18 Ways to Reduce Feedback in Live Sound Engineering
If you've ever engineered a band in a live situation or heard an engineer and feedback has ensued, you will know the horror. It's awful. There seems like a disturbance in the peformance's flow, audience members may cover their ears, people look for someone to blame - it really is the last thing you want. Here are a few helpful tips for setup and operation to help reduce the chance.
1) Mic positioning - near to source, bearing in mind pickup patterns
The below graphic helps to demonstrate the pickup patterns of microphones in relation to foldbacks. If you position a monitor directly behind the microphone (where the pickup pattern is least for a cardioid pattern), then there is less chance that the signal will be fed back into the microphone as it is less sensitive from the rear of the mic. Look at the depth of the red line into the cardioid pickup pattern (denoted by the black line). There is more sensitivity to the side foldback than that of the rear foldback. Granted, you won't always be able to position a foldback directly behind but bear in mind the pattern and this will help you visually understand the concept.
Furthermore, if you can get the microphone as close as humanly possible to the source then you won't need to set the gain quite as high. This is because closer to the source is a higher SPL level (basically, volume output). You can use an SPL meter (downloadable for free for iPhone) to see this visually. This is why mic technique is such a crucial thing: the closer you are to the source (in a singer's case, the mouth) the hotter the input signal and less gain needed thus less feedback chance.
2) Check high levels
A good idea is to make a mental note of troublesome mics or channels. Say you notice the BV or main singer's mic is a bit near the teetering point of feedback, be very careful when pushing it higher. Some channels you may have no issue with pushing higher, eg kick drum mic etc.
Another point on this is to ensure that no attenuation is taking place on the way from the signal source to the desk - say a dodgy lead, or a knocked out of position mic or -40dB padded DI. Recommendation: read the gain setting or soundchecking articles!
3) Make sure you know the "Point of Feedback"
Once you have SET your gain (ie, you don't touch it once it's set!) you can then ride the fader louder and louder until you spot the point at which it begins to feedback. Either make a mental note or if you must, draw a little line on some PVC tape next to the channel on sliders you'll be adjusting a lot to say don't push it past this point!
Sometimes when you're in the middle of a song you can sneak past this point slightly and get some more headroom without it feeding but then when the band dies down the feedback springs in. Test this in soundcheck, but I've found this can be useful to know and keep riding the sliders.
4) Harness the power of your EQ
You could get a feedback destroyer and put that in the signal chain to try and do this for you - essentially it automates this process by monitoring frequency levels and spikes and jumps on them before they feed too much. In some ways this feels a bit like 'cheating' as you're not learning the frequencies and how to control the sound...and to be honest, they don't always work well, or at all. They can add more noise into the system or make it sound weird.
Learn your EQ. Essentially you're learning to identify frequencies, sweep on your semi-parametric EQ or notch out on your graphic EQ. Sometimes you can set your gain quite high and then use the EQ to cut a certain troublesome high frequency instead of bringing the gain down - so you can have a channel that bit louder. Be careful you don't tweak the EQ after this though. Particularly useful for bad pickup mics eg tie mics under or ahead of speakers.
5) Watch your foldback mixes & levels and consider IEMs
Too much of a lot of instruments in many foldback mixes not only creates a lot of outputs of a mic source all over the place which is difficult to manage but increases stage volume. This means that there's a higher chance of feedback. Try and keep monitor levels as low as possible, and once a musician is happy you can usually get away with turning the volume down a little bit more when they're not looking ;). Furthermore consider In Ear Monitors (IEMs, head/earphones basically) - they can have as much as they want as loud as they want - it's their ears and I would be impressed if you managed to get it to feedback through IEMs!
6) Mute unused mics - eg when BV not singing
Muting mics that aren't being used will help you reduce the amount of pickup from the stage which will allow you to push the remaining mics a bit louder. Having lots of unused mics live on stage will pick up bleed from the stage noise, and you'll hear a slight noise through the main PA when nothing is being played.
7) Speaker positioning - FOH away from band and not behind, foldbacks directly behind mics, speakers not aimed at wall or band
At a lot of venues I've been to, whoever positioned the Front of House (FOH) speakers did an awful job of either choosing speakers and/or positioning them. They are frequently behind or inline with the band, sometimes pointing at the band or towards a wall. Many forget that speakers have a throw angle that is wider than just directly in front of them so the bounce-back from side or back walls is a common feature in feedback or causing comb filtering. Consider moving speakers to in front of the band if possible.
Also make sure wedge and stand foldbacks (don't need to worry about IEMs!) are behind mics (refer to point 1).
8) Mic technique. Mic technique. Mic technique.
A constant battle is not with the equipment but musicians! Inexperienced people who have poor projection as it is forget that you almost need to be kissing the microphone - keep reminding them. Do training sessions. Stick post-it notes on their car or door. Whatever it takes, this will be a massive life-saver!
9) Ring out your system before using a GEQ to reduce problem freqs using an RTA with flat-response freq mic
Push the main output fader(s) up until the system starts to feed - note the frequency and notch it down slightly on the GEQ until it stops. Repeat until you get some good headroom but make sure you don't make it sound weird! There are many good RTAs (Real Time Analyzers) out there, some free, some paid (eg SMAART).
10) Delay towers/speakers means you can reduce level of FOH near band
If you can, get some speakers installed halfway down the building (if it's long or wide) or outdoors put some delay towers in. These rely on delaying the sound based on the distance they're placed from stage - based on the "Haas" effect that is a short enough delay such that it doesn't sound like two signals: one coming from stage, one from the delayed speakers but long enough that the brain thinks it's coming from the stage. It ensures the delays are in phase with the main PA. It reinforces the speakers near the stage in a subtle but noticeable way. This means you don't have to have the speakers by the band quite so loud..particularly if they're placed badly (see point 7)!
11) Check for over-attenuation in signal path
For example: PAD button left on on the desk, DI box at -40dB attenuation, poor mic technique, low sensitivity setting on wireless mics, mic knocked out of position, dodgy cabling, EQs etc. All of these will give you a lower gain before feedback level.
12) Set your gain properly and don't adjust during performance - foldbacks issue
A whole post is devoted to this! I won't repeat myself because that's just boring. But a key point to note is if you do break this invaluable rule (which you may have to from time to time simply because a singer suddenly gets ridiculously loud compared to soundcheck) then note that if you increase the gain, this also is reflected in the foldbacks. This could cause it to feedback. Be very careful!
13) Look for phase issues
Stereo overhead mics on a drum kit, out of phase speakers via switched polarity wiring or the phase switch button pressed on the desk are all causes of this. It will mean two signals cancel out each other, a bit like an acid and alkali making a neutral solution. If you hear something sounding weird, check this out.
14) DI instead of amps eg keyboard amp
I've seen a church use a keyboard, plugged into a portable amp, which was then mic'ed up and into the desk. My comment: don't do this. Ever. Use a DI box (or if it's a balanced out of the keyboard, that into the snake or A/D box for digital situations). A DI box on a keyboard is probably the least likely thing to feedback this way!
15) Don't rush things and make sure you just focus on carefully mixing the front of house
Making rash changes in the setup or on the desk will mean you forget say your point of feedback and leaves you more open to mistakes. Take things a step slower and remain diligent - just focus on mixing from a front of house position.
16) Finger on the button!
Keep an eye out for people with live headsets walking in front of a PA stack, or an MC cupping the mic / waving it near a wedge... all common causes of a sudden howl of feedback.
17) Correct mic choice
Using an omnidirectional condenser where a dynamic cardioid should be used is ultimately going to end up in less headroom, and earlier point of feedback. Choosing the right mic for the job, and close miking, will help you immensely.
18) Remember acoustics change
Many things cause acoustical changes to an environment, but particularly if you soundcheck for an empty room and then it fills with people, you need to remember many extra bodies will absorb a lot of HF. Be prepared that the audience make a fair bit of noise so ensure you've got decent headroom!
That's all for now folks, might add more at a later date if I think of anything else to help you guys.