9 Tips on Avoiding and Eliminating Feedback in Live Sound.

One thing that can ruin a great live performance is feedback, especially the persistent kind that you end up fighting all night while slowly destroying your brilliant mix. You know it, you’ve been there, you’re happily mixing sound for your show when suddenly out of nowhere a WHOOOP, SHHHIIIIINNNG, or HOOOOOOT, comes howling out of the speakers.  If you were dozing off at the console you’re awake now.  What do you reach for, and how do you make it stop? 

Most of us have heard feedback at one time or another, but what exactly is it?  Feedback can be acoustic or mechanical.  Mechanical feedback is when a loop is created sending the signal back to itself via the electronics.  For example, sending an input to an Aux bus and then accidentally routing the output of that Aux bus back to itself. 

Acoustic feedback, which is what I will be focusing on here, is the sound a microphone makes when it hears itself.  Somewhat similar to the shriek I make when I take a look in the mirror the morning after a night of too much red wine. 

Feedback can be controlled and even eliminated with some proper techniques.  While there are tools such as feedback suppressors and spectrum analyzers that can assist or even do the job for you, it’s best to learn these techniques for when you are in a situation where those tools are not available.  And trust me, sooner or later you will be in that situation.

1- Know the frequencies.

You need to be able to identify the frequency feeding back to eliminate it.

If you can’t recognize 1KHz from 250Hz you’ll experience the illusion of time manipulation as time seems to slow to a crawl, seconds feel like minutes while you are hunting for the frequency that is wailing from the lead singer's monitor wedge as he/she glares at you.  You’ll also likely end up butchering the EQ or mix while you start grabbing for anything and everything to make it stop.  It can take time to learn how to identify frequencies but there are many ways to learn.

2- Proper speaker placement.

It’s just a simple fact of life, microphones pointed at speakers produce feedback.

Ideally, the speakers should be downstage of the microphones.  When microphones are in front of the PA system, you will likely need to do a lot of work on the system EQ to remove unstable frequencies, which can lead to an unnatural sounding PA.  This is called ringing out the mics which I’ll talk about next.  Careful attention should also be paid to the monitor speaker placement. If you’ve got a monitor wedge directly in front of a hyper cardioid mic, you’ll probably be fighting feedback since a hyper cardioid mic allows for sound to enter opposite the capsule.

Whenever possible keep the speakers out of the microphones.  I’m talking of course about the PA system speakers, not the talking heads walking around on stage.

Sometimes this just isn’t possible for whatever reason, in which case carefully ringing out the microphones in the system will be crucial.

3- Ringing out the microphones.

After tuning the system, it’s a good idea to ring out the vocal microphones in the PA.  This involves setting proper gain and bringing up the mic in the PA or monitor speakers until it begins to feedback.  Remove the offending frequency and bring up the mic until the next frequency feeds back and remove that.  Continue until you can get the vocal microphone/s loud enough without feedback.  Doing this prior to the soundcheck or show will greatly reduce the odds of feedback.

4- Get the sound right at the source.  

Let's take the example of a floor tom that has a terrible ring in it that takes off every time the drummer hits the kick.  You can either gate the heck out of it and/or butcher the sound of the floor tom to EQ out the ring, or you (or the drum tech) can tune the drum properly to remove the ring.  This allows you to EQ the drum for a more natural sound and open up the gate a bit to let the drum breathe.  That is if you like your drums to sound like drums and not blocks of wood.

5- Use the right mic.

If you’ve got a weak singer with less than stellar mic technique, (you know the kind who likes to wave the microphone around or doesn’t want it too close to their mouth because it will ruin their lipstick), you probably want a cardioid mic with high gain before feedback.  Similarly, if you’ve got a singer with great mic technique but incredibly loud stage volume, or who likes to be in front of the PA all night, a mic with a tighter pattern like hyper cardioid would work better.

Choose microphones with the frequency response that matches the sound of the source you are trying to reproduce. 

6- Proper mic placement.

Close mic’ing is common in live sound.  When you have a lot of instruments in close proximity to each other, they will tend to bleed into any nearby microphones.  The closer the mic is to the source, the signal from the source will be stronger than the signal from the surrounding instruments/amplifiers. This gives you more control of the sound and control is what we want.  Let’s say you are trying to mic a grand piano that is right next to the drum kit, you probably don’t want to use a stereo pair of overhead mic’s above the open piano.  A better option would be to put the microphones in the piano with the lid closed to isolate it from the drums.  This will help you to get a stronger signal from the piano, which ultimately leads to getting the piano loud enough in the mix without feedingback.

7- Use proper gain structure.

Understand and implement proper gain structure of your inputs.  There are several places on the console that act as a gain stage.  The most important is your head amp (or pre-amp) gain.  When gain is set properly at the head amp or pre-amp, you’ll go further in achieving good quality signal and less feedback than with pre-amp gain that is too low and which you are trying to make up for elsewhere.

8- Use subtractive EQ rather than additive.

When EQ’ing your inputs, in live sound the general rule of thumb is to cut first rather than boost.  If your vocals are muddy, instead of adding some top end to give them more clarity, try cutting the low mids.  Boosting high mid and high frequencies can lead to feedback if you are already struggling to get the vocal on top of the mix.  If you clean up the low mids first, you likely won’t need to add anything to get a clean and clear vocal sound. 

9- Understand signal flow

A thorough understanding of signal flow will help you avoid both mechanical and acoustical feedback.  Knowing how the signal travels through all parts of the audio system, including the soundboard will help prevent a mechanical feedback loop and allow you to set proper gain staging to get the most headroom from your system.

Understanding and implementing these techniques will help you to tame the feedback monster.  In the event that it does rear its ugly head, being able to quickly identify and eliminate the frequency that is feedingback is crucial. 

By: Michelle Sabolchick