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Someone asked me this question and I don't think I gave him an adequate answer (I was trying to think of the extreme case - that of neutron stars)

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Do you want an answer specifically for neutron stars, or do you want one in air on Earth? –  Mark Eichenlaub Jul 28 '11 at 9:49
    
Without gravity our atmosphere woud vanish to space, thus sound waves would be no longer possible. We would rely then on pressing our ears to a rail to hear a train aproaching :=) –  Georg Jul 28 '11 at 10:32
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@Georg: but the train wouldn't be attached to the rail anymore... –  Frédéric Grosshans Jul 28 '11 at 11:27
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@Frederic : Touche ! :=) –  Georg Jul 28 '11 at 11:31
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The wiki article has basic information en.wikipedia.org/wiki/… . Sound waves are pressure waves and depend on density so gravity which stratifies the atmospheric density affects sound waves through that. In solids and liquids to the extent that gravity stratifies them it will change the behavior of soundwaves. –  anna v Jul 28 '11 at 12:08

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OK, I'll make my comment into an answer.

The wiki article has basic information .

Sound waves are pressure waves and depend on density so gravity which stratifies the atmospheric density affects sound waves through that.

In solids and liquids to the extent that gravity stratifies them it will change the behaviour of sound waves.

Any configuration that can take density variations should have the possibility of sound waves propagating, so there could be sound waves in neutron stars.

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in this link: en.wikipedia.org/wiki/Sound_speed You find something on sound speed and pressure for gases: independent from pressure for ideal gas, very small dependence for real gases. Speed of sound depends on Temperature, but would You blame falling temperatures with height to gravity? –  Georg Jul 28 '11 at 20:27
    
@Georg I am answering on how gravity affects the speed of sound. Of course it is affected by other variables, some cancelling effects of others. There will always be the, weak maybe, effect of gravity even in gases, because no gas is really an ideal gas. In the earth's atmosphere temperature is also depended on height so it is still an effect of gravity ( not true for the sun though). –  anna v Jul 29 '11 at 4:25
    
In gases, speed is the sole figure if interest, because all other things follow from sound speed. –  Georg Jul 29 '11 at 8:01

To my understanding, as long as we are speaking about somewhat similar condition to earth gravity won't have an apparent effect on sound waves since the molecules composing air are too light for the gravity to have significant effect on them.

If you consider a much heavier planet, then I don't think that you'll have a significant gas phase and you are now dealing with fluids and solids. As you might now, sounds travels further (and faster?) inside those phases since the sound waves are less scatted. If you go to extream cases such as neutron star I don't think that you can describe this as anything similar to our comprehending of sound since you don't have atoms and molecules anymore (at least not in their classical definition). With a gun pointed to my head, I'll guess that in neutron star there is no sound.

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""since the molecules composing air are too light for the gravity to have significant effect on them. "" this is rubbish. –  Georg Jul 28 '11 at 20:27

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