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Tracking Down a Hideous Squeal

Discussion in 'Recording' started by skiffdriver, Jun 27, 2009.

  1. skiffdriver

    skiffdriver Active Member

    SM57, six inches from and slightly below the bridge, pointed up at the soundhole of my friend's very nice sounding Larrivee. I noticed that taking a little out of the 1-2kHz range improved the track quite a bit. When I swept through it with the parametric EQ at +18dB, this really horrible, almost feedbacky resonance jumped out, centered and most prominent at 1.5kHz and extending down to 1.1 and up to 2.2.

    It's not on the other guitar track I recorded that day with him (same guitar), it's not on the vocal track recorded at the same time (which has guitar bleed) and it's not on any of the other acoustic guitar tracks littering my hard drive (the ones I haven't thrown away).

    Obviously I'll be watching this frequency range like a hawk in the future, but it'd be nice to figure out possible sources for this noise. I've come up with a few:

    1) It's in the guitar. The track with the noise was in a different key from the one without the noise. In that key, the guitar resonates in a way that is not noticeable in the room but that interacts with the recording chain in an unpleasant way. I can check for this the next time he comes up to record.

    2) It's in the chain. Electrical noise from the computer (it's the only thing plugged into the wall; my Duet is powered from it), or interference from somewhere, or a crappy Garageband parametric EQ. But why would it just be on this one track, and not on the vocal too?

    3) It's in the room. Something (water heater, fridge, monitor) kicked on, and only the 57 was in position to pick it up. If this was the case, though, I'd expect to find the noise on more than one track, since this stuff kicks on and off fairly regularly.

    4) Aliens flew over my house, and what I am hearing is the tortured screams of their human lab rats. The guitar mic was pointed up, and the vocal mic was pointed down. This explains why the squeal isn't on the vocal track.

    Any ideas?
     
  2. Guitarfreak

    Guitarfreak Well-Known Member

    Lol, well without hearing it my first guess is that you had a preamp turned up too hot. Not input dB, it's different. You don't have to be clipping for a preamp to be pushed too hard. When you get to the top 5-10% of a preamp (lower for less venerable models) you will start to hear artifacts much like the noise you are describing. This can come in the form of white noise/hissing, other noises, or occasional audio dropouts.
     
  3. skiffdriver

    skiffdriver Active Member

    This is possible, I guess, since I didn't take notes at the time (which I resolve to do in the future), but I doubt it. I did roll the levels back to zero between the guitar takes, as I used the 57 as a dummy mic when we retracked the vocal, and so had to reset levels on the second guitar track (the one without the noise). Maybe they were better the second time around. But I've been conscious of where I'm putting the knob, and it's been landing around the upper forties/very low fifties decibelwise for most of the acoustic guitar tracks I've done. Apogee claims the Duet preamps can take 75 db gain, so I shouldn't be anywhere close to that.

    Unless I am.
     
  4. Codemonkey

    Codemonkey Well-Known Member

    I assume you're using a plugin for your EQ?

    Had it been analog you could maybe have blamed substandard circuitry for the problem.

    Is the noise only audible while the guitar is played, or is it permanent?

    Also, what are you using for speakers? It's entirely possible they might not like 1.5KHz much.
     
  5. Guitarfreak

    Guitarfreak Well-Known Member

    That too, I didn't think of that. I don't have a problem in that range, but my sub 'woofs' at any frequency peak between 130-250Hz. Down boy.
     
  6. skiffdriver

    skiffdriver Active Member

    Plugin

    It rings with the guitar. The more I listen to it, the more it sounds to me like an overtone that has taken an unwanted leadership role. It's a clear, whistling quality that feels a bit like feedback, with a little phasing, that starts strong with the strums of the guitar and disappears when the guitar isn't playing.

    On the track without the noise, the mic was in _roughly_ the same place, but probably not _exactly_ the same place. Could this be a classic demonstration of the importance of mic placement?

    No speakers yet. AT M40 phones. And they haven't balked at the same range on other tracks.
     

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