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understanding phase.

Discussion in 'Recording' started by vinniesrs, May 29, 2003.

  1. vinniesrs

    vinniesrs Active Member

    Has anyone here used multiple mic setups and placed them certain distances apart to create an intentional cancellation at a certain frequeny?
    I have tried this a few times with guitars and it sounded good. I have also tried this on other instruments with some sucess. Is this concept flawed?
    These figures are for easy figuring, but I'm thinking of using this concept in relation to drums by understanding the differences between phase with, as an example close mic to overhead mic. Close mic on snare @ 1.5" = 180* of phase at 4520hz with overhead @ 3 ft 4520 is in phase with the close mic at this frequency. Move the overhead 1.5" closer and it cancells.
    I am asking this just to be aware of, and possibly proactive with this.
    I realize that this can create variying degrees of cancellation across the spectrum.
    What do you think of this? Is it a waste of time to even consider this issue? I am going to work it out anyhow, just for the hell of it.
     
  2. falkon2

    falkon2 Well-Known Member

    Two waves have to be 180' degrees apart AND be of same amplitude to cancel each other out. With slight variances in frequency response of mics, and the fact that low frequencies lose less power when propagating the same distance, it would be hard to calculate with certainty. You'll be able to get the places which are theoretically nodes and antinodes for specific frequencies, but I assume that would be about it... uneven frequency response and the fact that the condensers would pick up a lot of room could mess with the expected results somewhat.
     
  3. vinniesrs

    vinniesrs Active Member

    Agreed. But now that I have some numbers, I'm gonna conduct a little experiment. I just want to see if it's noticeable.
     
  4. shaneperc

    shaneperc Member

    From what I understand (I remember reading this somewhere, but the equation is easy enough), when stereo recording, to use mic distance to move everything (well, a target frequency anyway) into phase, the microphones have to be the same distance from the source within less than 1/3 of an inch. That moves phase cancelation high enough into the bandwidth that it's less noticeable. Of course, that doesn't cover every frequency, and of course, how in the hell do you get a guitar player to sit that still???? :)
     
  5. vinniesrs

    vinniesrs Active Member

    Shaneperc, the equation is really very, very simple. Sound travel at about 1130 feet per second, depending on temperature and altitude. If you look at a 1hz wavelength it is 1130ft long according to this. Thus 1130hz has about a 1ft wavelength. Each wavelength represents 360 degrees of phase. One complete cycle.(hertz)
    With micing, flying overhead speaker arrays(live sound), speaker design and acoustics, this is the basic, basic math. If an 1130hz wavelength is 1 foot long-360 degrees, then 1130hz @ 6 inches would be 180 degrees. Phase canellation, if the wavelengths are equal in amplitude as falken2 pointed out. However if they are not EXACTLY the same amplitude, then cancellation will still occur, in proportion to the difference in amplitude. You can figure that out from the source.(see the inverse square law.)
    There are also variying degrees of phase, and thus variying amounts of canellation which are not related to amplitude. 1130hz at 3 inches is 90* and so on.
    If you already know this, then I apologize for expounding upon it so much. I think it's important for everyone to have a firm grasp on this concept. I am just trying to get fancy with it, and see if anyone has any tricks centered around it.

    cheers.
     
  6. shaneperc

    shaneperc Member

    No problem! I was just commenting (please excuse my lack of anything else to do. :) )

    So, it almost sounds like you're experimenting with changing the timbre by using phase adjustment. If signals are in phase, but different amplitudes, then the combination will just be a that signal at a different amplitude. But, if the signals are slightly out of phase (not exactly 180 or 360) and different amplitudes, then the timbre starts to change. Kind of like what happens with off-axis coloration. Is this what you're getting at?
     
  7. ummmmm, arent we forgetting something... surfaces need to be taken into the equation. relative walls/whatever will have some impact on phase relationship (especially on drum oh's)...


    i didnt read all the posts so excuse me if it was covered.
     
  8. falkon2

    falkon2 Well-Known Member

    Hey, you know, I just realized that walls and other surfaces could affect phase, especially on drum overheads.

    I didn't really read this whole thread so excuse me if this has been brought up.
     
  9. vinniesrs

    vinniesrs Active Member

    Shaneperc, this is pretty much where I'm going. I was just trying to see if anyone else had tried this on drums or different instruments and had neat sounds as a result. I also wanted to know if anyone has found danger areas with this. I have used this on clean guitar tracks(electric) in order to give them a more acoustic feel. Seemed to work pretty good.

    Creative soundblaster: You are cautioning me about reflections. My drum room is kind of small, but I have foam absorption panels on the walls on either side of the kit, I am of the impression that any reflected sound is so small I can't hear it off the panels. With respect to bass, the corners of the room are filled with loose foam, up each wall and along the roof. The walls in this room are unparalell, and the roof is treated as well. The only reflective surfaces in the room are the window (6ftx3ft) and the cedar paneling, which accounts for about 30% of one wall. The window is nearly 10 ft away from the nearest mic, and the cedar is 4-6ft away. sound from the kit would have to travel twice this distance to reverberate into the mic. With this in mind, I don't see how it can be that much of a concern. If I am mistaken, then please educate me, as I am always eager to learn.

    P.S. I have not had the room tested, but it sounds good. If I had to guess an rta I would say .3-4 overall with the 500hz and down range being the least lively.
     
  10. somehow i dont think fukin2 likes me very much...

    in regards to the surface interaction i just meant only true results (if possible) could only come from inside an anachoic (however its spelt) chamber...
    the main thing of concern i would think might be the roof. u mentioned the room was small so i gather the cieling is low? thats gonna be a problem on overheads with phase.
     
  11. falkon2

    falkon2 Well-Known Member

    Aww. I'm sorry if my eccentric sense of humor has made your panties tie up in a knot. ;P

    No offense was meant.
     
  12. falkon2

    falkon2 Well-Known Member

    Soundblaster:
    There's your answer. I think we can consider it more or less anaechoic (sp?)... The drum set itself would probably cause more changes in phase due to reflected waves than the surrounding surfaces.
     
  13. vinniesrs

    vinniesrs Active Member

    I agree the roof might be a problem, Tell me what you think. The ceiling sits at 7.5 ft.(fromm floor to the first layer) The building is constructed with 2x10 joists, which we have filled with r-20(pink). Beneath that we have a sheet of 58 drywall, and two mor 38 sheets. Beneath that are sheets of egg carton shaped matress foam glued to a hard rubber mat backing. Beneath that is a grid of thin lattice, filled with greenery. Beneath that is the drum kit.
     
  14. Kurt Foster

    Kurt Foster Distinguished Member

    Ok folks.. play nice.

    The concept of intentionally making something out of phase just weirds my morning out! :D I come from the old school where everything needed to be mono compatible. I think this is still valid today. My 2 cents. Kurt
     
  15. vinniesrs

    vinniesrs Active Member

    Kurt, would you pleas explain to me the mechanics of phas in a recoring and mono compatability? I am assuming that if there is a left right difference in phase, than when you mono the mix, that difference would cancell out it's counterpart, thus effecting the mix? Could this not be manipulated to your advantage if that was the case?
     
  16. Kurt Foster

    Kurt Foster Distinguished Member

    Steve,
    Yes, that is what happens. If things are out of phase, when they are collapsed to mono, the volume changes or in worst case the element completely disappears. IMO this is not a good thing and I can’t see as how that could be used to benefit. I want the balance of my mixes to be the same in mono or stereo.. Kurt
     
  17. vinniesrs

    vinniesrs Active Member

    Thanks, dude. :c:
     
  18. vinniesrs

    vinniesrs Active Member

    Maybe my little experiment will have an unintended benefit. I will now look into these differences in the context of stereo imaging. With my previous example of phase relationships from mic to mic, this could effect the attack, or resonance of each tom. Keeping in mind things like proximity effect, spl's, and off axis response, keeping a constant distance from each drum when close micing, or changing the distance of the overheads, from the snare, and using the angle of the mic to change the tone. For a different style say, metal to pop, one could have a standard distance for close mics in relationship to the overheads. Or vice-versa.

    This is just an idea. I haven't really thought it out yet. I'm off to another studio soon, and I'll work it out when I get back. Hope I don't sound like an idiot.
     
  19. falkon2

    falkon2 Well-Known Member

    I think it COULD be made to work if the sampling was done to a mono track, then re-imaged later. It would cause a comb boosting that could bring out a series of wanted frequencies to make a certain track stick out better in a mix. The peeps over at Acoustics probably could say this with a lot more certainty, but I think that objects reverbate at frequencies which are multiples of each other very well (guitar strings and harmonics, for example), and it could lead to boosting of those specific frequencies should they reflect sound waves.

    That could be what's being emulated here.
     
  20. vinniesrs

    vinniesrs Active Member

    With everything I do I think visually, not sonically. And in my mind I see this very similar to a parametric eq. The varying degrees of phase being akin to the q, and the 180* diff being the center freq. This is how it sounded to me when I did a clean guitar with two mics. I just move the far mic until I got cancellatio at around 600hz. Sounded good. Didn't notice mono probs. Who knows? I'd better get to work soon or I'm gonna start thinking some more! :D
     

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