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Is sound effected by air resistance?

  1. Nov 5, 2003 #1
    i was thinking... sound fades off over distance... does this happen because of air resistance? i know there are a ton of factors involved in this... but is this one of the more important ones in it?

    if anyone has any ideas please post them :D
     
  2. jcsd
  3. Nov 5, 2003 #2

    chroot

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    Nope. A source of spherical waves radiates its energy over all 4 pi steradians. A receiver captures only a fraction of that energy -- the ratio of the solid angle subtended over 4 pi. It has nothing to do with air resistance, which is a drag force experienced by objects moving through all fluids.

    And finally, this isn't quantum mechanics, so I'm moving the thread to General Physics. :smile:

    - Warren
     
  4. Nov 5, 2003 #3
    heh, sorry, i got confused about where it went
    so then what causes sound to not be heard over a certian distance?
     
  5. Nov 5, 2003 #4

    chroot

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    The fact that a receiver subtends a smaller solid angle when further from the source, and thus receives less energy.

    - Warren
     
  6. Nov 5, 2003 #5
    The fact that the waves spread out. The further away you are from the source, the smaller fraction of the total radiated power is intercepted by your ear.

    (Imagine putting your ear one centimeter away from a sound source; it might intercept half of the sound. One kilometer away, and most of the sound is going to places other than where you are at.)

    If you focused sound like a laser beam, then you would be able to hear it over a much longer distance, because all of it would be focused toward where your ear is (assuming you're standing in the beam). Of course, any beam will spread out eventually, just maybe slowly.
     
  7. Nov 5, 2003 #6
    but wouldnt gravity pull it down and air slow it? i mean if it were like a beam?

    i understnad that sound is a wave, thats how radios work
     
  8. Nov 5, 2003 #7
    ? You can form waves into a beam ... I'm not sure what your objection is. A beam is just sound waves pointed in a specific direction.

    In fact, if you just talk, the sound coming out of your mouth is directional; it's just not a very tight beam.

    I'm also not sure what the operation of radios in particular has to do with sound being a wave ...
     
  9. Nov 6, 2003 #8
    Air resistance as such does not have an effect on sound, but wind direction seems to.

    I live down the street from a football stadium. When a game is being played and the wind is coming toward me or there is no wind, I can hear some of the sound from the stadium, such as cheering crowds, the band, even words from the announcers. If the wind is blowing the other way, the sound becomes distorted or lost so I can't even hear the band.
     
  10. Nov 6, 2003 #9

    chroot

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    In fact, you could say that air resistance is required for sound to exist at all. Sound waves (pressure waves) take advantage of both an elastic and an inertial property of the medium through which they travel. The elastic property is the bulk modulus, and the inertial property is the density of the particles. In essence, the fact that the particles have mass and therefore have to have foces applied to make them move is part of the reason why sound waves exist at all.

    - Warren
     
  11. Nov 6, 2003 #10

    russ_watters

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    I'm not sure air resistance is the best choice of words, but there is another effect: since in real life there is no such thing as a perfectly elastic collision and there are viscous effects on air molecules moving past each other, there will be some sound energy lost as heat.
     
  12. Nov 8, 2003 #11
    I think what is meant by air resistance is impedence. All waves have 3 elements: Amplitude, frequency, and impedence. Impedence is due the resistance of the particles to change state/position which is dependant on mass and electrical attraction and stuff.
    It is like you have a perfectly still water in a swimming pool and start a wave. The waves bounce off the sides for a long time but eventuallly fade away and the water becomes still again. However, if you had a really really sensitive thermometer, you would observe a very very slight increase in the waters temperature.
     
  13. Nov 13, 2003 #12
    This should answer the question if you read throught it... It was enough for me...

    This is an elementary subject. ok first let me correct a statement made earlier sound wave aren't how radios work. Radio work by using radio waves not sound waves although eventually the radio waves are converted into sound waves sound isn't the whole process. How the frequency of sound changes as an object gets farther and farther away was described by Christian Doppler an Austrian physicist in 1842...
    ____________________________________________________________


    - World Book Encyclopedia (very descriptive)
     
  14. Nov 13, 2003 #13
    I shouldn't be such a space thief
     
  15. Jun 7, 2008 #14
    Sound waves are most affected by Air Resistance at 70 degrees F. At other temperatures the other variables that govern the speed of sound out weigh resistance.
     
  16. Jun 7, 2008 #15

    russ_watters

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    Heh - I read the first few posts without seeing the date and thought of another issue....then I saw I had already posted about it!

    Anyway, Simon, the thread is almost five years old! There really isn't any good reason to have dug it up.
     
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