Friday, August 31, 2012

Dear SNR: Fighting Feedback

We just got another email from our friend Michael. You may remember him from our podcast about acoustic guitars in tight places. It seems he's having some feedback troubles and would like us to weigh in on it. We'll likely tackle this in an upcoming podcast as well but I wanted to put a few ideas out there in print and see what our readers will put up in the comment section.

Michael mentioned feedback fighting devices at the end of his email and that he had disastrous results the one time he tried one out. Personally I hate them. Never trust a machine to do the thinking when art is the question at hand. When feedback fighters do work correctly they can then go on to take out things you'd rather have left in like flutes or guitar solos. Auto EQ can be a useful tool but more for learning than actually doing.

I'm going to start down on the stage because while you're micing up can be the best time to sort out potential monitor issues.  If you're using multiple wedges it's best to use one for music and one for a "me" wedge.  That lets you put more level through each one while the sound remains clear. If it's multiple wedges with the same signal are to be used then it's best to push them as close together as you can and treat them as a single "wedge array".

The reason for monitor positioning is that if they're producing the same signal and you splay them out (as people see done in concert videos) then you create comb filtering that can leave a singer in a hollow spot or with terrible feedback (or both). If the two boxes are as close as you can get them then they start to act more like a single enclosure.

As far as positioning wedges, a "band" wedge can go just about anywhere it's convenient to place it. A wedge for a vocal needs more careful placement. With a garden variety cardioid mic the best place for a wedge is right behind it. That's because directly off axis is where the best rejection is, the mic picks up the least. With that knowledge you at least have a starting point for where that wedge is most likely to end up working well.
Now that super and hypercardioid condensers are becoming more affordable though there's a trick to them that some people don't know and it comes around to bite them, making them feel that it's not possible to use them on smaller stages. It's not that they pick up too well in front, but that there is a spot directly behind them where rejection is only somewhat reduced, a second lobe visible on the polar patterns in the spec sheets.  Moving a wedge off to the side, somewhere between 160 and 120 degrees off center axis will put it at the point of best rejection.

Moving on, I expect you're waiting to hear about EQ. There's a lot that can be said so I'll try to keep it brief. An RTA can be a help, but that fact that you need four times the resolution of the EQ that you're going to use to really have it be effective is a bit of a sticking point. You need a 1/12 octave analyzer if you have a 1/3 octave EQ. That's so you can tell if feedback is occurring right on the center of a filter or slightly off to one side, maybe even split right in between two filters. Don't give up if you can't afford SMAART or don't want to bring a laptop to a gig at all. The RTAs available for smart phones these days can still help you out in a pinch. 
It's also very helpful to know the EQ you're using. With a traditional graph, say that 1/3 octave, the filter is only that wide when you're at full cut (or boost, but don't boost). Anywhere in between the filter is much wider. If you have a "constant Q" graph then then filters are much more consistent. One is better suited for shaping and the other for surgically removing feedback.

That said I like to get down on stage and take a listen. I'll try to hear if there's any sort of resonance from the room that's building up on stage and then turn on the wedges and start to EQ. There can be a lot of low end floating around at knee level on a stage so I'll usually high pass pretty severely when I start. Then I go after offending frequencies on vocal and guitar mics. The last thing I'll do after I get to the maximum volume I can obtain is go back and "warm things up a bit" in the lows and low mids so things don't sound cold and unnatural to the performers. 

There's so much more to be said and so many more ideas to try that I'll probably revisit this topic after next week's podcast. A couple things that come to mind are trying different sized and positioned monitors. Would a little hot spot on a stand do a better job than a 15" floor wedge? Could a performer sitting on a bar stool playing guitar get away with a headphone amp clamped to the mic stand? A lot of performers carry their own small splits and take care of their own monitors via in ear monitors (IEMs). The possibilities are nearly endless and we'll keep tackling the issue as long as the Brethren of the Knob and Fader keep asking.

No comments:

Post a Comment

You're the Scotty to our Kirk