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Feed back and bass frequencies

Discussion in 'Live Sound [BG]' started by Tedward, May 1, 2016.

  1. I have noticed in certain situations that when I play a note like C# we get I get a boomy sound or even feed back in the PA system. I have experienced this in other venues when playing different notes. It happened today in church, causing the sub woofers to feed back. Is this caused by room gain exciting that specific frequency? And, how can this be controlled?

  2. seamonkey


    Aug 6, 2004
    What do you have mic'd? Turn the rumble filters on. Watch the channel level meters to see if you can see which mic is ringing. Or turn off the mics one at a time until you locate the problem mic. If you have a digital mixer then you can look at the RTA of each channel.
    Feedback eliminators can notch out resonant frequencies. Notch filters are used quite a bit for acoustic instruments, like DB
  3. s0c9

    s0c9 Supporting Member

    Jan 9, 2014
    1964 Audio artist, Fractal Audio Beta Tester
    Are you going DI or thru an amp?
    Most feedback of the kind you experienced is due LF signals going into stage mics...
    I'd bet (being church) that the FOH guy doesn't have the 100Hz HP filters on the vocal mics.
    Last edited: May 4, 2016
    bebass and tbirdsp like this.
  4. C or C# can boom all by itself with no help from any mic when the ceiling is really low. Unlikely in church.
  5. All 5 vocals are mic'd, 2 guitar amps with shields and drums. The ceiling is high. I'll check to see if we have filters on the vocal mics. It could be one of the drum over head mics being it is close to bass amp.

  6. derrico1

    derrico1 Supporting Member

    Apr 12, 2005
    Charlottesville, VA
    I'd suspect either the floor tom mic or the kick drum mic.

    Acoustic drums are excitable little guys, especially in response to bass notes. If the drums are mic'ed up and there's a nearby floor wedge for the drummer, that's all you need for this kind of low-end soupy feedback loop.

    Sometimes taping the resonant head can help. If not, adjust the bass rig's level and position, bring the drummer's wedge level down, or get everyone on in-ears.
  7. The drummer is running the mics into a mixer within his reach and feeds that to the main board (Behringer x32). I did notice awhile back that the graphic eq was pushing pretty high, all at least 3/4 of the way up.

  8. s0c9

    s0c9 Supporting Member

    Jan 9, 2014
    1964 Audio artist, Fractal Audio Beta Tester
    you didn't mention it, but UGH.. double-ugh if the drum mixer is sending a 2-ch feed to FOH.. FOH will have no control over the mix.
    Drums should go direct.. even if that is just kick/snare.. the rest could go via the mixer onto a 3rd channel.
    If I'm running FOH, I absolutely want/need control over kick and snare.. I can deal with toms, etc in a 3rd feed, but get a full drum mix on 2 channels is an absolute disaster (if that is happening). The EQ the drummer is putting into the mixer will definitely impact wedges and (now that you mentioned the approach) probably causing the feedback with the bass.
    Johnny Crab likes this.
  9. Thanks guys, a lot of good info. I'll have to check with the drummer about his mix.

  10. Is that a hollow body bass?
  11. Jerry Ziarko

    Jerry Ziarko Supporting Member

    Feb 23, 2003
    Rochester, NY
    Here is a great reference to use to try and tame the offending frequency. Try knotching out in the house first, and then with a very narrow q, your bass channel. Make sure to have HPF engaged on all vocal, guitar, and snare mics as well.
    wmhill likes this.
  12. The bass is a solid body and thanks for the chart, it will come in handy.

  13. derrico1

    derrico1 Supporting Member

    Apr 12, 2005
    Charlottesville, VA
    Kind of counter-productive, given the mixer's capabilities. He could be running mics to the X32 and mixing his monitor aux out from the X32 w/ Behringer's phone or tablet app.
    bebass and s0c9 like this.
  14. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
    One thing you can try is to remove the mechanical coupling between the bass cab and the stage, using some kind of isolating pad.
  15. 73jbass

    73jbass Supporting Member

    Apr 17, 2004
    High pass filter. The magic tool.
  16. derrico1

    derrico1 Supporting Member

    Apr 12, 2005
    Charlottesville, VA
    ^ available on each channel of his digital mixer. Fairly elegant solution, especially if the drummer would route his drum mics' output directly there instead of sub-mixing first (and adding global eq) through a mini-mixer at the drumkit.

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