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Phase Cancellation Question (for school)

Discussion in 'Recording' started by mrkite, Nov 4, 2008.

  1. mrkite

    mrkite Guest


    I'm taking an audio class and I'm on a homework question I can't seem to remember the answer (even though it seems to be a pretty easy question).

    "If you cause phase cancellation at 1000hz, you experience cancellation at 2000hz, 4000hz, and 8000hz. What is this kind of phase cancellation called?"

    Could anyone tell me what this is?

    I know the overtones of a sound are multiples of the fundamental frequency (200Hz fundamental frequency, 400Hz, and 600Hz harmonics), which is kind of like the 1000Hz, 2000Hz, 4000Hz, etc.
  2. moonbaby

    moonbaby Mmmmmm Well-Known Member

    I'm trying to remember this one myself, said I, as I ran MY FINGERS THROUGH MY HAIR (that's a hint)...
  3. mwacoustic

    mwacoustic Guest

    Hmmm... I'd have to BRUSH up on my acoustics theory to answer this one...
  4. Space

    Space Well-Known Member

    I combed the whole web and didn't find the answer?

    edit: changed "at" to "end"
  5. hackenslash

    hackenslash Active Member

    Maybe we should ask Phil ter answer his question.
  6. Codemonkey

    Codemonkey Well-Known Member

    Talk about a straight up answer..

    Now, I was under the impression that 1000Hz actually had harmonics at 2KHz, 3KHz, 4KHz, - not on a x=2*(x-1) scale.
  7. natural

    natural Active Member

    It's a trick question. Or maybe an incomplete question.

    There's not enough information in the question to give an answer.

    Are all freq happening simultaneously? Or is it a recording of a 1K test tone, move on to the next tone 2K etc. Plus if the 1K tone is a sine wave there's likely no overtones at the upper freq.

    It depends if the Freq at 2k 4k etc are made up from the 1k source. If so then reversing the POLARITY of the source will eliminate all overtones.
    If 2k and 4k are separate freq having nothing to do with the 1k, then affecting the 1k has no effect on the others.

    That's my best guess so far.
    Send my degree in the mail.
  8. GeckoMusic

    GeckoMusic Guest

    x=2*(x-1) simply means that x=2

    Do you mean f=fo*(2^n) where n is an integer zero or greater and fo is the base frequency?

    That would be 1k 2k 4k 8k etc. Those are the octaves

    f=n*fo are the harmonics (1k 2k 3k 4k etc n is positive, fo is the fundamental)

    <hint> Note that all the octaves are contained in the harmonics. </hint>
  9. TVPostSound

    TVPostSound Guest

    I also combed the web, but couldn't filter anything from it!!!
  10. BobRogers

    BobRogers Well-Known Member

    For the benefit of Mr. Kite and all of the rest of us, isn't the type of cancellation being hinted at the filtering of all the odd harmonics of a fundamental frequency? As in...

    0 = Sin w(t+d) + Sin w(t-d) = 2 Sin wt Cos wd

    This implies

    wd = (2n-1)Pi/2 for all natural numbers n or

    w = (2n-1)Pi/(2d) ... odd multiples of Pi/2d.
  11. RemyRAD

    RemyRAD Guest

    This type of phase cancellation is called "COMB FILTERING" based upon its second harmonic. So comb filtering is the answer. Now take a bath and comb filter your hair.

    Chrome filtering is created with time delay effects.
    Ms. Remy Ann David
  12. GeckoMusic

    GeckoMusic Guest

    Trying to confuse things by moving into the time domain I see. I might need to comb though a text book to figure out what you are saying. You are correct that it is only odd harmonics effected by the comb filter delay. I had it in my brain that it was all harmonics.
  13. BobRogers

    BobRogers Well-Known Member

    Well, his question asked about octaves of 1kHz. No odd harmonics there.
  14. natural

    natural Active Member

    Oh, I see,
    SPACE had the answer the whole time.
    very clever
  15. GeckoMusic

    GeckoMusic Guest

    I think the first 4 did as well as a number of other posters.
    hmmm... those are even harmonics of 500Hz. I see your point. Sorry. So it's not a delay induced comb filter...
  16. GeckoMusic

    GeckoMusic Guest

    Oooh! I think I have the more specific answer.
    mrkite, are you studying room acoustics and standing waves?
  17. Space

    Space Well-Known Member

    It was a real clear night here @ Exit4, I just followed the moon baby!
  18. Codemonkey

    Codemonkey Well-Known Member

    Gecko: my bad, typo I think. I was trying to imply that I thought the octaves were simply at F = n*x where F = harmonic freq, N = harmonic number and X = base freq.

    Ofc that brought 4 posts of :S formulae.

    And now we seem to be making randomly emphasised puns at each other.
  19. moonbaby

    moonbaby Mmmmmm Well-Known Member

    In Scotland, phase confuses you... :)
  20. mrkite

    mrkite Guest

    Thanks a lot for all the great answers, I got my homework answer in just in time.

    Nope, just taking a class on how to record things. Nothing fancy just yet :p

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