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Comb Filtering???

Discussion in 'Recording' started by Brent D, Feb 26, 2004.

  1. Brent D

    Brent D Guest

    OK, this may sound stupid, but I was wondering if anyone could help me with this problem and lend some advice. It's another phase problem. In a situation when I mic up a guitar amp and choose to use a close mic, about 5" from the cone and a distance microphone 1-3m away, I find that I experience comb filtering as I draw the 'distant' mic back and forward (which makes sense) but I can't seem to find a 'phase free' position? Whats going on. The comb filtering sweeps across the spectrum as you move the distant mic backward and forward, so how do you find a position with no phaseing? HELP!

    Cheers :)

    Brent
     
  2. shaun

    shaun Guest

    Maybe try flipping the phase of one of the mics while moving it ( while moving it, listen till you get the most phase cancellation and flip the phase back. Works for me every time
     
  3. David French

    David French Well-Known Member

    You could also try putting a delay on the close mic to bring it into alignment with the far mic. Use the fact that the velocity of sound through room temperature air is about 0.3m per millisecond and set your delay accordingly.
     
  4. Screws

    Screws Active Member

    Start with the 2 mics equidistant from the cone. Have the guitarist repeatedly play a small section of the part to be recorded. If you have an assistant, tell him to pull the distance mic back slowly or you listen through the phones as you pull it back slowly.

    Don't listen for tone, listen for gain changes. It may take a minute to focus your ears on volume differences rather than the tonal differences.

    The out of phase spots will have the lowest aggregate volume and the in phase spots will have the highest.
     
  5. SME

    SME Guest

    Did you ever consider you are getting comb filtering from the indirect sources??? Like the floor, walls etc.
    That would be the reason you cant find a phase free distance from the close mic!!!
    Try putting the mic as close to, and perpendicular to the floor. (Boundary method)
     
  6. Brent D

    Brent D Guest

    Thanks for all of your help guys, I will take it on board,
    Cheers
    Brent
     
  7. shaun

    shaun Guest

    Little Labs IBPLittle Labs IBPLittle Labs IBPLittle Labs IBPLittle Labs IBPLittle Labs IBPLittle Labs IBPLittle Labs IBPLittle Labs IBP
     
  8. RecorderMan

    RecorderMan Distinguished Member

    ANYTIME you have two or more sources, there's will always be what we call "phasing". ALWAYS. Your hearing the result of adding those two source. As you move the one mic relative to the other, your hearing the center frequency change. ALL we can do is pick the spot where we like it best. I usually pick a spot that favors the fundamental..making it sound non-thin. You don't really need an IBP for this. One thing you can do..what I do sometimes. Wear headphones. pan the mics opposite and for the sake of movement, have both sources, while you move them, of equal loudness. Now you will notice as you move the distant mic that you will find areas/spots that the image between left and right sounds balanced , as if you had a short delay on a mono track panned hard left and right and it sounded "even". When your in wrong spots, the image in your head will sound skewed. The stereo image of these two source at these spots - and you can find multiple ones- is pleasing. You'll find these spot at intervals from the source, it relates to the 3 dimensional sound-field and is much more complex than sound on a string for you physics guys , so don't anyone come back with "is there a formula that I can use to compute the distance for the different nodes?"It's more complex than that (at least more complex than a formula that deals with one or two dimensions), and our ear/brain's are much better and faster.
    Now here's where I think it gets interesting. If you've just been moving the distant mic back and forth across the room , you will have found a spot that's sounds like 20ms , 40ms (approximations - what a source and a 20ms delay panned opposite sounds like). Once you pick a spot, try moving the mic on/off axis...sweeping it in an arc, You'll again find difference in inches that are much the same as the movements in feet you were making a few minutes before.
    So you will always have that comb filter...you pick it when it sound the best to you.
    I hope I communicated this to you.
     
  9. larrye

    larrye Active Member

    Recorder man and others,
    I can't express enough my appreciation for this site. I just found it a few months ago and this thread is just one of many. Great post!
    Larrye
     

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