Anyone who uses a sound system will encounter feedback troubles at some point.
Many times I've watched people trying to figure out which microphone is "feeding back", and while the relative positioning of a mic to a monitor or speaker can sometimes cause one particular mic to be the culprit; more often than not it is a cumulative effect. To put it technically, the incidence of feedback is exponentially related to the number of open microphones.
For example:
1 mic turned on = 1 chance of feedback
2 mics turned on = 2 chances of feedback
3 mics turned on = 4 chances of feedback
4 mics turned on = 8 chances of feedback
5 mics turned on = 16 chances of feedback
6 mics turned on = 32 chances of feedback
You get the picture. Therefore, in many cases the quickest way to eliminate feedback is to turn off any unused microphones (or just turn the volume off at the mixer) If you've ever listened to feedback, you've probably noticed that feedback occurs at different pitches. Sometimes it's a low-pitched howl or rumble, other times it might be a high-pitched squeal, or a mid-range howl. The point is that they are pitches!
If we think of the audio spectrum as if it were a piano, then we have somewhere around 88-90 pitches (or notes, or tones) that we can hear; with the lowest ones on the left, and the highest ones on the right. Now if you've ever looked at a graphic equalizer, you'll see it's laid out the same way. Lowest pitches to the left, highest to the right. The best graphic equalizer for controlling feedback is a 31-band EQ. Why? Because each slider controls only 3 notes or pitches (divide the 90 tones by 31 bands).
So here is the puzzle for the sound engineer: When you hear a feedback, which pitch is it? Which one of those sliders on your 31-band graphic EQ can you pull down to get rid of the feedback?
Well, here's a little process us old-timers call "ringing out the room". First, setup all your mics and turn them on to the approximate volume setting you'll be using. I usually do this first with only the monitors on, since they are the most likely to cause feedback. Start with all your equalizer's sliders at zero in the middle. Then, using the master monitor fader, gradually increase the volume until you begin to hear a feedback. Memorize that pitch and bring the master fader down only enough to stop the feedback and no more!
Now, starting with the lowest slider on your EQ, raise it and see if you hear that same pitch. If you don't, or if you hear a different feedback pitch, put it back to zero and go on to the next one. Continue this process, moving to the right across the EQ sliders until you are able to produce the exact same feedback pitch using one of the EQ's sliders. Once you find the correct pitch, pull that slider down a bit on the EQ.
Now you should be able to push up the master monitor fader a bit farther before encountering feedback.
Repeat this entire process 3-4 times until you can achieve a good bit more volume than you will ever need for your performance. Then, turn the monitors off and follow the same procedure with the mains, using your main graphic EQ.
You have just "rung the room out"! You should now be able to perform without that nagging feedback driving you crazy. Of course, if you're outside, and the temperature takes a dive in the evening you'll have to do it all over again because cool air is denser than warm air and will cause feedback issues more quickly. All part of the game. Just be prepared for it, and your evening will be much more enjoyable!
Some companies now produce equalizers with LED's that will tell you which pitch is causing a feedback. They're a little more expensive, but it will save you time in trying to find an aggravating feedback pitch.
Many happy songs to you!
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