Most
feedback problems are the result of a improper
gain structure (in otherwords--too high a
gain or improper
level setting) and also for poor or non-ideal placement of microphones in relation to the speakers.
EQ adjustments can only go so far in fixing some problems like this if the rest of things are not set properly. If you are placing your mic's in front of your speakers then start there--and if that is not possible to move them or move the speakers, and they are in front of yoru Subs only--a High Pass Filter (or low cut adjustment) would be your first course of action. What that will do is basically eliminate the low frequencies that are passed
thru those mics--if they are chorus mics or area mic's for a
stage or orchestra--you don't need the low end anyway. So if your
console has a HPF (high pass filter) set it to around 200-250hz for starters if its adjustable--if not just turn it on as a fixed setting may be around 80 hz. This control, if you have one, is usually labeled HPF or LOW CUT, and is next to or near the
GAIN/INPUT knob or next to your EQ knobs on each
channel. Go ahead and start with it ON as listed above...
Second would be to re-adjust your
gain structure on those mics and in your
system... For mics--you may have one mic
gain that is wide open and the rest may not be, so you need to make sure they are all similar to start--the idea being that to start your
gain structure you want the mic's to work collectively and equally, and not have one mic try to do the work of 3 or 4. The
GAIN is simply how 'sensitive' your
microphone is to what is around it and how much of that sound you allow in to be mixed. A Mic is just an electronic "ear" and it picks up everything and it is often made to
pick up specific or a range of frequencies depending on its type and application--but what you let come in to your
system to be mixed is what you adjust with your
GAIN knob and other controls. The higher the gain--the more sensitive the mic picks up and more the room noise and frequencies you let in--and the more prone to
feedback you will be. In an example (depending on your equipment), at a low
gain your mic may able to
pick up well only sounds that are within say a
foot or two..if you turn that
gain up to around 12 or 2-o-clock, your
gain may
pick up sounds very well that are 5 feet away--anything closer will be really really loud and strong and anything further will sound distant...and you will also notice a definite "air" about the sound as it picks up more and more. When setting your gains you want to listen but you also want to look at your
channel indicators--for
level. You do not want to be in the RED or else you will overload the signal you let in, and you will get "mush" and
distortion. Again without hearing or seeing your set up--its difficult to describe. After you get the mics' balanced for
gain inputs, you can adjust for mix levels later. Your
GAIN control is usually the very FIRST knob at the top of the channel--says
GAIN or INPUT....start with them all the way down and experiment from there with adjusting them up so they all balance out. Another thing you could try is to lower your gains on the mic's closest to the speakers and raise it more on the ones furthest away from the speakers.. There are many things you can try--without seeing or hearing your situation all this advice is just starting off points...
Another thing to consider and check while you do your
gain structure set on the mics--you also have
gain structure on the rest of your
system to consider..again you want to ensure that everything is balanced. The way to start is setting a
gain on a mic is to turn all your gains DOWN. Bring your MASTER
fader up to nominal -0db- or close to it, bring your
channel fader up to or close to nominal, and then open your
channel gain until you get
feedback on that mic--once you do back off your
gain to lose the ringing. Do the same with each
channel or mic. As you add more mics, you will need to back the gains down as you are collectively increasing things in your mix. After you get a couple of mics--you should have a general
point for your gains to be set at.. After you get this
gain structure started--then consider your EQ's... Again--this is a very rudementary starting
point and explanation...there is much more to this and any other sound guy will tell you of another dozen things to check too.. This is to give you starting points--not try to teach you sound....
Make sure ANY EQ that you have on that
channel or in your
system is not boosted to start with. For
CHANNEL EQ &
system EQ--start FLAT--no boost or cut, and do NOT boost any frequency to start....this could be another factor to check in those mics and in your set up. Occasionally someone will turn the BOOST/CUT to a full BOOST thinking they are making things sound better or are adjusting a frequency and they grab the wrong knob--seen it a billion times in schools...and suddenly that
channel is boosted at 2k or 800hz or whatever and no one knows why they are suddenly getting
feedback or why things sound like
butt. So flatten out any EQ you have to begin with on all your channels. Once you ensure your EQ's are flat--finish setting yoru
gain structure as above to where you have sensitivity but not
feedback.
You may wish to
purchase the
Yamaha Sound Reinforcement Handbook which is a very helpful guide. It gets a
bit technical for non technical folks and beginners--but its topics and sound related suggestions is thorough enough to explain how things work in great detail so you can get a much better grasp on things... You may find it helpful.
I hope this helps get you started...I'm sure others will have more to add which you can try.
-w