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Finding the RPM of a CD?

  1. Mar 7, 2013 #1
    In this video, a CD accelerates until it breaks:

    Here is one comment:

    The flaw that he made is that he assumed that there is one CD revolution per cycle in the sound wave, right?
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Mar 7, 2013 #2


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    One revolution is one cycle in the frequency, so it is probably correct RPM

    A 20x CD player revs about 12000 RPM as normal RPM at 1x, when the laser reads the inner tracks, revs about 640 RPM
  4. Mar 7, 2013 #3
    Why is one revolution one cycle in the sound frequency?
  5. Mar 7, 2013 #4


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    I can't find the comment [on YouTube] that this comes from, but the audio must refer to the mechanical noise made by the device that's rotating it, not to the sound encoded on the CD. I'd expect that sound to have a strong component corresponding to the rotational speed, from vibration caused by a slightly non-uniform mass distribution.
  6. Mar 7, 2013 #5
    It was actually a comment on reddit. Yes, the audio he was referring to was the audio from the video (which is captured using the camera and is due to the mechanical noise by the device that's rotating it).

    I'm not sure what you mean by something having a strong component.

    What exactly determines the frequency of sound? If I have a mass on a spring that oscillates from equilibrium from one maximum to the other and back once every second, will the frequency of the sound be 1 Hz?
  7. Mar 10, 2013 #6


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    Generally speaking, one revolution is one cycle in the frequency - generally because disalignments in the rotating mass will force the mechanism to viberate. However, there can be gear mechanisms, other mechanical things that might have viberations, gear noise, etc which generates other frequencies. Multi cylinder engines might generate frequencies higher than what the revolutions is as the pistons combined fires several times pr. revolution.

  8. Mar 10, 2013 #7
    What kind of disalignments are we speaking about here?
  9. Mar 11, 2013 #8


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    1Hz will be in the frequency spectrum, and probably 2Hz and some higher frequencies are there as well.

    The center of the CD is not exactly the center of the rotation, and the CD itself is not a perfect disk as well.
  10. Mar 24, 2013 #9


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    Just small disalignments. Even a flywheel which seams to be perfectly round, perfectly centered on the axis, will make a audioable viberation when the RPM gets high enough. And this small inaccuracies will force the rotating mass to wobble a tiny bit, and make a sound with a frequency (keynote) that is exactly the same as the RPM/sec.

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