Why this feedback?

Discussion in 'Live Sound [BG]' started by Klonk, Dec 15, 2013.


  1. Klonk

    Klonk Norwegian Wood Supporting Member

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    This question is bass-/DB-related:

    Last night I was sound guy for a big band who played a 3 set job, corporate gig. The bass player had both an upright bass and a 5 string electric bass, played through a small combo with Direct Out into the mixer. He mostly plays the upright, which has a piezo mic placed close to the "bass leg" of the bridge.

    We had a feed coming from the bass channel, only hearable through the subs of the FOH system. I asked if he could switch polarity on his pickup, which he did, and it helped a bit. I also gained the signal down. The feedback continued, however, and he eventually switched to his electric bass. This removed the problem, at least for the FOH, but band members complained about still hearing the feedback in on-stage monitors.

    Here are some of my thoughts on why this could happen, feel free to suggest and chime in:

    1. The sub speaker was standing on the stage, which is raised about one meter up from the floor, old, and made of wood. I am thinking it could be some self resonance thing going on, the sub making the stage vibrate (at least around where the sub was sitting). If so, it would have been wise to put the sub down on the floor instead of having it onstage?

    2. The piezo pickup for the upright was picking up vibrations from the floor (in addition to what it's supposed to pick up)

    I know it's hard to help without more details, but feel free to share any thoughts on why this feedback occured, it would be a great help since I'm fairly certain to do the sound for this band again, and I'd like to avoid this problem next time.
     
  2. derrico1

    derrico1 Supporting Member

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    I've seen that sort of low-end feedback more often than I'd like, but I'm curious about an apparent contradiction here.

    You say the feedback came from the bass channel—especially from the upright—but also that it continued when the bassist switched from doghouse to electric. When the player moved to electric was the upright bass disconnected, muted, or deselected (via Tonebone or somesuch)?

    Also, I'm assuming you were seeing metering on the bass channel light up when the feedback hit. No? If not, what was the evidence that the feedback was limited to the bass channel? Also, were the drums mic'ed?

    I ask b/c w/ low-end feedback, mic'ed up kick drum and floor toms are often the culprits.
     
  3. Major Softie

    Major Softie

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    The fact that you experienced feedback only on subs, and then only on stage, is a huge clue. Just because the feedback frequency is originating with the bass doesn't mean that it is included in the feedback loop: IOW's, the frequency can be fed to the system by input "1," fed to the speakers, and then the amplified signal can be picked up by a mic on input "2" and then create the feedback loop between input "2" and the speakers. I have to wonder if (in the second part 1/2 of your problem) if there wasn't some input that only was being fed to the stage monitors. That's the first place I would have been looking.
     
  4. Downunderwonder

    Downunderwonder

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    Have to go with non muted upright or else drum mics and stage boom. If not un muted bass then probably nothing to do with the bass at all in the origin of the booming. Yes the pickup does the boogie if the floor is alive.

    Mic's that aren't highpassed need to be gated. And get the sub the hell away from the bass!

    In the first place, DB doesn't really need to be in the sub anyway. Mic the combo next time and / or HPF it.

    In the 2nd place, what were sub frequencies doing in the monitors? :eyebrow: like you want mud and feedback! Probably "blame the bass syndrome" again, not actual bass feedback in the monitors.
     
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  6. derrico1

    derrico1 Supporting Member

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    Yes, BTW. Very much yes.
     
  7. Klonk

    Klonk Norwegian Wood Supporting Member

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    Thanks for the replies so far!

    I didn't send any sub frequencies to the monitors, that's what makes me baffled why there was a feed that was heard predominately in the sub (never in the satelites), but according to some band members, was heard also in the monitors...
     
  8. Downunderwonder

    Downunderwonder

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    Let's assume nobody was silly enough to leave an unmuted doghouse on the stage.

    As was explained, the sound origin may not be doing the original feedback. Since reversing the polarity partially helped, I suspect the instrument itself was only getting involved after the looping had started, with the original feedback continuing with the BG.
     
  9. Klonk

    Klonk Norwegian Wood Supporting Member

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    Sorry, I forgot to adress the muting/unmuting questions, the bass player has a small combo with 2 inputs and he can switch which channel is sent to the Direct Out of the amp, so it is, for all intents and purposes, muted.
     
  10. Downunderwonder

    Downunderwonder

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    Getting the sub off the stage and over against a wall should probably fix everything. You probably don't have gating. Highpass on most of the microphones and maybe less with the sub. Big thumping bottom end isn't needed for big band jazz.

    What made you sure it was the bass? Did you mute mics as well as the BG?
     
  11. Nagrom

    Nagrom

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    Mic stand stem resting on the stage?
     
  12. Klonk

    Klonk Norwegian Wood Supporting Member

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    Hm, normally I would mute all the input channels when set 1 ended, but this time, for some reason, I muted only the main out + monitor busses. This might well have screwed up my hunt for the feed... as you say, I can't now be entirely sure it had anything to do with the bass expect the loop was going through the bass channel. But might have originated from something else..

    They had a DJ there who ran on his own FOH, he had his subs on the ground, not on the stage.
     
  13. Downunderwonder

    Downunderwonder

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    When the BG was "feeding back" <insert eyeroll> was the bass channel definitely peaking? I can't see that happening normally. All I can think of is maybe a microphonic tube in the combo, but I never struck one myself.
     
  14. Klonk

    Klonk Norwegian Wood Supporting Member

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  15. derrico1

    derrico1 Supporting Member

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