feedback explanation

Discussion in 'Amps, Mics & Pickups [DB]' started by Steve Killingsworth, Aug 3, 2003.

  1. I played at a bluegrass festival last night and had an interesting experience. Every time I played a B on the 4th fret of the G string I got this rattling echoing feedback. It didn't happen on any other note but was very noticible even if I barely plucked the string. Eventually we moved to keys that didn't require This made me wonder about two things. What causes feedback and why would it only happen on certain notes?

    I await your wisdom.
  2. I'll try to do my best with my poor english...
    When we talk feedback, we talk about a loop where a part of the output signal is fed back to the input path.
    The feedback signal can be added or substracted (positive or negative feedback).
    Negative feedback is used in almost all audio electronics to reduce distorsion.
    Positive feedback is what gives an oscillator.
    When we're talking about amps and amplified acoustic musical instruments, that means the feedback signal (part of output signal, in this case speakers) arrives in phase with the input signal, so the two signals add together.
    This can produce howling but also a strong reinforcement of some particular frequencies.
    Those frequencies depend on many factors; room acoustics, coupling between the instrument and the room (stage in particular), type of input detector (mic, piezo pickup), non-linearity of the amp system, etc.

  3. There is nothing wrong with quality of your English. I wish everyone here had communication skills in English as good as yours!
  4. Thanks for the kind words, Bob!
    My first language is french, and it's sometimes a big task for me to explain things in a clear fashion.
    (the most difficult task being informal everyday talking, as strange as it may seem...)
  5. This may be a ridiculous question but I am a curious person. I have always thought that feedback was strictly the result of improperly "calibrated" amplification. If I understand Francois's explanation correctly, it seems that it can be caused by environmental conditions. So are there instances where feedback is inevitable and you just have to live with it?
  6. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator Gold Supporting Member

    Oct 19, 2000
    Louisville, KY

    There are many factors involved. Ay low volumes, feedback is NOT inevitable. As you get louder, certain frequencies cause your bass to vibrate excessively. Some of these frequencies may be exacerbated by the acoustics of the room you're playng in, your distance and angle from your amp, the type of pickup you're using, whether you have dampened the afterlength of your strings to help stop wolf ones,'s hard to tell where the problem comes from sometimes, except by process of elimination. If you have a parametric EQ on your amp, sometimes you can use it as a "notch filter" by zeroing in on the offending frequency and cutting it. The best case scenario is to somehow manage to get everyone else to turn the hell down so you can do the same. :)
  7. mje


    Aug 1, 2002
    Southeast Michigan
    Feedback happens, as Francois explained beautifully, when the output of an amplifier gets fed back into the input. So the question is, at what level does this happen?

    Below a certain level of volume, you don't get feedback. The output of the amplifier/speaker isn't powerful enough to start the instrument of microphone vibrating. And it doesn't happen at all frequencies at the same level, either.

    As Chris notes, there are various environmental factors. Your microphone, your bass, the room you're playing in, etc., all have various frequencies that they will resonate at. At these resonant frequencies energy is transferred more efficiently, and it's easier for feedback to occur. For instance, my archtop guitar seems to have a strong body resonance at Bb; if I start turning up the volume, that's where it'll start howling. Or if I'm playing at loud volumes, and hit a Bb, it's liable to get away from me.

    You can "calibrate" a room or system by setting filters, as Chris describes, to reduce the gain of the amplifier at these frequencies where feedback occurs most easily.
  8. fretlessman71

    fretlessman71 Still beats havin' a job Supporting Member

    Aug 8, 2005
    FoCo, NoCo
    Hi folks! Newbie here...

    I'd like to be able to play my upright in loud conditions and keep the feedback demons at bay. A notch filter sounds like a decent idea, owing to the idea that you'll probably have to adjust it for every different room. I was also thinking that since a lot of my feedback seems to occur when I'm not playing a note at all, maybe a noise gate would help. Also wanted to use a compressor, but can't seem to find the right one - when I run my bass through my Fender 15 watt practice amp, the built-in limiter makes my crappy bass sound HUGE. I'd really like to emulate that in the form of an effected signal that I could send to an amp or the board. Anyone got any bright ideas? Using an Underwood pickup - seem to like it okay, but maybe there's a better solution for me....
  9. fretlessman71

    fretlessman71 Still beats havin' a job Supporting Member

    Aug 8, 2005
    FoCo, NoCo
    Gonna Bump this thread. I think waiting 9 months for a reply is patience enough, don't you? ;)
  10. steve in tampa

    steve in tampa

    Jan 11, 2006
    Check out an Alesis 3630. It has compression, limiter and a gate.
  11. speedster


    Aug 19, 2005
    Ontario Canada
    Francois is right on with the feedback issue, interesting enough the loop can get started for all kinds of reasons. Being as you were getting it specifically on the B note of the G string would indicate that was the frequency causing the trouble.

    Moving away from the mains, moving from the monitors, turning the bass etc all might have helped out that situation. Also explaining to the sound guy which note and string might help in in dropping that frequency somewhat which would perhaps help out.

    If you were mic'ing the bass then feedback is always a problem.

    I run a small GK amp and line directly to it (placed behind me) and then line out to the main board and I ask the sound guy at the Bluegrass festivals to take the bass right out of the monitors.

    I use the amp as my monitor and I have never really had any problem with feedback except in situations at a Country/Bluegrass combined festival where inevitably the sound guys usually end up putting the mains on the stage or too damn close to the mains and the low 16k - 20k range starts to feed back.

    Other than that no problems at all even at really high volumes...

    Hope that helps
  12. Roger Davis

    Roger Davis

    May 24, 2006
    I use an Underwood via a Fishman Pro EQ into a GK. Pro EQ settings generally level but the GK bass cut button in. I take the GK treble control right off, get a great sound and no feedback under any circumstances - and that's with the GK on a mic stand right behind me. The secret seems to be to cut the treble but the great sound could be due to my Meinel.
  13. endika


    Oct 25, 2006
    I am playing in an electronic project that uses laptop loops as the core foundation. Everyone on stage (percussion,guitar,keys, and myself on upright) need to hear the laptop track clearly to sync with the beat. There is often lots of lows in the laptop tracks, which is making it very difficult to get loud enough without feeding back. Smaller venues is okay but larger venues where the band relies solely on the on-stage mix are very difficult. I run a realist through an ampeg STV-4pro with 4x10 & 1x 15. Any suggestions for me?
  14. Is it possible to EQ most of the lows out of the stage monitors? Or possibly send the foldback a different mix from the laptop with less of the bass lines and more percussion or whatever you keep time to.
    And the usual advice like don't let your amp blast straight at your bass, get your speakers off the floor, damp the tailpiece, avoid boosting the low end.
  15. Phil Smith

    Phil Smith Mr Sumisu 2 U

    May 30, 2000
    Peoples Republic of Brooklyn
    Creator of: iGigBook for Android/iOS
    I suggest getting a Yamaha Magicstomp, this effects box has a great feedback management system, you simply step on the pedal when you experience the feedback and it dampens that frequency and I think you can do that 5 times. I have tried it on a gig and it works well.
  16. bribass


    Jan 25, 2006
    Northern NJ
    Endorsing Artist; Arnold Schnitzer/ Wil DeSola New Standard RN DB
    Jeez, w/ a stack like that you're kinda beggin' for trouble, especially w/ a loud monitor mix as well. You've got to achieve a lower stage volume overall. Just too many loud frequencies in close proximity of your bass. Either cab alone should be more than enough if you signal is running tru the house as well. Also watch the monitor volume and the volume of your band mates on stage.


  17. Welcome to the world of bass playing.;)