EQ Propagation

The question of where to EQ is, of course, tied inextricably to what to EQ.

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On occasion, I get the opportunity to guest-lecture to live-sound students. When things go the way I want them to, the students get a chance to experience the dialing up of monitor world (or part of it). One of the inevitable and important questions that arises is, “Why did you reach for the channel EQ when you were solving that one problem, but then use the EQ across the bus for this other problem?”

I’ve been able to give good answers to those questions, but I’ve also wanted to offer better explanations. I think I’ve finally hit upon an elegant way to describe my decision making process in regards to which EQ I use to solve different problems. It turns out that everything comes down to the primary “propagation direction” that I want for a given EQ change:

Effectively speaking, equalization on an input propagates downstream to all outputs. Equalization on an output effectively propagates upstream to all inputs.

What I’ve just said is, admittedly, rather abstract. That being so, let’s take a look at it concretely.

Let’s say we’re in the process of dialing up monitor world. It’s one of those all-too-rare occasions where we get the chance to measure the output of our wedges and apply an appropriate tuning. That equalization is applied across the appropriate bus. What we’re trying to do is equalize the box itself, so we can get acoustical output that follows a “reference curve.” (I advocate for a flat reference curve, myself.)

It might seem counter-intuitive, but if we’re going to tune the wedge electronically, what we actually have to do is transform all of the INPUTS to the box. Changing the loudspeaker itself to get our preferred reference curve would be ideal, but also very difficult. So, we use an EQ across a system output to change all the signals traveling to the wedge, counteracting the filtering that the drivers and enclosure impose on whatever makes it to them. If the monitor is making everything too crisp (for example), the “output” EQ lets us effectively dial high-frequency information out of every input traveling to the wedge.

Now, we put the signal from a microphone into one of our wedges. It starts off sounding generally good, although the channel in question is a vocal and we can tell there’s too much energy in the deep, low-frequency area. To fix the problem, we apply equalization to the microphone’s channel – the input. We want the exact change we’ve made to apply to every monitor that the channel might be sent to, and EQ across an input effectively transforms all the outputs that signal might arrive at.

There’s certainly nothing to stop us from going to each output EQ and pulling down the LF, but:

1) If we have a lot of mixes to work with, that’s pretty tedious, even with copy and paste, and…

2) We’ve now pushed away from our desired reference curve for the wedges, potentially robbing desired low-end information from inputs that would benefit from it. A ton of bottom isn’t necessary for vocals on deck, but what if somebody wants bass guitar? Or kick?

It makes much more sense to make the change at the channel if we can.

This also applies to the mud and midrange feedback weirdness that tends to pile up as one channel gets routed to multiple monitors. The problems aren’t necessarily the result of individual wedges being tuned badly. Rather, they are the result of multiple tunings interacting in a way that’s “wrong” for one particular mic at one particular location. What we need, then, is to EQ our input. The change then propagates to all the outputs, creating an overall solution with relative ease (and, again, we haven’t carved up each individual monitor’s curve into something that sounds weird in the process).

The same idea applies to FOH. If the whole mix seems “out of whack,” then a change to the main EQ effectively tweaks all the inputs to fix the offending frequency range.

So, when it’s time to grab an EQ, think about which way you want your changes to flow. Changes to inputs flow to all the connected outputs. Changes to outputs flow to all connected inputs.