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Speed of Sound and Moving Vehicles

  1. Jul 11, 2012 #1
    When an object is thrown out from the front of a vehicle, its initial velocity relative to the ground is the velocity relative to the vehicle plus the velocity of the vehicle.
    Why is this not the case with sound traveling out from the front of a jet for instance? Or do the velocities add, creating a higher sound velocity which then falls to the normal velocity in air as the sound propagates outwards?
  2. jcsd
  3. Jul 11, 2012 #2
    The speed of sound depends on the medium that it is going through so whether the vehicle is moving or not the speed would be the same if the medium it is going through is constant.
    This question is similar to why the speed of light is constant relative to all frames of reference.

    However there are things that are not the same, that is the frequency of the sound waves observed from different observers. If source is stationary it is pulsing sound waves at a constant frequency

    Observer A ( ( ( ( ( ( ( Sound Source ) ) ) ) ) ) ) Observer B

    However if the Sound Source is moving to the right → then it will send more wave frequency to the right with resepct

    Observer A ( ( ( ( ( ( ( Sound Source ) ) ) ) ) ) ) ) ) ) ) ) ) ) Observer B

    NB: Just a rough conceptual diagram, Youtube or Google images on doppler effect to visually see java scripted application for a better diagram

    This effect is called the Doppler Effect. Haven't learnt it yet in school so correct me if i am wrong only interpreting this doppler effect from relativity that C is always constant regardless of motion of observer or the source only frequency is effected
  4. Jul 11, 2012 #3


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    It doesn't really have to do with the Doppler effect as you describe it.

    Sound simply cannot travel through the air faster than the speed of sound in air at those conditions. No matter how fast the vehicle moves, the sound will always move away as fast as it can in the medium. This does mean that (for a subsonic vehicle) we will se the Doppler shift occur.

    Just remember, sound speed is a property of the medium. Sound is not an object, but a disturbance in a given medium, so it will always travel at the rate the medium prescribes. The only way to change its speed is to change the medium somehow (typically temperature in air).
  5. Jul 11, 2012 #4
    At the atomic level, close to the surface which is creating the sound, is it the case however that molecules in the air are being accelerated by the vibrating surface and they reach a velocity higher than that of sound because of the motion of the vehicle, even though the wave propagates at the speed of sound?
    Also, unlike light, is the speed of sound relative to the observer in the moving vehicle less, as they travel in the direction of propagation of the sound?
  6. Jul 11, 2012 #5
    I think that you're right, that is possible. And that is unlike light, as far as we know: nothing propagates faster than light.
    I suppose that with "relative to the observer in the moving vehicle", you refer to a reference system transformation. At 300 m/s is the effect of the Lorentz transformation negligible, so that the measurement of the relative speed of sound and vehicle practically the same is with the "air reference system" as with the "vehicle reference system".
  7. Jul 11, 2012 #6
    The speed of sound is dependent in the medium it is travelling through the motion of the source does not affect its speed however doesn't it affect the frequency the waves are propagating if the source is travelling.
  8. Jul 11, 2012 #7


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    of course the individual air molecules must move as fast as the vehicle, so for a supersonic vehicle, the molecules will need to move faster than the speed of sound to keep up with and ultimately get out of the way of the vehicle. After all, the no-penetration and no-slip conditions must still hold. However, actual sound waves cannot propagate faster than the speed of sound.

    Keep in mind that the speed of sound is relative to the motion of the medium itself, so in front of a vehicle, sound can technically move faster than the speed of sound were the medium stationary in front of the vehicle. It still abides by the speed of sound relative to the moving air in front of the vehicle. This basically occurs out to the shock, at which point the waves start to coalesce into the shock wave. This is what happens in front of a blunted object. On a sharp object there is no stand-off distance, so the sound waves effectively only propagate backwards and have no small area where they actually move forward like in the subsonic region in front of a blunt object.

    This is of course somewhat of a simplification of the situation, but you should get the idea.

    Sure it affects the frequency, but that doesn't affect the speed of sound in air, which is non-dispersive. Pure CO2 would be a different story since it is a dispersive medium and thus on Mars, you would see a change in sound speed, but on Earth, no such phenomenon.
  9. Jul 15, 2012 #8
    If a wave is traveling in a medium, the added velocity of particles in the medium have very little to do with the speed of the wave.
    The speed imparted to the molecules by the vibrating surface is usually much less than the velocity of the sound wave.
    The sound wave does not significantly affect the speed of the molecules. Note that the average velocity of an air molecule without a sound wave is about the speed of the sound wave. The change in speed caused by the sound wave is much less than the speed of sound.
    As someone else pointed out, the speed of sound actually depends on the properties of the medium, not the properties of the thing that generates the sound wave.
    The situation changes if that vibrating surface is moving faster than the speed of sound. In that case, the waves being given off are not called sound waves. They are called shock waves. Shock waves do not satisfy the usually laws of acoustics.
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