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Thread: Sound travelling faster/slower than the "speed of sound" in the same medium?

  1. #1 Sound travelling faster/slower than the "speed of sound" in the same medium? 
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    Speed of sound - Wikipedia, the free encyclopediaWiki says sound travels faster in denser medium, but why/why not faster/slower? It didn't say anything about it.


    After all, scientists recently discovered Slow travelling light?
    http://www.youtube.com/watch?v=EK6HxdUQm5s&gl=SG&hl=en-GB
    http://www.news.harvard.edu/gazette/1999/02.18/light.html
    http://en.wikipedia.org/wiki/Slow_light


    Then couldn't slow/fast sound exist?


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  3. #2  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by ryanawe123 View Post
    Wiki says sound travels faster in denser medium, but why/why not faster/slower? It didn't say anything about it.
    As it says, the speed of sound is determined by the stiffness and density of the medium. So unless those change, the speed won't change.

    The speed of light changes when it travels through different mediums. The same is true of sound.


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    Ophiolater Nehushtan's Avatar
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    Quote Originally Posted by ryanawe123 View Post
    Speed of sound - Wikipedia, the free encyclopediaWiki says sound travels faster in denser medium, but why/why not faster/slower? It didn't say anything about it.


    After all, scientists recently discovered Slow travelling light?
    http://www.news.harvard.edu/gazette/1999/02.18/light.html
    http://en.wikipedia.org/wiki/Slow_light]Using BEC to Slow Down Light - YouTube[/URL]Then couldn't slow/fast sound exist?
    Sound does not travel faster in a denser medium. Where in the Wikipedia article is it stated so? The mathematical relationship is


    where is the speed of sound, the density of the medium it is travelling through, and a quantity called the bulk modulus.

    Bulk modulus is a measure of the materialís resistance to uniform compression and is defined by where is volume and is pressure. (Think of it as the (infinitesimal) ratio of the increase in the external pressure applied to the relative decrease in volume produced.) Materials with a high bulk modulus tend to be hard and rigid; those with a low bulk modulus tend to be soft and spongy.

    It is high bulk modulus, not high density, that leads to a higher speed of sound (if anything, sound actually travels slower in a denser medium). What the Wikipedia article actually says is this:


    All other things being equal (ceteris_paribus), sound will travel more slowly in spongy materials, and faster in stiffer ones.
    Really, it would be nice if people actually took the time to read what they were linking to instead of quoting nonexistent quotes.

    Furthermore, be careful when comparing sound waves and light waves: they are very different in a number of ways. Firstly, sound canít travel through a vacuum whereas light can. Secondly, sound waves are longitudinal waves, meaning that the atoms of the medium the sound is travelling through vibrate in the same direction as the waves are travelling. In contrast, light waves are transverse waves.
    Last edited by Nehushtan; March 15th, 2013 at 10:56 AM.
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  5. #4  
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    so cant we create like 600m/s sound in our atmosphere?(the norm is ~340m/s)

    &why not?
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  6. #5  
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    Because the speed at which waves travel is determined by the stiffness and the density.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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  7. #6  
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    Quote Originally Posted by Nehushtan View Post
    All other things being equal (ceteris_paribus), sound will travel more slowly in spongy materials, and faster in stiffer ones.
    rephrase, sound travels slower in airy materials, and faster in less air, more solid materials?

    since air = travel slower when less dense, and solid = faster in more dense,

    why am i wrong to say
    sound travels faster in denser medium
    ?
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  8. #7  
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    Quote Originally Posted by ryanawe123 View Post
    why am i wrong to say
    sound travels faster in denser medium
    ?
    Look at the speed-of-sound equation again:


    Doesn’t it tell you the speed of sound is inversely proportional to the square root of density for a constant bulk modulus? In other words, if you have two media with the same bulk modulus but different density, sound will travel slower, not faster, in the denser medium. And the fact that sound travels faster in water than in air is not due to the greater density of water but to its high incompressibility compared with air.

    So stop saying sound travels faster in a denser medium. That statement about sound is not a sound statement.
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    oh.... spongy/stiffer, not dense/less dense...

    you meant whether its more/less compressible in another words?
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  10. #9  
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    No guys, not good!

    In Condensed matter (Liquids, Solids) Sounds is mediated by phonons. Or in the absence of any crystiline build-up, atomic vibrations.
    The easier, and more efficient these vibrations propagate (disragarding modulation due to difference in phase and group speed of the waves), the quicker sound will propagate.
    Dense, materials, with difficult to move atoms, hands most often stiff, are generally good conductors of phonons. Crystals naturally being the best. Taking it as its stiffness is a good approximation. But it must be noted that the order of atoms play a very strong role here.

    Nehustan, don't rely too much on Wikipedia, they offer a general form of the speed of sound, for air. In general since sound in air is propagation of pressure fluxuations, it isn't a good formula for really dense materials. The problem with air is that if diffuses more likely when it is denser. But this isn't the case for water.
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  11. #10  
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    are
    phonons
    particles?
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  12. #11  
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    no, they are lattice vibrations, but they act as bosons.
    In the information age ignorance is a choice.
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  13. #12  
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    are
    bosons
    particles?
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  14. #13  
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    Quote Originally Posted by ryanawe123 View Post
    are
    bosons
    particles?
    No, Bosons and Fermions are the two groups of secondary quantized entities that behave in a certain way. (Bose-Einstein condensation for bosons for instance)

    Why do you want things to be particles so badly?
    In the information age ignorance is a choice.
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  15. #14  
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    Because Bosons and Fermions end with 's'
    like electrons
    protons
    neutrons
    quarks
    muons
    atoms

    etc. it could be the plural of a particle
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  16. #15  
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    oh darn..

    wiki says
    List of particles - Wikipedia, the free encyclopedia
    List of particles - Wikipedia, the free encyclopedia

    wiki says they are particles.

    or are they exhibit both particle and lattice vibrations?


    i dont know what im saying above this line :3
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  17. #16  
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    Everything is both particle and wave! The vibration in atomic lattice called phonon should be same as the vibration of air for sound, but since phonon is much much smaller than average sound (is small to quantum level) then everything at that size is weird and exhibit both particle and wave properties.

    And there's no point in caring about whether they are wave or particle.
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  18. #17  
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    Not quite on topic, but interesting: Supersonic Stereo
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    This gives me an opportunity to put in a favourite, super, magical video on sound and water - even though it has nothing to do with the OP. I love it anyway - it's mesmerising.

    Amazing Water & Sound Experiment #2 - YouTube
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  20. #19  
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    wow its supermagical..
    awesome video

    its like... wdf. looks like antigravity, last bit
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    Quote Originally Posted by Strange View Post
    The speed of light changes when it travels through different mediums. The same is true of sound.
    Nitpick:
    The speed of light does not change. No matter the medium.
    There can be, however, a delay caused by interactions between photons and electrons...
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  22. #21  
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    i thought i read somewhere that scientists found "slow light"?
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    Quote Originally Posted by ryanawe123 View Post
    i thought i read somewhere that scientists found "slow light"?
    Not 'found.' Affected with artificial means.
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    what do you mean? they affedted the speed of light with artificial ways?

    means light can travel slowly?
    Last edited by RamenNoodles; March 21st, 2013 at 07:54 PM.
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    Quote Originally Posted by ryanawe123 View Post
    what do you mean? they affedted the speed of sound with artificial ways?
    No, light.
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    oops, typo, i typed light instead of sound.

    thanks for correcting me.

    how many % of light's speed did they slow down?

    to what speed?
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    Physicists Slow Speed of Light

    Usually, using supercooling and a laser but when I went to find a more detailed article- that one above was one of the hits that came up. Interesting read.
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  28. #27  
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    Light can be slowed, but what about sped up?

    What if they reversed the factors in the experiments, will light speed up?
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  29. #28  
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    Quote Originally Posted by ryanawe123 View Post
    What if they reversed the factors in the experiments, will light speed up?
    No.
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    What is a pulse of light?
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  31. #30  
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    Quote Originally Posted by theorist View Post
    What is a pulse of light?
    A short burst of light, a flash, light that is on for a short duration, a pulse.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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