Fighting Back Feedback

Feedback loop block diagram.

One of the most common problems with a lot of young musicians beginning their progression as live stage performers is their ability to reduce mic feedback coming from the PA – or should I say their inability to combat feedback. Yes, we are all know that signature screech that comes from an improperly set mic and PA system but not too many know why it happens; let alone how to avoid it. Today, we here at the PAL blog would like to take the time to not only explain the causes of that annoying feedback but also plenty of tips on how to avoid it. It might be a bit confusing for those out there who don’t really know their way around a PA and mic system but trust me – if you’re going to be performing live without a sound engineer, you’re going to have to learn this sometime in order to get the best sound possible from your system. Let’s get started.



What is Feedback and Where Does it Come From?

First of all, there are a few different types of feedback in the music world such as that annoying hum that single coil pickups tend to commonly suffer from which is caused by electric-magnetic interferences coming from anything with a transformer (such as lighting systems, a computer monitor, a radio, etc.). But the kind of feedback we’re talking about today occurs when microphones pick up amplified sound from the loudspeakers.  Sound from the speakers enters the mics, is re-amplified, and goes around in a feedback loop. Almost instantly, the sound builds up until a loud tone occurs -- usually at a single frequency. It really is as simple as that, and although pretty much everyone who has experienced mic feedback can deduce that it gets worse the louder the PA is set, few realize that last bit about the single frequency which is important to understand if you want to be able to tackle any feedback situation which comes your way. Anyways, let’s get on to the solutions; we will start with the quick fixes first and then get into the nitty-gritty.


Messing with Settings

The first thing you should do before any show to prevent mic feedback is turn up each mixer fader to the point where that annoying screech starts to come in (it should sound more like a slight ringing tone). Mark that point on each fader and do not exceed them during the performance. If you still happen to hear feedback during the show, turn down the mixer’s master faders a little until the feedback stops. If you happen to know which mic is causing the feedback, turn down its fader slightly. And better yet, cut EQ at the frequency which is feeding back (this might be a bit tougher for newer musicians but a little trial and error will go a long way). Another way to prevent feedback is to use as few mics as possible. True, depending on how many singers a band has they will need a certain amount of mics but the fact remains that the more you have in use, the higher the chance of feedback. Even if your entire performance calls for – let’s say – four mics at one point or another, always turn down mics when not in use; this noticeably increases clarity as well as reducing the potential for feedback.

Before we go on, we should explain “gain before feedback” which is the amount of amplification a sound system can handle before it starts to feedback. A sound system with high “gain before feedback” can amplify a mic’s signal quite a bit before if begins create feedback which for you means pretty loud vocals. Conversely, a system with low “gain before feedback” needs to be kept relatively quiet because the faders can’t be pushed too far without causing feedback. The “gain before feedback” decreases 3 dB each time the number of open mics doubles.  Two mics have 3 dB less gain-before-feedback than one mic, four mics have 3 dB less gain than two mics, and so on. To reduce the number of open mics simply turn off or mute any mics not in use at the moment.


Messing with Placement

But with that said, there are assuredly plenty of you out there with bands that require two or more simultaneous singers (for those lush harmonies) so turning off mics might not be the best possible solution. If that sounds like it might be the case for you, another way to combat feedback without having to switch off mics is by placing the speakers far apart from them. This weakens the sound traveling from speaker to mic so in turn it diminishes the feedback loop. Don’t forget to keep the speakers directed towards the audience.  And speaking of speaker placement, if possible, try placing them in front of the mics, or rather, in front of you. We want the mics to reject the speakers so try to place the house loudspeakers toward the audience and away from the stage, that way the mics don’t pick up the sound as well.  

Also, if the venue’s space allows, you can also always try placing the speakers closer to the audience. Close speakers sound louder than distant speakers (I know, shocking!) so this increases “gain before feedback.” Not only will this reduce the chances of feedback but with close-up speakers, the audience hears a lot of clear direct sound, straight from the speakers. With distant speakers, the audience hears a lot of muddy-sounding room reflections. Close speakers sound more intelligible than distant speakers. A fair warning though – the closer the speakers are to the audience, the farther they are from you, meaning you’re going to have to feel the dynamics of how loud or soft your singing. While veteran players are able to feel out exactly how loud their voice is being projected, even if they can’t hear it themselves (mainly through muscle memory after loads of practice), newer players seem to commonly suffer from the headphone effect (we naturally tend to control the pitch of our voice by the how loud we can hear it which is why people who are hard of hearing – or wearing headphones – tend to yell things like a crazy person). As you can imagine, this can easily cause the vocals to either sound confusingly soft or annoyingly loud. A common method employed by most professionals is the use of personal floor speakers pointed towards the singer to help them figure out the correct loudness they must project. And yet another way is by using in-ear monitors which is just a fancy way of saying wireless headphones to hear yourself. Alright, back to feedback.


EQ to the Rescue

A handy tool for removing frequencies that are causing feedback is a graphic equalizer. The equalizer has a row of controls that affect the level or loudness of various frequency bands from low to high. Most mixers should already have a form of this but a dedicated graphic equalizer comes with plenty of more control. You connect this device between the mixer output and the power-amp input. Basically, you find the frequencies that are feeding back, and turn them down on the equalizer. Automatic feedback suppressors, such as made by Sabine and Shure, will do this for you. They quickly sense feedback and determine its frequency. Then they assign a narrow notch filter at the same frequency, which eliminates the feedback. Several filters are assigned for different feedback frequencies. A word of warning: some budget feedback suppressors use wideband notch filters which affect the tone quality. Also, since they cut out a wide band of frequencies, they can actually reduce the gain-before-feedback. If you want to use a graphic equalizer, there are a few things you should keep in mind when putting them to use:

Set all the graphic-equalizer controls to their center position (“flat”). The rows of volume controls toward the left of the equalizer affect low frequencies; those on the right affect high frequencies. If you push a control up (apply boost), the level or volume increases at a particular frequency. If you push a control down (apply cut), the volume decreases at that frequency. Now slowly turn up the mixer’s master faders to bring up the volume in the PA speakers. The system will start to feed back, sounding like a musical note or tone. Try to find this note on the equalizer by cutting (pushing down) each control in turn. The control knob that stops the feedback is the correct one. Lower this control just to the point where the feedback stops. Then turn up the mixer’s master faders until the system feeds back again (usually at a different frequency). Lower the control for that frequency until feedback stops. Repeat this procedure several times, turning up the overall volume as feedback is suppressed, so that three-to-five frequency ranges are cut. You should be able to play the house speakers louder without feedback than you did before equalization.


And finally, if all else fails or you simply want a very quick fix – you can always reduce feedback by lowering competing sounds from the guitar, bass or what have you. Sure, this tip technically doesn’t reduce the cause of feedback but it should let you turn up the mics much louder than you had before, and isn’t that what you probably wanted in the first place? Anyways, have a great weekend and good luck fighting that feedback!

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