I’ve used this metaphor before, but mixing is like putting together a complicated puzzle. Every track has a perfect spot in the mix which is created by level setting and adjustments via processing. A great sounding mix is one which has all of its tracks arranged in just the right way relative to every other track, and a big part of this is addressing and resolving frequency conflicts. Complementary EQ is the key to resolving frequency conflicts, so let’s talk how to use it to fix your mix.
What is Complementary EQ
First let’s define what we’re actually talking about here – what is complementary EQ?
Complementary EQ refers to using EQ to clean up frequency conflicts between two or more tracks.
A frequency conflict is just that – when two or more tracks feature similar fundamental frequencies, particularly when the two tracks are sharing a close proximity or similar location in the stereo field:
As the arrow suggests, a track’s or audio clip’s fundamental frequency is its lowest frequency core, that area in its frequency profile which is the loudest and/or widest peak. While the above frequency profile of the low E string being plucked on an acoustic guitar shows overtones at ultimately smaller peaks higher up, its fundamental frequency occurs around 82Hz which is literally the frequency of that string (when in tune).
One of the many secrets to a clean mix is minimizing or altogether eradicating frequency conflicts between the many tracks which make up your mix, and as mentioned, complementary EQ is a big part of that.
How to Use Complementary EQ
Let’s cover a few different ways to use complementary EQ in your mix to better demonstrate how to use it to resolve frequency conflicts and achieve a cleaner mix.
Mixing in Mono
Before we get into the EQ itself, let’s talk the importance of mono for finding problems.
I’ll talk about the obvious tactic of panning later on independent of complementary EQ, but the easiest way to identify frequency conflicts is to force your mix into mono.
Different DAWs have different ways of doing this. In my Ableton Live I simply drop a Utility plugin (see my overview of Ableton’s stock plugins) on the master bus with the mono feature engaged.
When mixing in mono, all of the frequency conflicts which were disguised when the affected tracks were separated in the mix are now in plain sight as the tracks are lumped on top of one another.
With them easier to hear, it’s easier to target and address them via complementary EQ.
Admittedly you’re not going to bounce your mix in mono, but the key is that the better you can get your mix to sound in mono, the better its going to sound in stereo.
Complementary Cuts
I’ll begin with the most obvious and simplified type of complementary EQ – a complementary cut on one track to clear up space for another track.
This is the ultimate “taking one for the team” cut here.
A classic example for complementary EQ is the kick drum and bass – classic in that it’s two instruments which share a similar fundamental frequency in every mix in that sub/around 100Hz range, but also two tracks which sound their best when centered in the mix.
The fundamental of the kick typically exists in that 50-80Hz range, so a complementary cut on the bass gives the anchor of your mix that is the kick the room to assert itself.
With regard to kick and bass there’s arguably a better way to approach this which I’ll talk in a minute, but that’s the basic premise of complementary EQ, simply cutting one track in deference to allowing the other track to play unimpeded and creating clarity simply through the act of removing the conflict.
How do we find the conflicts, though?
While you can certainly simply go by ear, an easier and expedited approach is to use an EQ like FabFilter Pro-Q 3 which will literally show you any tracks which are competing for the same frequency space:
I talked about this in my FabFilter Pro-Q 3 review and cited it as one of many reasons it’s my favorite plugin in general.
Shown in red, Pro-Q 3 refers to these conflicts as “collisions” and will point them out amongst any and all tracks which have an instance of this plugin on them which share fundamental frequencies.
What’s more, it makes it incredibly easy to use sidechain EQ between the two tracks to treat the complementary EQ more surgically. Speaking of which…
Sidechain EQ
Sidechain EQ is the most transparent form of complementary EQ to resolve frequency conflicts.
Sidechain EQ refers to EQing, specifically dynamic EQing, a specific frequency based on the behavior of another track:
Dynamic EQ by nature, meaning it proportionately cuts the band you want to affect more or less as the behavior of the other track dictates.
When the kick swells at 70Hz for example right in that fundamental “meaty” thud of the kick, we can create a complementary sidechain EQ cut on the bass at 70Hz to pull down only in those instances where the kick triggers, giving the kick full domain over the low end when it triggers.
The bass is otherwise unaffected, and only the 70Hz range on the bass is pulled down when the kick triggers, making for a much more transparent solution in getting the two to work together and cleaning up that frequency conflict.
You can even filter out other frequencies to dictate the behavior. This is useful when doing the aforementioned sidechaining your bass to the kick drum.
In other words, we can limit the part of the kick which triggers the dynamic EQ on the bass to just everything under 100Hz. This helps to ensure no bleed from the rest of the kick is pulling 70Hz out of the bass needlessly when the kick ISN’T triggering.
I put together an entire overview on how to sidechain EQ your bass to your kick to seriously improve your mix, so refer to that for more information.
EQ Filters and Shelves
If we’ve got a frequency conflict even after panning as is the case in a crowded stereo field, we can think about sending one of those tracks farther back relative to the other(s).
While we typically think of reverb when we want to send a track deeper into the mix, an EQ filter or shelf can be used with a similar principle in mind.
Because of the difference in speed in how low frequencies versus high frequencies travel, we perceive higher frequencies before we hear lower frequencies.
We can exploit this by adding a low pass filter on one of the two conflicting tracks to make it sound like its coming from deeper in the mix:
Rolling that filter point down will cause the ear to perceive that track is coming from deeper in the mix, helping it get out of the way of the other track.
For a more subtle adjustment in the same vein, try a high shelf cut at a similar cutoff point to instead attenuate (instead of boosting as shown) uniformly above that point by a few dB:
Either of these are a great way to address a frequency conflict via perceived depth differences between conflict tracks when we don’t want the issues associated with reverb.
Other Ways to Address Frequency Conflicts
I touched on this when talking about using complementary EQ a moment ago, but aside from sculpting frequencies of multiple tracks to complement one another, the other reliable way to address frequency conflicts is to make full use of the stereo (and three dimensional depth) field.
Refer to my audio panning guide which shows a complete and effective panning setup with frequency conflicts in mind, keeping similar profiled instruments on opposite sides.
Aside from the aforementioned complementary EQ filters and/or shelves, reverb can be used to send instruments farther back by simulating natural (or artificial) depth in your mix regardless of how “dry” or where the tracks were recorded (see my many reverb tutorials for endless tips on how to get the most out of using reverb in your mix).
