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  1. #1

    phase cancelation

    I was wondering if all of you who understand phasing could explain to me in detail why and how phase cancelation occurs because it jsut doesnt make sense to me the way that I've been explained phasing and phase canceling. I understand that if you take signal "A" and flip it 180 degrees out of phase and then add the 2 sounds together, they will cancel them selves out, but I'm just not understanding how placing mic "A" that picks up a sound that another mic "B" is picking up can cancel the sound out totally or at all if both line phases are the same and how they get put out of phase with eachother.

  2. #2
    Senior Member SC358's Avatar
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    Quote
    Quote: andrewr
    Snip.... but I'm just not understanding how placing mic "A" that picks up a sound that another mic "B" is picking up can cancel the sound out totally or at all if both line phases are the same and how they get put out of phase with eachother.
    Hi Andrew,
    You have part of the equation so far. The key factor is how far the 2 mics are placed away from each other that will start the cancellation. Due to the distance, the process of the signals will have a slight delay (from each other) sufficient enough to be out of phase.

    It's a great experiment if you have a few minutes to try out - then you'll have a better understanding.
    SC358
    Relationships are based on compromises - behavior accepted is behavior repeated.

  3. #3
    Administrator Dave's Avatar
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    Yep, that's it - the phase cancellation happens because the sounds waves hitting each mic are at different phase points, because the mics are different distances from the source.

    Another way to demonstrate phasing is in reverse. Play some audio tone through stereo speakers and slowly move your head around as you listen to it. You will find spots where the tone is louder and quieter - in phase and out of phase.
    Dave Owen
    MediaCollege.com

  4. #4
    so, I've struggled with words for about half an hour and then I just decided to keep it simple and make a JPEG and upload it for you guys to explain how I am interpreting phase cancellation right now. So, if you click on the attachment I've attached and it is correct, then I belive I understand phase cancellation, and yes I have the second wave starting 1 m/s after the first starts and the senario I was thinking about when I created this model is this: on a drum kit, mic 1 is the snare, and mic 2 is an improperly placed Hi hat mic that's aimed at the edge of the hi hat and mostly at the snare (yes, minor detail that sound waves don't travel that far of a distance from the snare to hi hat in 1 m/s but this is just a model, so here we'll just assume that sound waves travel like light speed... whats this world coming to? lol... kidding...) But yeah, if you guys wouldn't mind taking a look at the JPEG I made and tell me if I am understading phase cancellation correctly I would greatly appreciate your time and effort in helping me understand phase cancellation. Also, one thing I'm not understanding is on the audio mixers that have the option to flip the phase on each channel, what exactly it does becuase I've been told that it flips pins 2 and 3 (reversing the polarity) and I was wondering if it only flips the polarity of the audio until it gets to the mixer and then flips it back or if the polarity is reversed and stays 180 degrees out of phase (reversed polarity- sorry to keep calling it different things) and stays 180 degrees out of phase as it goes down the eq's and through the faders and then out to the house. Thank you to all of you guys who are and have helped me understand audio better and thank you for your time and willingness to help me with this stuff, I really appreciate it... BTW, those waves that you see, I copied from this site in the intro to audio page if any of you are wondering where I got them from...
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    Last edited by andrewr; 5th Jun 2005 at 06:51.

  5. #5
    Senior Member SC358's Avatar
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    You most definitely have it down right - congratulations!! At times I also have a hard time understanding concepts unless I can make it happen. Your better off struggling to understand this now than being in a hot seat on a situation you never came across before.

    There is one thing I'd also like to clear up at this time. In reference to your jpeg - if one sound source appears on 2 seperate mics/buss/tracks AND is kept seperate as Left & Right Channels then there is no cancellation. It is when the 2 tracks are MIXED that creates the cancellation. Whether they are mixed to a mono channel or stereo won't matter at that point - some type of phase cancellation will occur.

    Quote
    Quote: andrewr
    Also, one thing I'm not understanding is on the audio mixers that have the option to flip the phase on each channel, what exactly it does becuase I've been told that it flips pins 2 and 3 (reversing the polarity) and I was wondering if it only flips the polarity of the audio until it gets to the mixer and then flips it back or if the polarity is reversed and stays 180 degrees out of phase (reversed polarity- sorry to keep calling it different things) and stays 180 degrees out of phase as it goes down the eq's and through the faders and then out to the house.
    Your last statement is correct, once the phase switch has been activated, it remains out of phase throughout the system unless it is switched again before being mixed in.
    SC358
    Relationships are based on compromises - behavior accepted is behavior repeated.

  6. #6
    Sweet... Thank you dave and SC358 for helping me understand phase cancellation. Sorry it took me so long to thank you guys but I was like everywhere for a couple of weeks right after you replyed and I came back and started posting replys and giving back to media college and I remembered that I forgot to thank both of you, so thank you again and I look forward to helping other people in need of advice.

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