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Mic feedback

Discussion in 'Safety' started by Stephen, Dec 16, 2006.

  1. Stephen

    Stephen Member

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    In Godspell, which we just did in my theatre, the first number involved a set of wireless mics, one with an echo effect and the other without it. We knew the mics were unstable, and we had to guard it with our hand on the master slider if something happened... anyway... during the second show, (second show is always worst), everything seemed to be going well..it was a quiet part of the number and then....

    KA-FOOOOOOOOOOOM!!!!!!!!


    feedback.

    I can't explain to you how loud and startling this was, we gave up on the mic for the rest of the number.
     
  2. Peter

    Peter Well-Known Member

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    If i have a mic that I know is very prone to feedback, I (after trying everything else) try to put a fairly high ratio compressor in the line so the feedback is not allowed to grow exponentially, instead it is squeezed down and kept at what is hopefully just below a painful level.
     
  3. SHARYNF

    SHARYNF Well-Known Member

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    Many times the problem is that the mic is omni and gets picked up by the main speakers. If you don't worry too much about the possibility of altering the sound some, a feedback eliminator can work quickly, Sabine FBX901's go used for next to nothing and can work well in this situation

    If you have a RTA on the system with a display, and you have a graphic eq you could use the graphic to correct the problem vs reducing the entire signal level. that said, a Feedback eliminator could save the day.

    One thing to remember if you have any sort of cardioid mic and you cover over the sides say with your hand or clothing etc, you effective convert it to Omni. Ringing out the mics and monitors if you use them is important but you need a graphic eq typically to accomplish this


    Sharyn
     
    Last edited: Dec 17, 2006
  4. highschooltech

    highschooltech Active Member

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    What headsets were you using on the mics. Countrymen E6's are terrible with feedback and enjoy causing issues. The best way to deal with this it to ring the system out for them and remind actors to stay away from speaker and take the mics out of monitors. Also, a compressor could be used to keep feedback from doing damage.
     
  5. SHARYNF

    SHARYNF Well-Known Member

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    Not to be picky but technically you would need a limiter, not a compressor, the compressor would simply make your feed back with a lower dynamic range

    In an effort to point out an approach, the correct way to eliminate feedback other than mic and speaker placement is via eq either graphic, parametric, or audo feedback eliminator, you don't want to reduce the entire signal, just the offending frequency range
    Sharyn
     
    Last edited: Dec 18, 2006
  6. silvrwolf

    silvrwolf Member

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    I totaally agree with SHARYNF. A cool product that you can use to eliminate feedback on just one mic or on your whole system is a Behringer DSP110 (also known as the Sharc). Its a neat little device that has a self learning feedback suppressor, noise gate, compressor, phantom power, low-cut filter and a mic preamp. All of this @ $80 is a deal. Unfourtinatly you can only use it for one mic unless you got two and ran them between your mixer and amps in a stereo hookup.
     
  7. Peter

    Peter Well-Known Member

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    You must be careful with limiters however because some of the cheaper ones simply chop off the top of your signal when it gets to loud leaving a harsh 'knee' that can be damaging to speakers if your amps are under rated for your speakers. A compressor at a very high ratio will essentially do the same thing, but will do it in a much softer way, especially if your compressor has a 'soft knee' option.

    The only reason for the compressor or limiter in the line is to set a 'cap' on the volume level so that it is not quite so painful. You really want to do an EQ beforehand to eliminate the need for a compressor or limiter in the first place (that's my last resort as i said above)
     
  8. jbeutt

    jbeutt Active Member

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    As far as last ditch efforts go, I'd throw in a feedback eliminator rather than a limiter.
    Obviously, using either of these things can be avoided with good placement and EQ technique which would be my primary focus in dealing with an issue like this. Although, I realize what section this is and that the OP was probably just letting off some steam.

    Keep in mind, too, that regardless of power ratings, a limited signal can hurt your speakers.
     
  9. TupeloTechie

    TupeloTechie Active Member

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    I too use these, but you can use it for more than one mic if you assign more than one mic to a submaster then use the INSERT to run thru the sharc.
     
  10. SHARYNF

    SHARYNF Well-Known Member

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    While this works, it is not the best way to go.

    First of all, it alters the audio on all the mics when only one has the problem, but more importantly, you easily wind up using up all the automatic filters quickly.

    Sharyn
     
  11. TupeloTechie

    TupeloTechie Active Member

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    hmm, that makes sense, but we use all the same mics on each sub, we had a sound company come in and thats how they wired it, I guess to save money, we don't have much funding as its a public school. I'm not quite sure, as I'm just learning this stuff, but some of the graphic eq's made now how a built in feedback reducer, one of those might help, as it would (I think) give you the power to cut down the on the band(s) thats causing feedback and also have the feedback reducer in case anything thats unexpected happens.

    Sorry if this is all wrong, I've never actually had anybody tech me anything about tech, I had to learn it the hard way, trial and error...
     
  12. SHARYNF

    SHARYNF Well-Known Member

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    Basically a feedback eliminator is a combination of a RTA real time analyser and a narrow band filtering program. The new ones work by converting your analog signal to digital, doing the analysis and the filtering, and then convert it back to analog. Behringer has one, and Sabine is probably the leader in this type of technology.

    Problem with when sound companies set up stuff for schools is that they do the quick and dirty, one size fits all type of deal, with the idea that the operator will be totally a Noob.

    Feedback comes more from placement of the mic with regard to the speakers, and also clothing, hands etc by the mic element. Sure some of the non linearities of the mic come into play but they are minor.

    The problem with the auto systems is that they are non intelligent and can have a tendancy to over correct or get confused with a frequency in the program vs a feed back frequency. So most pro setups use the FBE as a last resort, emergency protection AFTER they have properly run out the monitors. You will hear people talking about a systems GBF Gain before feedback, which really will determine how loud your system will actually be able to operate at on live mic'ed inputs, vs pre recorded.

    I suggest you ring out the system first, using a Graphic eq, and make sure you resent and clear all the pre set filter notches so that the FBE is going to work on exceptions on your system.

    When you ring out the system make sure you work mic by mic first in all the positions it is going to be used, and then check by bringing up and adding the additional mics to make sure you are still ok.

    A trick on some mics is if they are directional is to have the mic face DOWN on the person especially if you are not using monitors

    Sharyn
     
    TupeloTechie likes this.
  13. saxman0317

    saxman0317 Active Member

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    One little thing that ive been doing latly is i started place the moniters hanging intstead of on the floor. Ive found that this gives the mics alittle less to pick up on on the stage. Also, with our house we would have a heck of a time trying to keep the moniters from over powering the house because of the construction of the stage and wings, this seems to deaden it better. Another problem ive had with feedback recently is that soemone on stage was usuing a wireless quarter inch receiver and didnt mention it to me, as i scurried to find the "feedback" Happens the sets they had were the same as a few of my mics... I wasnt happy considering that i was already in a bad mood for haveing to rewire the house with new speakers and amps the night of the show...
     
  14. SHARYNF

    SHARYNF Well-Known Member

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    One trick if you have access to it is to put a TINY delay in the monitors, our ears will typically not pick it up, but it will drastically reduce feedback
    Sharyn
     
  15. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    Let's clarify that a bit...putting a delay in the monitors will in some cases drastically reduce feedback. In other cases, it can drasticallyincrease feedback, and in others it won't do much of anything at all. Is it worth trying? Sure. Is it a magic cure-all? Nope.

    All adding delay to the monitor is doing is electronically creating the equivalent of moving the speaker slightly further from the microphone that is (potentially) feeding back. It's basically just shifting the feedback frequency a tiny bit; with a mic that never moves (ie, on a stand), it might help move it to a frequency that either the mic is less sensitive to or the speaker is less responsive to, but it might not. And if it's a handheld mic, it's pretty pointless, since the mic will be constantly moving, anyway.
     
  16. howlingwolf487

    howlingwolf487 Active Member

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    Did it really sound like that? Because that kind of sounds like your transmitter frequency was getting steeped by something else (stupid local television stations!!!).

    I may be wrong, but the feedback I'm used to usually involves a howling sound...
     
  17. astrotechie

    astrotechie Member

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    I have had the same problem.

    Turn on a wireless mic, all of a sudden, KA - FOOOOOOOOOOOOM!!.

    Then everybody looks at me like i caused the feedback. The real answer was that somebody wanted to change the switch from Drama(only soundboard) to PA(soundboard and stage rack) [it is a high school auditorium] so that they can screw around with the tech team.
     
  18. SHARYNF

    SHARYNF Well-Known Member

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    This is were having the monitor system under control, and turned down helps a lot. Having a complete split so that the monitor system have a complete mixer that you can mute each input channel also helps a lot.

    Sharyn
     
  19. PhantomD

    PhantomD

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    I too wouldn't expect feedback to just come out like that out of the blue unless there was a seriously messed up PA system.

    That sounds like what happened when a tech last year couldn't get any volume out of the choir mics, and just kept upping the volume with no result. Then flipped on the phantom power without turning it all down!
     
  20. Stephen

    Stephen Member

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    Funny thing about that, after the show closed, months later, we concluded that it was brain waves interfearing with the radio waves of the wireless mic (or the TD's watch, we're not sure), because it would never happen when I was in the booth alone. (which got pretty lonley) :S

    And yes, the feedback was literally a loud ka-foom, almost as if it had to take a breath before the actual feedback occured, afterward, following with the most horrendous blowing noise you've ever heard. Now, even looking at that soundboard or listening to that number gives me the jitters.

    By the way, this week I'm stuck all rehersal in the booth running the CD player (our stupid rehersal pianist never shows up). Anyone have any idea on what I can do to kill time while sitting in the dark by myself?

    Edit: Also, another reason we thought of was that we were using a serrrriously big echo effect, as in the opening number, our "God" was speaking into a mic with the echo offstage, and there was another mic onstage, very close to the monitors, we even had to turn off the onstage ones and use the pit monitors because they were causing feedback as well.
     

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