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Phase Cancellation Question (for school)

Member for

21 years
Hey,

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.

Comments

Member for

13 years 9 months

Codemonkey Wed, 11/05/2008 - 10:48
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.
Great.

Member for

21 years

Member Thu, 11/06/2008 - 02:32
Thanks a lot for all the great answers, I got my homework answer in just in time.

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

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

Member for

15 years 2 months

natural Tue, 11/04/2008 - 23:50
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.

Member for

21 years

Member Wed, 11/05/2008 - 01:21
Codemonkey wrote: 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.

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)

Note that all the octaves are contained in the harmonics.

Member for

15 years 5 months

BobRogers Wed, 11/05/2008 - 03:46
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.

Member for

21 years

Member Wed, 11/05/2008 - 05:13
BobRogers wrote: 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.
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.

Member for

21 years

Member Wed, 11/05/2008 - 06:04
natural wrote: Oh, I see,
SPACE had the answer the whole time.
very clever
I think the first 4 did as well as a number of other posters.
BobRogers wrote: Well, his question asked about octaves of 1kHz. No odd harmonics there.
hmmm... those are even harmonics of 500Hz. I see your point. Sorry. So it's not a delay induced comb filter...

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