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Asymmetry in the Doppler Effect for sound

  1. Jan 30, 2016 #1
    I'm a bit confused about this. So say there's an observer and a source of sound. If the observer moves towards the source of sound, the frequency seems to increase because he encounters more wavelengths in the same amount of time.

    In a second case, if the source moved towards the observer, the wavelengths get sort of 'bunched up' and it makes the effective wavelength shorter, again increasing the frequency.

    But even if their relative velocities are the same in both cases, the perceived frequency is different. My textbook (and the internet sources I looked at) didnt really explain this, though one place off handedly said it's because sound only propagates in a medium.

    I get how it is numerically different. Can someone help me understand kind of quantitatively how it's different? And why the same doesn't occur for light waves (where both cases would give the same answer?)

    Thanks for your help and time!
  2. jcsd
  3. Jan 30, 2016 #2


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    Why do you expect a symmetry? The sound travels in one direction through a medium, the situation is not symmetric.

    Light in vacuum does not have a medium which would fix its relative speed. Light in air is similar to light in vacuum, but if you are fast enough it will show an asymmetry there as well.
  4. Jan 30, 2016 #3
    I don't understand why the presence of the medium makes it unsymmetric. Why isn't the speed perceived by an observer the only thing that matters?
  5. Jan 30, 2016 #4


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  6. Jan 31, 2016 #5
    Thanks for the link! I'm not done reading it yet, but still thanks.
  7. Jan 31, 2016 #6


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    Consider the situation where the observer is moving away from the source. And, assume, faster than the speed of sound. No sound from the source reaches the observer.

    But, if the source is moving away from the observer faster than the speed of sound, then the sound still travels towards the observer at the usual speed.

    That's perhaps the ultimate asymmetry of the situation for sound.
  8. Jan 31, 2016 #7
    The observer would still hear sound from the source, but in reverse order. An observer moving away from the source at twice the speed of sound would hear a musical piece in correct time and tune, but backwards.
  9. Jan 31, 2016 #8


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    You are welcome. It is one of my favorites, and I think addresses your concern pretty well.
  10. Jan 31, 2016 #9


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    Imagine you want to achieve infinite Doppler shift:
    - How fast must the source move through the medium to compress the distance between wave crests to zero?
    - How fast must the receiver move through uncompressed wave crest, to collapse the encounter period to zero?
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