You’ve all heard it, that squealing, irritating sound that rises in volume until everyone has to cover their ears and the speaker rolls their eyes. Everyone looks to the soundman in disgust and the entire mood of the event, from a wedding to a Sunday morning mass, is destroyed. Hopefully you haven’t blown out the tweeters. This is an example of problematic audio system feedback.
So… what is feedback exactly?
Feedback starts by a sound entering a microphone, traveling through the electronics and then to the loudspeakers. Since the system is designed to take a small sound and increase it to a loud sound, the sound emerges significantly louder from the speakers. Now that sound is out there bouncing around. If enough of it gets back into the microphone, then the process starts again, each time increasing a little bit until the squeal is heard. This is known as a feedback loop.
Each room and each sound system have certain frequencies that are accentuated. These frequencies tend to be the core frequencies that feedback on a regular basis. The amount of loudness a system can produce before the feedback begins is known as gain-before-feedback. Every system has its tipping point, but there are things that a savvy operator can do to prevent feedback from ruining the day (or at least the sermon).
How can you prevent feedback?
Properly tuning the room should always be the first thing that you do. This means finding a qualified consultant to determine which acoustical absorbers to use, how much to use, and where to place them. I can’t emphasize this enough in Houses of Worship, which tend to forego wall treatments in favor more electronics. There are many materials available which can be placed which will both complement the interior design of any facility and help curb the sound energy from bouncing around.
Purchasing the right microphone is the next thing. Good professional microphones will have a pickup pattern that is more in the front of the mic, called a cardiod or supercardiod pattern. They will also reject as much sound as possible from entering the rear of the mic element so that your monitors will not cause feedback. Additionally, the better the mic, the more even the frequency response. Uneven frequency response adds to the imbalanced resonating frequencies.
Next is not only choosing the right monitors, but more importantly placing them correctly. Try to avoid speakers with a Piezo electric tweeter as these are harsh and uneven in their frequency response. Then place these speakers so that your speakers will be about 60 degrees off axis from the microphone. This tends to be the place where microphones pick up the least.
If you’ve done these previous three things, then the next place to look is at your electronics. Usually on most mixers there is the High Pass (or Low Cut, this is the same thing) filter. Engage this switch on all the vocals. This will at least remove the low end rumble and possibility of low frequency feedback.
Parametric Equalizers are your friend with regard to feedback. If you don’t have one as a piece of outboard gear, then you should have at least one in the EQ section of your mixer channel strip. There will be at least two knobs, the frequency selector and the gain cut/boost. The next paragraph explains how to use them to your advantage.
How to make feedback your friend.
The process of using feedback in a positive manner is known as “ringing out the system”. You force the system into a feedback mode, and then notch, or cut, those offending frequencies. This should be done in the room before anyone comes in since it can be a painful process to a listener. The procedure is as follows.
Set everything up on the stage. Turn the input gain on the channel strip of the mixer down as low as it will go. Bring the mains up to the 0 mark, and bring the channel fader up to 0 as well. The EQ section should be flat, with the High Pass/Low Cut switch engaged. Slowly bring up the gain knob until the system begins to feedback, and then bring it back down just until the feedback is gone. Look to the parametric EQ section of that channel. Turn the parametric gain knob up about three to six db. Sweep the parametric frequency selector until you find the feedback frequency. Take the parametric gain knob and turn it to three to six db below 0. If there are more parametrics in your channel strip, do the same thing for those frequencies. You should now be able to get three to six more db of gain out of your microphone, which is about twice as loud!
Feedback Destroyers are useful for systems that have no dedicated operator. Usually, they find the first feedback frequencies and cut them, and then there will be other frequencies that are searching for the next frequencies that pop up. Their use should be limited however, since these roving filters cause a sweeping and undesirable sound as they are doing their job.
Feedback is inherent in every sound system, but knowing what it is and how to work with it can help you get the maximum performance out of your equipment. If you need help figuring this all out, please contact Proaudionet Consulting by visiting our contact page or send us an email! We'd love to hear from you.
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