Sound wave attenuation


by silversurf
Tags: attenuation, sound, wave
silversurf
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Nov27-13, 01:08 PM
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Why does water attenuate sound waves more than air? Is it because of it's bulk modulus? This is what I understand. Speed through a medium is determined by the elastic component over the inertial component. Since water has a much greater bulk modulus (elastic component) than air, it makes the wave travel faster in water. What I don't understand is how is it that the speed is faster yet it's attenuation is also greater. Since a sound wave travels faster through water than air shouldn't it also be more resistant to attenuation?
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davenn
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Nov27-13, 03:45 PM
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hi silversurf

you are coming from an incorrect assumption

attenuation of sound in water is much less than in air for a given freq

here's just one example from some tables

Attn in Air dB/km @ 1kHz = 14dB/km @ 10% RH; 5.3 dB/km @ 90% RH ( Relative Humidity)

Attn in seawater dB/km @ 1kHz = 0.07 dB/ km @ 0 deg C 0.05 dB/km @ 30 deg C

http://www.kayelaby.npl.co.uk/genera...2_4/2_4_1.html

hence why whale song can be heard over huge distances


cheers
Dave
tiny-tim
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Nov27-13, 03:49 PM
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Quote Quote by davenn View Post
hence why whale song can be heard over huge distances
that's why the whales left the land, to live in the water!

davenn
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Nov27-13, 04:08 PM
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Sound wave attenuation


Quote Quote by tiny-tim View Post
that's why the whales left the land, to live in the water!
haha


D
sophiecentaur
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Nov27-13, 05:38 PM
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Quote Quote by silversurf View Post
Why does water attenuate sound waves more than air? Is it because of it's bulk modulus? This is what I understand. Speed through a medium is determined by the elastic component over the inertial component. Since water has a much greater bulk modulus (elastic component) than air, it makes the wave travel faster in water. What I don't understand is how is it that the speed is faster yet it's attenuation is also greater. Since a sound wave travels faster through water than air shouldn't it also be more resistant to attenuation?
It is true that water attenuates the level of sound that you can hear, when you are immersed in water. This is not due to losses in the transmission through the water but in how well the sound power is matched from the source into the water medium and then how well it is matched to your ears.
You mention modulus. This (and the density), affects the impedance of sound waves in a medium (high in water and low in air). Your ears evolved to match sound in air, using very flimsy diaphragms and tiny levers (the ossicles). The displacement in air (for a given power level) is much greater than that in water and the movement at the entrance to the cochlea will be very much less for waterborne sound. I imagine that dolphins have a similar problem hearing sound when they are out in the air - for the converse reason.


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