They say monsters aren’t real, but I’m not so sure. You can check under the bed or in the closet, but the monsters in your studio are invisible. Hiding in plain sight, messing with your mix.
Maybe you didn’t know it, but the same process you use to weed out feedback in a live venue can be used to identify and eliminate those monsters. What we’re talking about here, of chorus, are acoustic standing waves. And to make my point, let’s first define feedback.
On the one hand, there’s controlled feedback. That’s what you might use to dial in a little bit of delay on your guitar pedal, and that works by including a little bit of the delay signal back into the delay input. As long as it’s less than the original, you’ll get additional repeats as the signal gets quieter and quieter.
But a feedback loop occurs when the signal at the point it’s being picked up is as loud or louder than the signal at its source. That’s what’s responsible for that dreadful squeal. When this mic hears the amplified version of my voice as loud as my actual voice or louder, well, you’ve got a problem.
The feedback itself isn’t the monster. The room is. The feedback is just revealing it.
Every enclosed space has resonant frequencies. Just like movement in a tub of water, there are certain frequencies that are going to resonate more than others. Those are called standing waves.
In theory, they’re easy enough to calculate. You have to account for the way of traveling from one side to the other. Your first problem frequency is the speed of sound divided by double the room length.
Sound is around 1,125 feet per second. We’ve got about 15 feet here, so 30 into 1125 gives you somewhere around 37. That’s the first monster.
It’s a thunderous low note at the bottom of your mix. You’re hearing it louder than everything else, and it’s causing you to make bad decisions. And there are more of them, because every multiple of that frequency is a harmonic.
37.5 times two is 75, 112.5, 150, 187.5, and that’s just one dimension. Not to mention, we’re assuming a rectangular room with nothing in it. There are easier ways to do this.
Room correction software can measure and create an EQ offset, but most people will tell you it’s best to address it with room treatment first. Sometimes you can’t. Now, if you stuck around this long, you probably don’t remember.
We started all this talking about monsters and feedback, and you’re probably wondering why. It’s because this is a good opportunity for me to teach you a skill that every live engineer should know how to do, which is how to ring out a room. Ringing out a room is the technique to get a massive amount of amplification without feeding back.
It’s those same resonant monster frequencies that are gonna be the first ones to start squealing at you when the microphone feeds back. The procedure is really simple. We’re gonna start turning up the control room volume until we hear the first frequency start to ring.
We find it and pull it back. Then we increase the volume some more until we find the next one. On stage, you’d probably be using a graphic EQ for this.
In the studio, we’re more likely to use a parametric filter. Think of it as a blueprint for where to address problem frequencies. In other words, proceed with caution before you start making critical mix decisions through an EQ that looks like an exotic mountain range.
