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Feedback Eliminators - Eliminating more than just feedback?

Blake Engel, All Church Sound

Do you have problems with feedback? Well, have I got a solution for you! Yes, it's called a feedback eliminator. This amazing black box contains computer chips which listen to the sound and when they hear feedback, they stop it in its tracks! Everytime there's a ringing in your system a very narrow audio notch filter is set to eliminate that frequency of feedback. Nothing more, and nothing less. Use of this device will give you an amazing 6 to 10 decibels more gain from your system!

For some of you, this sounds all too familiar. A magic black box which will solve all of your feedback problems forever! Well, let's get to the point right away--there's no device that will eliminate feedback completely. There's really nothing special about a feedback eliminator besides the fact that it tracks the audio and makes automatic changes. Any well trained audio engineer with the proper test equipment (ears and maybe a computer or two) and a quality parametric equalizer can do the same thing as a feedback eliminator.

OK, before we get too far into this, what exactly is feedback, what's a feedback eliminator, and why can people claim they're so great? First, feedback is a ringing sound typically associated with the condition when sound coming out of a speaker re-enters a microphone and is re-amplified over and over many times. This ringing can be barely noticable or can be deafening. It can occur at any frequency (pitch) of sound. Every room has specific frequencies it resonates at before other frequencies--these are typically the first frequencies at which feedback will occur. A feedback eliminator is an electronic device that uses tight parametric filters which allow the sound engineer to turn down specific frequencies such that the nearby frequencies are affected very little. For example, a typical 31-band graphic equalizer has a filter every 1/3 octave. If there’s feedback at 4,327Hz, the closest filters are at 4,000Hz and 5,000Hz. Turning down these filters enough to affect the 4,327Hz feedback location will result in a large loss of the surrounding frequencies. On the other hand, a parametric equalizer allows the engineer to select the specific frequency and vary the width of the filter often up to 1/40 of an octave wide! That's so narrow, the filter can be used to help reduce the feedback at that frequency and yet leave the nearby frequencies unaffected.

Most feedback eliminators have some sort of computer chips in them which track the audio and watch for feedback. When feedback is sensed, a filter is automatically inserted at the proper frequency and is adjusted to the right amount to help eliminate it. This is part of the reason many people claim feedback eliminators are so great--they do all of the work for you, automatically! You don't need to hire an audio expert to properly adjust this gem, it does everything by itself. Oh, did I mention how fast these units work? Once feedback begins, it's usually only a second or two before a filter is placed--that's quite a bit faster than any live person can make the adjustments needed. This means the unit isn't limited to being set before a program or service, but it can be used during the program or service! A feedback eliminator which has been set with dynamic (changing) feedback filters will operate in the background during an event and will help eliminate any chance of feedback you may encounter.

So far, this all sounds great, right? Let's all go out and buy a bunch of these devices! Hang on a minute--how does a feedback eliminator know the difference between feedback and a musical or sung note that's held? Is the computer chip so smart it knows the difference between feedback and a key held down on an organ or someone whistling into a microphone? I hate to break it to you, but no, the computer chip doesn't know the difference. (Many of the newer feedback eliminators do a better job of deciding if the sound is feedback or not than the early units.)

Just as a sound operator can think he or she hears feedback (when it's actually just the organ, a specific voice on the keyboard, or someone's hearing aid squealing), a feedback eliminator can be fooled easily. Thus, if a song contains notes in it which are held for several beats, they may be notched out! If you're doing drama and someone whistles, the feedback eliminator can be confused and will notch out those frequencies as they occur. I once heard of a school who complained that their sound system never had enough volume. Well, after many visits the sound contractor found out that before each use of the system in the gym, the school fight song would be played on a portable tape player into the sound system. The feedback eliminator thought all those tones were feedback, and notched them out as best it could! The result was a system where many frequencies were turned down way too far and thus the overall gain was next to nothing.

There's no argument that feedback eliminators are a great tool to quickly find several offending frequencies and control them. The problem is mainly with the dynamic filters. To avoid your feedback eliminator from taking out too much, it should be allowed to find the first five or six offending frequencies and then lock all of the filters so they can't change on their own. Another solution is to only use the feedback eliminator on individual microphones or on groups of similar microphones (none of which are used for music).

If you'll be using a feedback eliminator in a situation where the filters will be set and left, a better solution would be to use a quality parametric equalizer.

On the other hand, the need for a feedback eliminator or parametric equalizer with many deep notches in the audio spectrum is a sign of other problems. Acoustical problems and system design or installation problems can produce problems with feedback (more than there should be).

Before you go out and buy a feedback eliminator thinking it will solve all of your problems, keep these things in mind. They may do more harm than good, you may be better off with a different product, or maybe you should have the acoustics of your room or your existing sound system checked out for proper design and installation.