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Mic phasing questions.

Discussion in 'Recording' started by sshack, Apr 29, 2008.

  1. sshack

    sshack Active Member

    I'm not certain I'm even asking this correctly, but if/when a mic is used alone, by default, is it in or out of phase? Or is that even a valid question?

    What I mean, or more so, what I'm experiencing from time to time is that when I use my condenser mics I'm not certain if I should have the phase switch on my preamps IN or OUT. It seems that regardless of which I choose, there's still an element of a 'hollow' sound that I keep getting. It doesn't happen all of the time, but when it does (as an element of troubleshooting), I'll usually record twice, one with the phase IN and one OUT, then go back and listen to the differences to see which one is less swirly.

    Could it be as simple as me just having lame mic position or improper gain structure on my preamp?

    As always, any help and/or clarification is greatly appreciated.

    Thanks.
     
  2. danbronson

    danbronson Guest

    As far as I know, being "in" or "out" of phase can only be determined in relation to another source. You can still change the phase of a single mic, but it won't affect any phase relationship because there is none. That is to say, with one mic, you are always "in phase" because there's nothing to be out of phase with. So if your song is recorded with one mic, clicking that phase button will have no effect on what you hear, just what the wave literally looks like. Reverse it and your speakers will vibrate "forth and back" instead of "back and forth". Hope that makes sense.

    When you have two mics together on one source, you have to be careful that sound either reaches the two mics at the same time, in the same direction, or that the mics are far enough apart that it doesn't even matter. You will never get them perfectly in phase, but you can get closer by being smarter about positioning and that phase switch can help you (especially when the mics are pointing in different directions).

    Someone correct me if I'm wrong or if I left out anything important. Hope this helps clarify things.
     
  3. BobRogers

    BobRogers Well-Known Member

    In the simplest way of thinking about phase, it is not a valid question. Phase relates to the relative position of two wave. The issue usually comes up when you have two mics. For instance, if two mics are different distances from the sound source the trough of the sound wave could hit mic one at the same time that the peak of the wave hits mic two. The signals from the two mics would cancel even though they are recording the same sound.

    Another case of phase problems is when you mic a snare drum from both the top and the bottom. The two signals are essentially opposite - the bottom mic experiences a peak exactly when the top mic gets a trough and vice versa. (Since the drum moves toward one mic at the same time it moves away from the other.) The two wave forms look almost like mirror images of each other. This is when you hit the phase switch on one of the mics and flip one of the wave forms. Now the waves compliment each other instead of cancel.

    With a single mic, what you are probably experiencing is "room modes" and resonant frequencies. The keys to dealing with this are treating the acoustics and positioning the mic. The better you treat the acoustics, the easier it is to position the mic. The phase switch should not make any difference in this. The difference you are hearing is probably due to slight changes in position. Do some reading in the acoustics forum for more info on this.
     
  4. danbronson

    danbronson Guest

    You bring up a good point. This is a phasing issue as well, but it can only be solved with positioning and room treatment. What happens is your sound source hits the mic, but also bounces off a wall and hits the mic again. The two sources arrive too quickly for your ear to distinguish the difference between the two (in a large room it just sounds like reverb) and whatever phase they are in is simply added together. So some frequencies end up in phase and others are out of phase, depending literally on the length of their wavelength. What you end up with is comb filtering, it can make things really, really weird sounding and it's terrible for monitoring since it lowers the accuracy of what you hear.

    The solutions are better positioning (get your mics and sources away from walls and try to get things 38% into the room where they are less likely to fall into peaks and troughs of the natural modes of the room) and sound diffusion or absorption.
     
  5. sshack

    sshack Active Member

    That makes perfect sense, thanks.

    So then, when using two microphones and trying to balance out phasing issues, is it simply one or the other with respect to the phase switch being ON or OFF? Is basically the only way to determine which is which, by listening?
     
  6. Codemonkey

    Codemonkey Well-Known Member

    Pretty much.

    With drum overheads, consult the Recorderman OH technique.
    Otherwise keep things the same distance and symmetrical, is one way. Failing that get down to millisec level in your DAW and drag the tracks around. (Worst case scenario).
     
  7. BobRogers

    BobRogers Well-Known Member

    The phase switch is designed for the situation where two signals are exactly half a wavelength out of phase. The snare is one common example. Another would be two cables with their positive and negative terminals wired in the opposite way. You can fix a more subtle phase shift by "nudging" one of the tracks in a DAW so that the waves line up. This only works with a "one note" source like a drum.
     
  8. sshack

    sshack Active Member

    Interesting Bob, thank you for the tip and the education. I'll give it a try.
     
  9. Kev

    Kev Well-Known Member

    really, really good question

    the other guys have given good practical answers

    however I'd like people to think more about polarity rather than phase
    ... phase can be a continuous variable with frequency

    phase can be an issue as it relates to delay or time taken to get to the mic
    ... speed of sound stuff

    back to the direct question
    polarity

    historicaly there has been a difference of opinion or point of view with the definition of positive polarity for both microphones and speakers

    without getting into all the history
    positive pressure and the element goes into the basket and the voltage out is positive
    or
    positive pressure and the element goes into the basket and the voltage out is negative

    a similar situation for speakers

    Although most microphones today seem to be on the same standard
    do check
    because there some older ones that adopted the reverse
    AND
    wiring mistakes at manufacturing can happen

    as for speakers
    Some manufacturers still have a reverse standard to the majority

    hope that made sense
    for more historical stuff
    the early 1900's radio engineer books and radio communication and telephony books
    and mid 1900's Wireless World has good stuff
    AES papers and old Patents
     
  10. BobRogers

    BobRogers Well-Known Member

    Kev's right. The "phase" switch really changes the polarity of a wave. (Turns it upside down.) And the original switches were put in because of the lack of standardization of polarity long ago. With a pure sine wave or some other very symmetric waves that is the same as a phase shift. But in general it isn't the same. Still the switches are still there because they are useful in situations like the top and bottom of the snare drum that are not connected to polarity. So effectively they are used to correct phase problems more than they are used as originally intended.
     
  11. RemyRAD

    RemyRAD Guest

    This is one of my favorite subjects. I'm relatively unfazed by everyone's explanation. People actually confuse phase & polarity. Phase is a state of polarity. But I think my answer will polarize many into listening differently? It's hard to teach people how to listen.

    In my control room & others, I always began by listening to some of my favorite, previously recorded works by others & myself. Once I have my sonic reference I can begin.

    When recording many instruments during a tracking session & such, proper phasing is critical. More than a single microphone means sounds will be picked up at different points in time. Generally you'll find greatest low-frequency energy is reinforced when you are closest to being in phase. If all of your low frequencies disappear, chances are you have some phase issues. That's when you start punching that button. But changing just one channel will affect all. So we don't do it willy-nilly.

    Flipping phase is a useful function when decoding Middle-Side encoded stereo tracks. Plus other stereo tricks can be accomplished by manipulating phase.

    But flipping phase is not the same as flipping polarity.

    Some folks believe that if you are recording something with a single microphone that changing the phase polarity of that microphone does not present a noticeable difference. I disagree. It's all based upon perception. And perception for everybody and everyone is different. So you always have to listen. Carefully. You have to decide what you are listening to and how you are perceiving it. I frequently play phase tricks when mixing, anything.

    When using a console, some phase switches are only active for microphone inputs. Whereas others are further downstream and will affect a signal after it has been recorded.

    That hollow sound you mentioned is a phasing problem. But it's not of an electronic nature as much as it is a physical problem. A problem with time in a space. Yes, you'll hear a difference in that hollow sound when you invert the phase. It simply inverts the problem to be an inverted problem. That can only be fixed through proper studio design & acoustics. Microphone type, placement & proper gain staging can't completely compensate for a lousy space.

    Don't even get me started on proper speaker polarity in the control room. That gets me mad.

    Bipolar
    Ms. Remy Ann David
     
  12. Kev

    Kev Well-Known Member

    another situation where polarity can bring a different result is in electronic circuits where the positive transient response id different to a negative transient response

    BAD choice of words Kev ... cos transient response has a specific meaning to an electronics engineer

    shoot to the chase
    different topologies can give unexpected results
    .... ClassA circuits can give rise to a different slew rate on one cycle over the other
    ... also some can have more headroom in one direction

    it could be argued that there is something wrong with the circuit
    even so

    changing the polarity into the Mic-pre can give a different feel to the sound

    some Woodwind and Brass instruments can give a different shape to the waveform above and below the mid line

    it IS all very interesting and do take the time to look and experiment


    Remy's speaker polarity in the control room is a ripper of a subject for discussion
    and also for PA systems and Foldback

    even IF there is a right answer
    there are times when the wrong answer can work a treat
    :)
     
  13. sshack

    sshack Active Member

    This is really good insight. I can't say that I follow and understand everything in its entirety, but it has certainly helped me to be more cognizant of what to do and what to listen/look for.
    In retrospect, I do believe the times that I had my phasing problems was before I treated my room. Since then I have spent some time and money to treat the room about as good as I can (given my set up), also I've fashioned some surprisingly effective sound absorption 'panels' that work really well when I need them to.

    To that end, I just recorded some acoustic guitar last night that included a new SDC mic mated with my 4033 and it turned out quite nicely I think, and without phase issues.

    Remy, feel free to rant about anything you want...I"ll just pick up the crumbs until I have enough to build my own pie.



    :cool:

    Thanks again everyone.
     

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