out of phase

Discussion in 'Recording/Live Sound' started by rod horncastle, Aug 16, 2006.

  1. rod horncastle

    rod horncastle Member

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    I'm still trying to understand the Pros & Cons of Out Of Phase recordings. When is it good, when is it bad? Most of all "what is it?. Is it a cancellation of sound when picked up by two sources?
    I own a few C.D.'s by major artists where the cymbles are white noise & cancelling each other out. Is that what your trying to avoid?
    I more concerned about recording my guitar cabs with two mics. I just use two sm57's & blend them together. Can out of phase happen with this. Do some people like things out of phase?

    Any advice or opinions would be insightful.
     
  2. LSchefman

    LSchefman Member

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    Phasing problems cause frequency cancellation, and when things are 180 degrees out of phase, the sound disappears when summed from stereo to mono. This is the principle used to reduce noise in balanced audio lines and humbucking pickups.

    Since people still hear audio on mono systems, like portable radios, old TVs, etc., phase cancellation of what you're trying to lay down to tape or disk is not what you want.

    Drum recordings often have phasing due to the movement of the cymbals back and forth as they're being hit, and this is hard to avoid.

    My feeling is that if you want a certain amount of phasing when recording with mics (as opposed to electronic phasing boxes), you're simply going to have to experiment a whole lot.

    It's easy to check your mixes for phase problems, just sum the mix to mono and see if something disappears from the mix or significantly changes in timbre. Most consoles have a "mono" switch for this purpose.

    As for miking a cab, use the 3:1 ratio rule as a starting point.
     
  3. Greggy

    Greggy Member

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    When double micing a sound source, maintain a distance between mics at least 3 times the distance from the mics to the sound source. Les may correct me if I'm wrong, but that's what I recall. BTW, I often double mic a cab and have ignored the 3:1 rule when doing so. Not sure how relevant the rule is to cab micing, at least when both mics are on a single speaker.
     
  4. JingleJungle

    JingleJungle Member

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    To expand the above... if the first mic is 1' from the grille, the second would have to be 3x that distance, or else...

    You will also have phasing issues when doing live takes with multiple mics. It can be interesting t hear a little bit of bleed in the other mics - especially if they will be placed elsewhere in the stereo field - as this can yield some interesting pseudo-stereo aperture. But phasey, flangey, nasal tones can also arise (both in stereo and in mono), so you always need to doublecheck your takes right away, before you pack everything up, mix and then print the result for posterity ;)

    JJ
     
  5. LSchefman

    LSchefman Member

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    Yes, the second mic needs to be 3x the distance of the first mic.
     
  6. MichaelK

    MichaelK Member

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    Sometimes a little taste of deliberate phase collision is nice, like a chorus effect. If you like it, it's "good." If you don't, it's "bad."

    One thing you might try when summing to mono is filpping the phase on one channel and seeing which way sounds better. That might be all you need if there's a problem. If you don't like it either way, then try moving one mic farther back or closer to the source. Just an inch or two might make a big difference.

    The aforementioned 3-1 "rule" is a good way to avoid problems, but breaking that "rule" doesn't necessarily meant you will have problems. It's only a problem if you don't like it the result, and it never hurts to experiment.
     
  7. suprotennessee

    suprotennessee Member

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    Yeah the 3:1 rule is just a rule of thumb. You can also be careful to place the two mics exactly the same distance from the source. Theoretically that should work with no phase issues. One more guide line is a 9dB rule: the two mics can pick up eachother's signal as long as each mic's primary sound source is 9 dB louder than the secondary sound source. But these are all just rules of thumb.

    More than anything listen to the two mics panned to the center (mono). Listen to how each sounds on its own, then with just one of the mics at a nominal level, fade the second in. Sometimes you will hear a dip in certain frequencies as the second signal increases. That means they are not in perfect phase. In the best situation you should hear the two complement each other, and each mic will retain its qualities alongside the other.

    Minimal phase incoherance in guitars is no big deal, though I feel you can usually get a better result with just one mic. Phasing in the drums is a nightmare, and is pretty much never good. The best way to hear the effects of bad phasing is to place two cardioid mics, preferably the same model, in X/Y (the capsules at 90 degrees to one another). Pan one to the left and one to the right. This should result in a near perfect phase relationship. Then flip the phase on the channel (the O with a slash through it button). The result is highly unnatural and will nearly throw you off balance. That's 180 degrees out of phase. The most problematic phase situations sit somewhere between 0 and 180 degrees. To correct them mics need to be moved.
     

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