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Speaker Phasing?

Discussion in 'Sound, Music, and Intercom' started by The_Terg, Jan 14, 2004.

  1. The_Terg

    The_Terg Active Member

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    After fiddling around with the pinguin audio meter, and miscellaneous programs and mics, I am growing curious about Phase inbalances.

    I have a pretty good idea how an audio signal can become out of phase, (being wired incorrectly, specifically inverted, reverb or other other effects that echo the sound out of phase....etc...) However, how does incorrect phasing effect speakers, and/or the overall sound of a system? Why is bad phasing looked down upon?
     
  2. DMXtools

    DMXtools Active Member

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    Bad phasing has several bad effects on the overall sound. Specifically with speakers, think of a simple stereo system with just one speaker on the left and one on the right. Now imagine a bass instrument center stage (phasing effects are more noticeable at low frequencies). If it's mixed at the center, you should have equal amounts of bass from both sides. But if one of the speakers is wired backward, it will be pushing while the other is pulling - the sound from the two speakers will cancel out, causing the bass to sound really weak when you're exactly in the middle between the two speakers.

    Now imagine you're playing a recording of a full orchestra. Instruments at the center of the stage will be weak and lacking bass, while instruments at the sides (where the sound only comes through one speaker) will appear to be much louder - like there's a hole on the middle of the orchestra. With proper phasing, the instruments appear in their proper places, instruments in the center are at the appropriate levels and the bass is clean and powerful.

    Proper phasing lets the whole system work at its peak efficiency. With the speakers in phase, the sound from them adds - most of the power you put into the speakers gets to your ears. If they're out of phase, it subtracts: you can dump a lot of watts into them and they can be working really hard, but the sound is weak.

    For multi-way speaker systems, phasing is important to flat, smooth frequency response. Crossovers, the gadgets that aim the bass at the woofers and the treble at the tweeters, don't make the transition from one speaker to another instantly: there's a small band of frequencies surrounding the nominal crossover frequency where the sound goes to both speakers. For example, if the crossover frequency from the subwoofer to the main cabinet is 150 Hz, frequencies between about 100 to 200 Hz will be going to both speakers. If the sub and main are out of phase, the sound from the two in that frequency range will cancel, scooping out that frequency band just as if you pulled the 150 Hz fader on your graphic EQ all the way down.

    With microphones, there are situations where being out of phase can be good. For example, a band has two singers, each using fairly good technique, singing close-in on an SM-58. What will happen if the two SM-58s are out of phase?

    Well, because each singer is close to just one microphone, there's no cancellation of the signal you want - the voice. But sources farther from the mics, like band instruments, floor monitors and feedback from the mains, will tend to be picked up by both of them and, because they're out of phase, tend to be cancelled out.

    Some sound engineers will go as far as to put two mics on each vocal mic stand, one to sing into and one just below/behind it, where it won't get much of the singer's voice, connected out of phase, to cancel background noise and feedback.

    But this only works for close vocal mics. For distance miking, where, for example, a stereo pair needs to pick up a vocal chorus of several people, the mics should be in-phase for best sound quality.

    John
     
  3. seanb

    seanb Member

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    here is a related question. or (s) since now I thought of another...

    one: if a sound (voice) is picked up in equal levels by two vocal mics, is the sound reproduced in the PA? What if the phase is reversed on one of the mics?

    two: what's with the headroom thing... that when you double the number of open mics on stage your headroom falls. I mean it makes sense in practice, I'm just wonder what the theory is behind it.
     
  4. DMXtools

    DMXtools Active Member

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    Assuming the mics are in-phase, yes, it will be reproduced in the PA, and louder than if only one mic was on (assuming they are the same distance from the signal source) because the signals from the two mics add. If they are out of phase, i.e.- pins 2 and 3 reversed on the XLR of one, the signals subtract. Theoretically, if the two mics are identical, placed exactly the same distance from the signal source, with exactly the same gain and EQ settings, they would entirely cancel out and you'd have no sound coming out of the PA. In practice, they'll never be entirely equal - you'll never get to the point where they cancel completely. But having mics out of phase can really cut down on the amount of signal that gets through.

    I'm assuming by headroom you're talking about how loud you can make things before feedback starts, rather than the more normal usage as the range between nominal level and clipping.

    With multiple microphones on stage there will be certain frequencies where two or more mics are roughly in-phase even if they are different distances from the speakers. At those frequencies, the signals add, meaning the signal that gets to the PA is stronger... meaning that the sound out of the PA is louder,,, meaning that more of it gets back to the microphones and if the system gain is high enough... SCREECH! With only a single microphone the gain before feedback is a little higher because you don't have multiple signals adding up.

    John
     
  5. The_Terg

    The_Terg Active Member

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    Thanks for the comprehensive info. its not really an issue, but I was just curious.

    *Extending my curiosity*

    If you were to run a mic out of phase to pick up backround noise, and mix it in so as to 'cancel' out the backround noise, would it work?

    I know that, theoreritcally it would, but is it a common practice?

    (Thinking about that nasty heating fan noise backstage...)
     
  6. DMXtools

    DMXtools Active Member

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    As I mentioned, it's fairly common practice for close vocal mics. A second mic of the same type is mounted to the stand just below/behind the primary mic specifically to pick up and cancel background noise. The thing is, it's got to be very close to the primary mic but not picking up much of the primary signal. It won't work for any kind of distance pickup, where both mics will pick up similar amounts of the primary signal, and it won't work, especially, if the primary mic is moving around and the out-of-phase mic is stationary - it's likely that at some time the different distances between the primary mic and the noise source will result in a situation where its signal is in-phase with the secondary mic, increasing the noise pickup.

    John

    P.S. - speaking of extra mics on stage, a neat little trick I use is to put an extra mic on stage, usually above and behind the drummer - an omnidirectional mic that never gets put into the mix. By soloing it into my headphones, I can listen to the monitor mix from the performers' perspective (I'm a small-time operation - monitors are from the aux sends of the FOH console). It helps a lot - Every show I get performers telling me "wow! it sounded great up there!"
     
  7. dvsDave

    dvsDave Benevolent Dictator Administrator Senior Team CB Mods Fight Leukemia

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    awesome technique... exactly what mic do you use?
     
  8. DMXtools

    DMXtools Active Member

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    Right now, the only omni in my collection is an old Peavey PVR-1 condenser. But it doesn't have to be a condenser, or even an omni - one of my SM58's would do as well, I'd just have to make sure it was pointing in the right direction. The important thing is it lets me hear what the musicians are hearing, so I can make sure they're hearing what they need. Soloing out a monitor send lets me listen to what's going to the monitor speakers, but doesn't give me any idea of how loud it is or how it's blending in with the sound of the drums and instrument amps on stage.

    John
     
  9. Nephilim

    Nephilim Active Member

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    IIRC the numbers are you lose up to 3dB of gain before feedback every time you double the number of open microphones, with all else remaining the same.
     
  10. The_Terg

    The_Terg Active Member

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    Wouldn't that also depend heavily on the acousitcs of the room, the type of mic, the EQ, the placement of mic.... ETC?
     
  11. Nephilim

    Nephilim Active Member

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    Yes. Thus the 'up to'. I guess it's more like a guideline, since (again, IIRC... it's early) 3dB is twice the air pressure.
     
  12. DMXtools

    DMXtools Active Member

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    It depends somewhat on the room, EQ, and mic type and placement, but it's a good "rule of thumb." It's more like directional mics and good EQ give you more gain before feedback to start with, or give you back some of the gain before feedback you lose by having multiple mics open.

    John
     

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