Sunday, February 24, 2008

Run! Hide!

What's the loudest thing in the ocean?


Though the blue whale produces the loudest noise of any individual animal in the sea or on land, the loudest natural noise of all is made by shrimps.

The sound of the "shrimp layer" is the only natural noise that can white out a submarine's sonar, deafening the operators through their headphones.

Below the layer they can hear nothing above it and vice versa. Hearing from below can only be accomplished by raising a mast up through it.

The noise of the collected shrimps amounts to an earsplitting 246 decibels, which even adjusting for the fact that sound travels five times faster in water, equates to about 160 decibels in air; considerably louder than a jet taking off (140 dB) or the human threshold of pain. Some observers have compared it to everyone in the world frying bacon at the same time.

The noise is caused by trillions of shrimps snapping their single oversized claw all at once. Snapping shrimps, members of the various Alpheus and Synalpheus species, are found in shallow tropical and subtropical waters.

But it's even more interesting that in sounds. Video shot at 40,000 frames per second shows clearly that the noise occurs 700 microseconds after the claw has snapped shut. The noise comes from burst bubbles, not the shutting of the claw itself, an effect know as cavitation.

It works like this. A small bump on one side of the claw fits neatly into a groove on the other side The claw is shut so rapidly that a jet of water traveling at 62 miles per hour squirts out, fast enough to create expanding bubbles of water vapor When the water slows down and normal pressure is restored, the bubbles collapse, creating intense heat (as high as 20,000 degrees celsuis), a loud pop, and light - this last being a very rare phenomenon called sonoluminescense, where sound generates light.

Shrimps use this noise to stun prey, communicate, and find mates. As well as ruining sonar, the sharp, hot intense noise makes dents in ships' propellers.

Shrimp: The Endless Quest for Pink Gold
Shrimp: The Endless Quest for Pink Gold

--Taken from "THE BOOK OF General Ignorance" by John Lloyd and John Mitchinson. (I got it for Christmas, thanks Chris!)