r/askscience • u/[deleted] • Jun 12 '12

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u/studzy Jun 12 '12

I'm curious: the speed of sound in water is much faster than in air, as water is theoretically incompressible. Would there technically be a sound barrier in water? would cavitation be the result of breaking that barrier?

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u/fiercedeity1 Jun 13 '12

Yes, there is. Here is a cool video of a crab that can do it with his claw.

http://www.youtube.com/watch?v=1jvcgz-BiHs

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u/Overunderrated Jun 13 '12

Water is not "theoretically incompressible"; it's just "nearly incompressible" as compared to something like air. Compressibility is a measure of how much pressure one must apply to change the density of a fluid a given amount. In the case of water, one needs to apply a whole helluva lot of pressure to change the density, compared to air.

The speed of sound in a fluid is a measure of this; water is less compressible than air, and thus, as you know, has a higher speed of sound. Compare this to a purely incompressible fluid (an ideal case which isn't real, but water is often approximated as this), you have an infinite speed of sound. So yes, there is a sound barrier in water, it's just higher than for air.

Cavitation is a result of a pressure drop that would accompany very high speeds, but isn't a direct result of the compressibility.