Noisy bubble clouds.
The ocean is a noisy place. One of the chief contributors to the underwater cacophony is the oscillation of bubbles, especially in the top few yards of the ocean's surface. These bubbles, generated by breaking waves, raindrop splashes (SN: 1/4/86, p. 4), living organisms or organic decay, are often easily set into motion. The oscillating bubbles, like tiny balloons, rhythmically expand and contract, generating acoustic waves that can travel thousands of miles before fading away.
Mechanical engineer Andrea Prosperetti of Johns Hopkins University in Baltimore has been studying theoretically how different types of bubble oscillations may contribute to underwater noise at various frequencies. He suggests that at low frequencies --up to tens of hertz--bubbles may be driven into oscillation by turbulence in the water. At frequencies from 1 to several kilohertz, bubbles oscillate freely at a natural frequency dependent on the bubble size.
However, noise in the range of hundreds of hertz may be the result of large clusters of bubbles pulsating together, says Prosperetti. In a sense, each bubble "knows' what its neighbors are doing. Such bubble clouds, probably created by breaking waves, would collectively oscillate at a lower frequency than that of individual bubbles oscillating freely.
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|Title Annotation:||underwater noise caused by oscillation of bubbles near ocean's surface|
|Date:||May 31, 1986|
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