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The sound of air bubbles.

The sound of air bubbles

To human ears, the tune may be inaudible, but an underwater air bubble jolted by a burst of intense light will "ring,' radiating sound waves into the surrounding liquid. This effect is set off by the push that reflected light gives to a surface from which it bounces. Thus, a bubble, which reflects a large amount of the light that strikes it, is initially compressed. The bubble begins to expand and contract, vibrating rapidly at its resonance frequency until the oscillations die away.

The hard part is ensuring that the vibrations are strong enough to produce detectable sound waves, says physicist Philip L. Marston of Washington State University in Pullman. Marston and graduate student Bruce T. Unger were the first to observe this effect. To detect it, the researchers illuminated tiny, individual gas bubbles with green light from an argon-ion laser, and the resulting sound waves were focused onto a microphone designed to operate in water.

Typically, an illuminated bubble, 0.1 millimeter in diameter, naturally rings at close to 30,000 cycles per second, a frequency the human ear can't hear. Larger bubbles oscillate at lower frequencies. The researchers found that by sending the light in pulses at a rate that matches a bubble's resonance frequency, they amplified the oscillations, making the resulting sound waves even "louder.'
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Publication:Science News
Date:May 4, 1985
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