Noise at sea: cries of infant microbubbles.Noise at sea: Cries of infant microbubbles
Breaking waves and rainfall splattering the ocean surface create vast numbers of invisible, microscopic bubbles. Several studies now suggest that these tiny bubbles generate most of the noise a diver might hear while swimming just below the surface.
"There is a lot of ambient noise in the ocean, and bubbles are very efficient generators of sound," says Lawrence A. Crum of the National Center for Physical Acoustics in Oxford, Miss. "It is our contention . . . that a great deal of the sound in the frequency range from about 100,000 hertz down to 10 hertz is generated by bubbles."
Studies of the sounds created by oscillating os·cil·late
intr.v. os·cil·lat·ed, os·cil·lat·ing, os·cil·lates
1. To swing back and forth with a steady, uninterrupted rhythm.
2. bubbles or scattered by quiescent quiescent
at rest; latent; the G0 stage of the cell cycle. bubbles may prove useful for tracking waves and near-surface ocean currents, for remote detection of precipitation over the ocean, and for monitoring a variety of processes, such as mixing in the ocean and the exchange of gases between ocean and atmosphere.
"Scientists are beginning to recognize that acoustical methods can help them enormously in tackling difficult problems that are of environmental significance," says David M. Farmer of the Institute of Ocean Sciences in Sidney, British Columbia Sidney is a town located at the northern end of the Saanich Peninsula, on Vancouver Island in the Canadian province of British Columbia. It has a population of approximately 11,300. . "There's a huge world of underwater sound Underwater sound
The production, propagation, reflection, scattering, and reception of sound in seawater. The sea covers approximately 75% of the Earth's surface. -- both natural and artificial -- that provides a window on the ocean, which has not previously been exploited."
Bubble noise was the main subject of a session on acoustical oceanography Acoustical Oceanography is the use of underwater sound to study the sea, its boundaries and its contents. History
The earliest and most widespread use of sound and sonar technology to study the properties of the sea is the use of an echo sounder to measure water depth. at this week's meeting in San Diego San Diego (săn dēā`gō), city (1990 pop. 1,110,549), seat of San Diego co., S Calif., on San Diego Bay; inc. 1850. San Diego includes the unincorporated communities of La Jolla and Spring Valley. Coronado is across the bay. of the Acoustical Society of America The Acoustical Society of America (ASA) is an international scientific society dedicated to increasing and diffusing the knowledge of acoustics and its practical applications. History
The ASA was instigated by Wallace Waterfall, Floyd Watson, and Vern Oliver Knudsen. .
When a wind-driven, breaking wave crashes at sea, it traps large volumes of air, which quickly break up into tiny bubbles, perhaps only tens of microns in diameter. The newly formed microbubbles start their lives as pulsating sources of sound close to the ocean surface. Although they radiate ra·di·ate
1. To spread out in all directions from a center.
2. To emit or be emitted as radiation.
ra sound for only a few milliseconds before settling into a passive state, these "screaming" bubbles contribute far more to the sounds heard underwater than does the splash generated by the impact of water on water.
Although researchers have known for more than 50 years that the amount and characteristics of underwater sound seem to depend on wind speed, they have only recently uncovered the link between the extent and strength of breaking waves and the number of microbubbles produced, which in turn generate the noise heard underwater. "The detailed mechanism of how [microbubbles] are formed is not well known, but they are apparently responsible for practically all the natural sound from 500 hertz to 20,000 hertz," says Herman Medwin of the Naval Postgraduate School The Naval Postgraduate School is a graduate school operated by the United States Navy. Located in Monterey, California, it grants primarily master's degrees plus some doctoral degrees to its students, who are mostly active duty officers from U.S. and foreign military services. in Monterey, Calif.
To account for the noise heard at even lower frequencies, Crum and his colleagues have been investigating the role of collective oscillations oscillations See Cortical oscillations. of "bubble clouds." Any individual bubble large enough to generate sound waves at 20 or 30 hertz would get quickly torn apart. But high-speed photographs now show that a sufficiently large In mathematics, the phrase sufficiently large is used in contexts such as:
"We've been able to demonstrate in the laboratory that a whole cloud [of bubbles] will resonate and produce low-frequency sound," Crum says.
Because breaking waves create the noisy microbubbles, researchers can use arrays of hydrophones designed for detecting underwater sounds to listen to and track individual breaking waves as they move across the ocean. "That kind of information could be used for understanding the mechanisms governing wave conditions," Farmer says.
Microbubbles are so pervasive and persist for such surprisingly long periods that they can provide information about ocean currents. "Bubbles injected into the ocean by breaking waves tend to get organized into long rows, like furrows on a plowed field, which are lined up with the wind," Farmer says. "By looking at the way the bubbles are organized and the way they move, we can learn something about the circulation near the ocean surface."
Like breaking waves, raindrops striking water also produce tiny, briefly active bubbles. "We have found that the sound generated by raindrops is caused by bubbles whose diameter depends on the diameter of the raindrops," Medwin says.
The studies reveal that a small raindrop makes a crater in the water surface small enough that its sides come in faster than its bottom comes up, closing off the crater to create a tiny bubble. In contrast, a large raindrop produces a bigger cavity in the water surface and throws up a curtain of water that smashes together at the top to create a canopy above the water. The closing of the canopy generates a downward jet, which punches through the cavity's bottom, dragging with it a tiny bubble of air.
Because bubbles of different diameters oscillate To swing back and forth between the minimum and maximum values. An oscillation is one cycle, typically one complete wave in an alternating frequency. at different frequencies, this discovery opens up the possibility of using "long-range listening" to determine raindrop size distributions and, ultimately, to measure rainfall rates over the ocean.
"Is it a fine mist or the large, heavy drops that come from a thundershower thun·der·show·er
A brief rainstorm accompanied by thunder and lightning.
Noun 1. thundershower - a short rainstorm accompanied by thunder and lightning
rainstorm - a storm with rain ?" Medwin asks. "That can all be deduced from the sound that you hear underwater."