection program. To improve detection of the new breed of quieter submarine, the Navy hopes to employ a low-frequency active sonar array. As opposed to passive sonar, which simply listens for unnatural sounds, active sonar produces a low-frequency signal (between 75 and 1,000 Hz) to create echoes off of hostile submarines. These signals, which would cover vast tracts of ocean, are to be produced at 230 decibels (dB), well above the noise level of a jet engine (120 dB).

On the Pacific coast, scientists studying global warming are also adding to underwater noise while hoping to get accurate readings of the oceans' temperature. But gathering this data requires the use of sonar on a large scale. The $40 million acoustic thermometry of ocean climate (ATOC) program of the Scripps Institution of Oceanography in La Jolla, California hopes to capitalize on the fact that sound travels through cold water faster than through warmer water. By tracking the variations in the time it takes a 75 Hz tone (within the hearing range of many marine mammals) to travel from a sonar transmitter in California to a receiver 6,000 miles away, the ATOC program can begin to plot the changes in ocean temperature over time.

At their source, ATOC'S sonars, in action since December 1995, produce a deafening 195 dB. In humans, permanent hearing loss can result above 150 dB. Even at half-a-mile's distance, the transmissions register at 136 dB, still louder than a jet engine. In addition, although the decibel level will drop with distance, the frequency remains the same. This means that marine mammals in the sound's path may not be subject directly to a loud noise, but suffer consistent exposure to low-frequency sound waves. …

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