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Avoiding Microphone Feedback

Discussion in 'Miscellaneous [BG]' started by wulf, Jun 17, 2003.


  1. wulf

    wulf

    Apr 11, 2002
    Oxford, UK
    Another band practise coming up tonight and I want to hear the vocals nice and clear. However, we often find that we can only push the vocals up so far before getting unwanted feedback via the PA.

    The room we use is about 7m x 4m (maybe a little bit larger). The arrangement tends to be:

    Bass       Drums     Guitar

    horns                  keys

    speaker vocal vocal speaker


    Bass and guitar go through individual amps, drums and horns are unamplified and vocals and keys go through the PA. The speakers are towards the corners and pointing more or less straight into the room. Every one is looking into the centre of the room (so the mics are pointing towards the vocalists and then the wall)

    I attempt to keep the overall volume of the instruments low and sometimes adjust the EQ of the higher frequencies down a bit on the vocals, both of which help a bit. However, does anyone have any other positioning suggestions that might help us get a stronger vocal signal without feedback?

    Wulf
     
  2. fastplant

    fastplant

    Sep 26, 2002
    Connecticut
    You should be able to EQ the feedback out of the vocals. Make sure the speakers are far enough away from the mic and not facing it. Just keep playing with the EQ and you should be fine. There are a ton of factors that can lead to feedback but EQing it should eliminate most of it, good luck.
     
  3. wulf

    wulf

    Apr 11, 2002
    Oxford, UK
    Okay - I'll play around with the EQ a bit more tonight. Any particular frequences you'd target?

    I was wondering if there was anything around mic positioning that might help? Although the walls and floor are carpeted, which will absorb some of the sound, I'd imagine there's still a lot bouncing around and I wondered if there might be some trick so that the mics and speakers could be positioned so that the mics are in a relatively quiet zone, not the key point where all the sound waves bounce together and combine!

    Wulf
     
  4. Tsal

    Tsal

    Jan 28, 2000
    Finland, EU
    Also, there are feedback destroyers available. Behringer Shark comes to mind, it's a small box with mic pre/feedback destroyer/something-and-other combined and costs around 100€.

    I've been craving for one since the place we rehearse in has severe feedback problems with the room and the vocals setup.
     
  5. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Hi Wulf,

    What about monitors? They're a common source of feedback if you don't have them set correctly in relation to the mics.

    Make sure you use directional mics for vocals and instruments. Cardioid mics are good for miking the backline, but for vocals and frontline instruments, hyper- or super-cardioid is usually better because the null isn't 180 degrees off-axis, but more like 120 to 140. That helps put the null more in line with the monitor wedges.

    Make sure the main loudspeakers are downstage of the band. Make sure that no mics point toward them.

    It's often helpful when playing live to cut the 200 to 250 Hz region somewhat in your EQ to help counteract the natural boost you get in those frequencies from floor bounce.
     
  6. wulf

    wulf

    Apr 11, 2002
    Oxford, UK
    Tsal,

    I'm looking for a free solution rather than a gear solution - not only because of the cost, but also because I don't want to be lugging more stuff to each rehearsal. However, maybe I'll ask the owner of the rehearsal studio if he's got an tips - if he recognises the problem, perhaps he might want to experiment with a feedback destroyer to prevent feedback destroying the rest of his PA setup.

    Bob,

    No monitors - it's a small rehearsal studio. I know enough to avoid having the mics pointing at the speakers, but I'm wondering if the feedback is from sound that's hitting the far wall, bouncing back to hit the wall behind the vocalists and then bouncing back into their mics. That's why I was wondering if a bit of positioning would get the mics into a spot where the sound doesn't reach or is cancelled out.

    Is the 200-250Hz range something to cut just for bass or does it affect all the signals?

    Wulf
     
  7. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Hi Wulf,

    Okay, I wasn't aware it was a rehearsal studio. You said the loudspeakers are pointed toward the middle of the room. Where is the band in relation to the middle of the room?

    The 200-250 Hz cut should be in the system EQ.
     
  8. Eric Cioe

    Eric Cioe

    Jun 4, 2001
    Missoula, MT
    I find that in my basement, there tends to be some high end feedback. I cut out 4K and 8K a bit and it works like a charm.
     
  9. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    As a sometimes sound guy, I can tell you there are 1000 different reasons why you're getting feedback. I'm not going to go into the reasons because the answer is always the same - eliminate the offending frequencies. Best way to do that is with EQ. There isn't a professional PA out there that doesn't have a 15 or 31 band eq on each foldback send that's primary job is to cut problem frequencies.

    As for lugging gear, that Behringer shark device is tiny. I mean really tiny. From memory you need 8 of them to fill a single rack space. A nice 31 band EQ is only 1 rack space. Both weigh next to nothing.

    The problem here is that these devices must be fed a signal that's before the amplifier - and some rehearsal studios use those powered mixers. so there's nowhere practical to insert them into the chain where thay can catch all the mics.
     
  10. We added the Peavey Feedback Ferrett to our system a couple months ago. Absolutely no feedback from our monitors anymore. We can turn the monitors wide open now if we want to. We have the one for monitors and mains but so far we haven't needed to patch the mains in. I would have to say this is the hardest working piece of equipment you can put in your system. It reads the frequency that is feeding back and adjusts it without changing the way your monitors or mains sound.
     
  11. wulf

    wulf

    Apr 11, 2002
    Oxford, UK
    Like the diagram in my initial posting. The bass, drums and guitar are along one long wall. On the opposite wall you have a speaker in each corner (pointing almost straight across - slightly angled towards the centre point) with the vocalists in the middle. Horns and keys fit at either end, and everybody is looking roughly towards the centre.

    Yep - powered mixer, so I don't think one of these devices would apply.

    However, with a bit of EQ fiddling, we managed to get a decent sounding and reasonably loud vocal signal without feedback - that included a dip at 250Hz and again on the 4KHz and 8KHz bands. Therefore, problem solved - although I'd still be interested if there are any insights around positioning.

    Wulf
     
  12. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Your positioning looks fine. It's pretty much how I would recommend it.

    Here's a little EQ tip. Sometimes when you get problems in octaves as you did with 4k and 8k, it can be the octave one lower sending them off. So even thought 2k itself isn't feeding, it could be triggering the feedback at 4 and 8k. This doesn't always work but try cutting a bit of 2k and see if it helps settle 4 and 8. If so, you may even be able to put some 4 and 8 back in.
     
  13. wulf

    wulf

    Apr 11, 2002
    Oxford, UK
    Okay - that's another thing to play around with next week. All these tips are valuable.

    Cheers,

    Wulf
     
  14. enzyme

    enzyme

    Feb 4, 2003
    All you need is a graphic eq. To tune the PA to the room:
    Reset the graphic so the faders are back to zero.
    Take the main vocal mic and point it directly at the speaker where you would be standing when you are singing.
    Goto the mixer and turn up the fader until the pa starts to take off.
    This is the tricky part.
    Find the frequency ton the graphic and pull it down till it stops feeding back.
    Keep turning it up and taking out frequencies until you get the volume you want.
    Simple.