SOUND SYSTEMS & PRO AUDIO
ACOUSTIC FEEDBACK can be caused intentionally—but for most of us, feedback is usually the undesirable result of turning up the monitor feed too loud for Sarah the Singer who's getting drowned out by Buddy the Bass Blaster. At the onset of feedback, Peavey's patented Feedback Locating System® (FLS®), indicates which slider on the graphic equalizer you need to adjust to make the dreaded brain darts go away.
Well, that's HOW it works, but WHY does it work?
Feedback is caused by a singer's microphone picking up sounds from the monitor, or at times the main loudspeaker system. If we analyze what's going on logically, we'd expect feedback to sound like sounds that are already playing, just more of them. Feedback does follow the rules of logic, but also those of physics. Yes, feedback starts to amplify all sounds, but the sound frequencies that are accentuated (boosted) by the system will be amplified much more. After hundreds of loops occurring in fractions of a second, feedback is a single, very angry note.
If you've ever studied the frequency response of a speaker or a microphone, or better yet a speaker's response added to a microphone's response, you could see how a few frequencies would dominate feedback. About the only straight lines on those plots are the edges of the paper (just kidding transducer engineering). The fact is, electro-mechanical transducers are the hardest to get ruler flat. Maybe that's why we have big speakers for the low notes and small speakers for the high notes—but I digress.
If we use an equalizer to smooth out the frequency response peaks so that the combined response of the microphone, loudspeaker, and room (yes, the room makes a difference) is as flat as possible, you will be able to get the maximum monitor level before feedback. Feedback can never be completely eliminated. When pushed hard enough, even a perfectly equalized system will break into an unearthly growl with multiple frequencies attempting to feedback. Changing where you stand, even by a few inches, can cause huge differences in the sound of this feedback. The good news is, with proper equalization, you can run your monitors 6 dB (or more) louder than an unequalized system. 6 dB is a lot, just ask Sarah.
Now you know more than you need to know to fix feedback. With an FLS-equipped equalizer in your monitor signal chain, all you have to do is turn up the gain until you hear feedback and pull down the slider indicated by the glowing LED. Pull it down just enough so another frequency tries to feedback, and then suppress this new feedback mode. This is called "ringing out" the room. You may get to the point where one of your first feedback modes starts up again, in that case, just cut it a little more. As you approach the limits of what you can get, you'll hear a growly multi-frequency feedback. By this time, you've got the monitors so loud that they're parting Sarah's hair, so set your stage levels for rock and relax and enjoy the show, knowing you've got plenty of headroom to turn it up later if you need to.
Another good use for FLS
Do you remember when you were just starting out and thought bass was a fish and treble was a kind of hook for catching it? Well, wouldn't it have been nice if the graphic equalizers back then came with little lights above the frequency bands to tell you where that sound was. When there's no feedback to report, FLS indicates the frequency band of the loudest musical sound. If you want more or less of the sound you just heard, look at which LED it illuminated and simply boost or cut. It's no spectrum analyzer, but it's a lot faster and easier to use—not to mention, it's practically free when you consider it comes with a graphic equalizer attached.
Don't be dismayed if you spent years, like many of us did, memorizing frequency sounds. FLS is like a pocket calculator—even though we can do the math with paper and pencil, why bother? Who can see those little numbers in the dark anyway?
