Doppler effect

• + - What

  • The change in frequency of a wave (or other periodic event) for an observer moving relative to its source

    When source is approaching listener, the wavefronts are compressed and frequency is perceived as higher

    When source is moving away from the listener the wavefronts are more separated so perceived frequency is lower

    If source is moving around the listener in a circle, there is no change in frequency, no matter the speed!

• + - How

  • Useful equations

    The important part of the velocity is the one moving towards or away from the listener

    The perpendicular velocity is irrelevant (no Doppler on a circle)

    • Unless the speed is really really fast! (the frequency of rotation approaches audio frequencies ~ 20Hz)

    When movement is not directly towards or away, the velocity used to calculate Doppler is the projection on the line connecting the listener and the source

    Software implementation

    • + - Synthesis
      • Simple
      • The frequency from the generator can be modified
      • So it is just a matter of calculating the changing frequency
      • However timing might not be that simple!
    • + - Doppler on Audio file

      • Audio is stored in a buffer in memory

        Instead of reading buffer linearly, the speed of reading is modified according to the relative speed

        If doing it in real-time must be be stored in a buffer, otherwise we would need to look into the future when source is approaching!

• + - ref
  • ~ http://en.wikipedia.org/wiki/Doppler_effect