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See Inside September 2011

Less Bang, More Bubbles: Curtains of Air May Protect Fish from Noisy Human Activity




Courtesy of James Piper University of Texas at Austin

Noise pollution in the oceans has risen dramatically because of an increase in commercial shipping, oil and gas prospecting, and other activities. Evidence is mounting that low-frequency noise from these and other sources can pulp delicate organs in squid, octopuses and cuttlefish.

One way of protecting ocean dwellers would be to raise solid, heavy and potentially expensive barriers around either the sources of sound or the areas one would want protected. Acousticians now think they might be able to use bubbles instead of barriers, and several are experimenting with light curtains of air that absorb and reflect sound waves.

Low-frequency waves have long wavelengths, which means you would need big bubbles—10 centimeters or larger. But freely rising ones—like those pumped into home aquariums—that are wider than about 10 centimeters break up into smaller ones. To keep the bubbles big, investigators encapsulate them in thin latex and string them together like balloons. Tests performed on these latex bubbles inside laboratory tanks show that layers of them could muffle sound by 44 decibels—the difference between a busy city street and a library. Mark S. Wochner of the University of Texas at Austin and his colleagues presented that research at a recent Acoustical Society of America meeting in Seattle. They now plan to test latex bubbles around a barge at a lake in Texas and, down the road, on larger seagoing ships and offshore wind farms.

The bubbles alone may not fully solve the problem. They may dampen sound traveling through the water from above,  but about 10 percent of the noise from underwater pile driving would still get transmitted up from the seabed, says acoustician Peter Dahl of the University of Washington. Dahl and his colleagues are analyzing the nature of this sound to find ways of suppressing it as well. 

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