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Out of Phase: how can you tell?

Discussion in 'Amps, Mics & Pickups [DB]' started by MingusAmongUs, Aug 13, 2007.


  1. I might be opening myself up to ridicule here, and this subject applies to other instruments as well, but how do you know if your line is "out of phase" and you need to reverse polarity or "phase shift" ?
    Is it always obvious by the sound alone?
    What causes this?
    In what kind of DB->pickup or mic->amp scenario might this occur?
     
  2. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    No cause for ridicule at all. First, it's important to recognize that the human auditory system is essentially "phase-deaf." That means it essentially does not care about absolute phase. For example, if you are listening to music via a single loudspeaker, there will be no discernible difference if you simply invert the phase. (Humans are exquisitely sensitive to phase/timing differences across ears but that is another matter).

    With musical instrument amplification, the phase or polarity of the amplified signal matters only to the extent that it interacts with the instrument itself and other sources of sound. The DB, as we all know is a resonant cavity that creates its own acoustic energy. When you amplify it, the wavefront emanating from the speaker cabinet can relatively reinforce or cancel the acoustic output of the bass. Reinforce it too much and you could end up with feedback. Cancel it too much and you can impede the vibration of the bass itself. So, you change the polarity of the amplified signal to achieve the best compromise. Ideally, you would want a continuously variable phase shifter so that the best result could be achieved but usually one or the other phase orientation is sufficient.

    If you have a device that allows you to invert the polarity of the signal, you simply choose the one that gives you the better result in terms of how it sounds.
     
  3. BEEF

    BEEF

    Apr 16, 2007
    Naperville, Illinois

    And these devices DO exist (I own two for recording purposes)...

    [​IMG]
     
  4. BEEF

    BEEF

    Apr 16, 2007
    Naperville, Illinois
    Two ways to tell if your polarity is out:

    1- Record direct to recording software. Look at the wave on screen. The first 'swoop' of the wave should go up.

    2- Look carefully at the cone of the speaker on a bass amp. Thump a string to make that 'thump' sound. You should see the cone move 'out' first. If 'out of polarity' it will move 'in' first.

    VERY difficult (if not impossible) to hear this on a single source/single speaker listen.

    It's when there are TWO sources (or more) that it becomes a little bit of a mess.
     
  5. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    Well, I disagree. There is no physical reason at all why the first bit of speaker cone motion should be a compression (outward movement) rather than a rarefaction (inward movement). Neither should the first displacement in a waveform necessarily be "positive." There is nothing more "correct" about either and, as a listener, it is of no consequence at all. By the way, when considering complex physical resonators and complex signal chains, phase often shifts with frequency. So, while the speaker cone or waveform might move in or out at the onset of a sound within a particular frequency, it could likely do the opposite at another.

    As for phase-shifters, I sure do know that such devices exist. In fact, decades ago, I used in my research rack-mount vacuum-tube, Grason-Stadler broadband continuous phase shifters complete with OC3 gas voltage regulators. I still have them. They are now 35-40 years old. By the way, I went digital a long time ago. :)
     
  6. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    [KY resident translation]

    Try 'er both ways, then use the one that sounds better, y'hear?

    [/KY translation]


    FUNGUSAMONGUS - I've been through this more than once with the techies around here, and they always talk over my head when answering my question (not because they're being condescending, but because my poor wee little brain doesn't understand the tech talk). With my Full Circle pickups, I noticed that when using a preamp with a phase switch, I always thought they sounded more natural when they were switched to the "out of phase" position. In layman's terms, "in phase" sounded a bit too fat in the bass frequencies and slightly rubbery in the bottom end, while "out of phase" thinned out the lows a little bit and balanced the sound better, especially at higher volumes.

    Since purchasing the Focus series III amp head, I 've just been using the HP filter to achieve the same basic effect, and I've been happy enough with that. But now that FLIGHTDECK has started making an affordable HP filter with a phase switch, it looks like a no-brainer to get one for on the road and see what it does with the Full Circle and the Focus. If I do, I'll be sure to report the results in one thread or another.
     
  7. Hmmm

    If you wanted other people's opinion...why are you arguing?

    Current feel, in both the academic and field arenas, is that it should be positive/upswing in the metering that you are looking at. If you knew different then why hammer someone that is trying to give you advise?

    Not a good way to go about getting information.
     
  8. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    I'm not the one who asked for advice. I was answering the question. By the way, I am certainly in the academic arena. The "feel" that there should be an upswing is little more than convention. It has little, if any, practical basis. What you are referring to is the notion that one should preserve absolute phase through a recording and reproducing chain. That is, if the microphone was hit with a compression, the speaker that ultimately is the reproducer should also produce one. The truth is there is no reason that this needs to be the case and it is often a crap-shoot anyway, considering all of the phase-altering devices that are employed in a typical recording chain. In addition, anecdotal claims to the contrary, it is well-understood (at least by academics) that altering the absolute phase of a running, complex waveform (such as music) is of no perceptual consequence. The only credible studies that have found any effect of absolute phase were those in which highly asymmetric, temporally-restricted signals were employed.

    This is not to say that phases should not be aligned across devices. This is the impetus for time-alignment in multiple-driver loudspeakers and across microphones in recording setups.

    As far as musical instrument amplification goes, there certainly is no "right" or "wrong" orientation of phase. As I mentioned above, the issue is important only insofar as it affects acoustic interactions between devices (e.g., speaker output and double-bass, two speaker outputs, etc.)
     
  9. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004

    Precisely!!!!!!!!!!!!
     
  10. I stand corrected.
     
  11. That's about the best answer I could've gotten.
    But thanks all, it's definitely helpful. I always muck around with my gear and I've never really been able to tell whether it sounds better in phase or out, only that there is a slight difference. And I have noticed it's more distinct at higher volumes. So long as I know there isn't a right and wrong per se.
     
  12. stefandisgust

    stefandisgust

    May 28, 2006
    Boston, Massachusetts
    Spector Basses-New Artist
    Cut a speaker cord and take both contacts of a 9v battery to a bare end while the other is connected to the input jack of the cab.Just watch the speakers carefully its only one quick jolt that theyll make but youll find the one out of phase
     
  13. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    ...or put the two cabs face-to-face while they're playing anything with low-frequency content. If you hear the bass "suck out," they're out of phase.
     
  14. BEEF

    BEEF

    Apr 16, 2007
    Naperville, Illinois
    I wuz just answering the above question.

    The only thing 'correct' about polarity is the design intention of the piece of gear.

    If it's polarity is one way on input and the opposite on output, something is wired wrong (unless it's purpose is to reverse polarity).

    The 'convention' of positive polarity from a source (a speaker compression/outward movement as opposed to the opposite) is just a standard. All microphones (as well as ears) hear things by rarefection. The opposite of the source.

    You can't tell from a distance. It only matters up close (where you hear/feel the 'sucking' vs outward pressure. And it's only a problem with MULTIPLE SOURCES.

    I can think of a GREAT example where BOTH of these things occur at the same time and cause trouble.

    When recording/listening to any instrument with a direct input and a mic at the same time. The mic hears only ONE polarity (The capsule never goes 'out' first, just 'in'. Like an eardrum.) shortly AFTER the sound is made (it takes time for sound to travel). The DI signal is heard 'instantly' (no time delay between the 'pickup' and the source) and either has positive or negative polarity.

    I understand that we are arguing semantics here. I apologize if I inferred a 'correct' polarity when I should have stated 'conventional' polarity.

    :)




    Everyone go wire their stereo's the opposite way now. :eek:
     
  15. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator

    Oct 19, 2000
    Louisville, KY
    While we're on the subject, what about those cabs that use banana plug connections on the back. Can you switch the phase by turning the banana plug the other way? If so, is that the same basic effect as hitting the "phase" switch on a preamp?
     
  16. I think if you plug the red wire into the black hole and vice versa, you will have the same effect. Or a different effect by the same name. So I've read.
     
  17. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    Please accept that I am offering the following in a friendly spirit. My intention is not to be argumentative. Rather, I'd like to clear up a few things.


    This is not true in general. For example, many op-amp devices will invert phase. The output of such devices has polarity opposite to the input yet they are not wired wrong and there is no specific intent to invert phase.


    I agree. It has become convention that when a positive voltage is applied to the "hot" side of a loudspeaker connection, the cone should move outward. When you play some source of sound through it, the first voltage change need not drive the cone outward or inward.


    Something is amiss here. Sound waves are longitudinal waves consisting of compressions and rarefactions. Neither microphones (be they piezo, moving-magnet, ribbon, electret, condensor) nor ears "hear" by only rarefactions. It takes both in order for anything but a DC pulse to be transduced. That is, you gotta wiggle it back and forth (that's technical talk :) ).

    Such "feeling" has nothing to do with hearing. If you are considering audio frequencies, you cannot discern the individual "pushes" and "pulls" of the transducer (the speaker) re what you are hearing.

    With all due respect, this is completely incorrect. The microphone capsule absolutely moves in and out and so does the eardrum. In the case of a microphone, that's how you get positive and negative voltages with respect to the neutral or resting position of the diaphragm. Delays that occur via a microphone are not a result of the time it takes to move the diaphragm. Rather, they are a result of the group delay that stems from their filtering properties.
     
  18. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    Yes, indeed. In fact Bob Gollihur once posted here that he carries inverting and non-inverting speaker cables so that he can do just that (in his case, he's using Speakon or phone plugs so there is no dual-banana to just flip).

    [So long as the ground on the speaker cabinet is not in common with the ground in any other part of the circuit which, 99.999999999999999999999999% of the time, it is not. If it were, when you flipped the dual-banana, it would short out the amp.]
     
  19. bribass

    bribass

    Jan 25, 2006
    Northern NJ
    Endorsing Artist; Arnold Schnitzer/ Wil DeSola New Standard RN DB
    BTW, Chris, the switch that turns on the HP filter on the Focus 2r III reverses the phase as well.

    BG
     
  20. bolo

    bolo

    May 29, 2005
    Apex, NC
    Brian - Actually the notch filter reverses the phase. The high pass filter reverses it again, effectively putting it back where it was.

    From the last AI newsletter:

    Phase Reversal
    When the notch filter is engaged, there is a 180-degree phase shift in the amp output. ...

    When the low cut filter in engaged, the phase shifts another 180 degrees which brings you back to the original phase relationship.​
     

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