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I Can the speed of sound in air be faster?

  1. Jun 11, 2017 #1
    Sound is vibration of air molecules knocking into each other. Intuitively, it seems the faster a molecule travels before hitting another, the faster the propagation of sound.

    Can't you hit air molecules harder, so the air molecules will travel faster and spend less time between collisions, and hence the sound will travel faster?
     
  2. jcsd
  3. Jun 11, 2017 #2

    Drakkith

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    Hmm. I don't think so. While the speed of sound in a gas increases with higher temperatures because the molecules are moving faster, I believe that any increase in the speed of the particles from collisions with a high speed object is quickly lost as the energy is spread out over large numbers of molecules moving in many different directions. In other words, the molecules bounce off of the object and quickly impact nearby air molecules, which are then scattered in random directions.
     
  4. Jun 11, 2017 #3

    davenn

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    no, that just produces a louder sound

    you can prove that yourself
     
  5. Jun 11, 2017 #4
    Yes, in theory. I, however, cannot generate enough heat through my mouth to change the temperature a significant amount that the change in speed is noticeable.
     
  6. Jun 11, 2017 #5
    I thought louder sound just means the air molecules travel further from equilibrium, AKA larger amplitude?

    Ok, so there is by some mechanism that sort of prevents the air molecules from speeding up too fast. But what if I push the air molecules slowly? Is there some mechanism which keeps air molecules from going too slowly?
     
  7. Jun 11, 2017 #6

    davenn

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    think about it ... I even said you could prove it for yourself
    tap a piece of metal, then hit it hard. What is the difference ? the only difference is in the loudness of the sound

    The speed of sound is constant in a specific medium. But can be a different speed in different mediums.
    Generally higher density mediums will have a higher speed .... eg speed of sound is faster in water or rock than it is in air. Density isn't the only factor tho ......

    https://www.nde-ed.org/EducationResources/HighSchool/Sound/speedinmaterials.htm

    http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/Soundv.html


    Dave
     
    Last edited: Jun 11, 2017
  8. Jun 11, 2017 #7

    Drakkith

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    They're already moving quickly, bouncing around all over the place. Moving an object slowly doesn't change that.
     
  9. Jun 11, 2017 #8
    I think the crucial difference here is that you are talking about molecular interactions, but sound is a much more macroscopic effect that happens through pressure variations. You may be able to affect a small number of molecules, but that in the end just translates into a minute increase in pressure that then propagates on with the usual speed. As pointed out, you have to affect the medium as a whole.
     
  10. Jun 12, 2017 #9
    The molecules of air are already moving in the absence of sound. If anything, this speed can be related to the speed of sound in the medium. For air the in standard condition this average speed is about 500 m/s.
    As you increase the temperature, the thermal speed increases and so does the speed of sound in air.

    Now, when you excite a sound wave in air, the molecules gain some extra speed, on top of their thermal motion speed. This is not a constant value but its value oscillates, like the other parameters of the wave (pressure, displacement The relationship between sound intensity and the velocity amplitude is
    $$ I= \frac{\rho_0 c v^2}{2} $$
    where I is the intensity in W/m^2, ## \rho_0 ## is the density of the medium, c is the speed of sound and v is the maximum value of the extra velocity induced by the sound wave.
    For a loud sound of 100 dB, the intensity is ## 10^{-2} W/m^2 ## (in air) and one gets ##v \approx 7 mm/s ##.
    So it is quite clear that this is negligible when compared with the actual speeds of the molecules which is actually responsible for the speed of the propagation.
    Even for a sound that is at the pain threshold (about 130 dB) the extra speed is just a fraction of m/s.
     
  11. Jun 18, 2017 #10
    A saturation diver's 'heliox' air-mix has a significantly higher speed of sound than the usual stuff. Further afield, the speed of sound in a hydrogen-based atmosphere is even faster...

    IIRC, you can have a non-explosive yet breathable hydrogen + helium + oxygen mix. Speed of sound should be significantly faster than in 'heliox' but due care, please ??
     
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