Mythbusters: Subtractive vs. Additive EQ
If you’ve been out and about on the internet then you’ve probably read that subtractive EQ is generally better than additive EQ. In other words, it’s better to attenuate than to boost.
And you have probably read all sorts of reasons for this: less phase shift, sounds smoother, more accurate, doesn’t require active gain, etc.
Well, I’m here to tell you that a great deal of that is false.
Let’s start by dispelling a few myths.
Myth #1: Subtractive EQ incurs less phase shift, or less noticeable phase shift
This is untrue. The degree of phase shift is 100% controlled by the amount of cut or boost. In fact “phase shift” is almost synonymous with “equalization.” They are inherently part of the same process.
Now, are the artifacts of said phase shift less noticeable? In objective technical reasoning, no, they are not.
And here’s why: If you exclusively use boosting to create an EQ curve on a source and then recreate the exact same curve using only attenuation, and then level match those two outputs exactly, you will get identical signals.
A simple way to test this is by taking any sound source and a shelf filter. Mult the sound source. On one, use a hi-shelf to boost up some high end. On the other, use a low shelf, set to the same corner frequency with the same slope and attenuated by the same amount. Then boost the output volume of the low-shelfed signal up to match the hi-shelfed signal and flip the phase. They will completely null.
What’s important, is to use an EQ where the parameters match up equally. For example, this will work with the Waves Q EQ, but will not work with the Waves Ren EQ.
What this demonstrates is that the phase shift and artifacts produced by that shift are based on the curve and degree of equalization, not whether the equalization is positive or negative.
Myth #2: Subtractive EQ Sounds Smoother
Ultimately the truth to this is based more on application than reality. It tends to be easier to mix additively – boosting up things you want more of.
The problem with this is that it leads to a lot of compensational boosting. By that I mean boosting up lots of frequency ranges when really we just wanted to hear less of one frequency range. Or we will boost up a frequency because we aren’t hearing enough of it, when in reality there’s something from another instrument that’s getting in the way.
These issues aren’t inherent to additive or subtractive EQ, rather, they stem from using additive EQ when subtractive EQ would be a better approach.
Myth #3: Additive EQ is Less Accurate
This is something I hear a lot, and quite honestly I’m not even sure what it really means.
I believe this myth stems from the idea that it’s better to “cut narrow and boost wide.” In reality you should boost or cut as narrow or wide as needed.
I think it’s easier to over extend the width of an EQ boost because that means more stuff is getting louder. And louder has that instant gratification effect of sounding better. Again, this is a result of application and not inherent to how an EQ works.
Myth #4: Subtractive EQ Doesn’t Require Active Gain
This one is based on the idea that because you are “taking something away” you don’t need to add power to do it. This is really only true in the simplest filter setups.
While it’s true that a simple resistor-capacitor circuit doesn’t need any power, most more complex circuits do. That’s not to say there aren’t complex subtractive circuits that work completely passively — there are a lot. But many EQs, no matter what you are doing, are drawing on active power. It completely depends on the EQ itself.
Lastly, digital EQs don’t use power. There’s no such thing as an “active” digital EQ, outside of plugins that emulate hardware.
Myth #5: Subtractive EQ Frees Up Headroom
This is half true. Subtractive EQ can and often does reduce overall amplitude, but the process is actually not amplitude based. You’re not exactly adjust the level of frequencies – you are adjusting the phase, and then getting a change in level at the band you are adjusting. But that’s not the same as adjusting the signal amplitude. In fact, on occasion you will use subtractive EQ and the result will be a boost to the signal amplitude! I kid you not.
So what’s the difference?
The major difference in subtractive vs. additive EQ is the thought process.
When you are using subtractive EQ, you are thinking “what can I take away to make this better.” Taking something away lends itself toward degrading a sound — so any time you take something away and it improves the sound you pretty much know you are making a good EQ move.
Additive EQ, conversely, is enhancing a signal, which naturally tends to sound better no matter what (particularly if you are doing it in solo mode). The real key to using subtractive or additive EQ is to have a good idea of what you’re trying to accomplish. The “solo” button can be very useful here.
Lets say you want to hear more upper mids in your guitar lead. Solo the guitar — does it sound like it has enough upper mids? If not, then use some additive EQ and boost those mids. If it does sound like it has enough upper midrange in solo, maybe there’s something else in the mix that has too much upper mids like a piano or acoustic guitar. Or maybe you really just want to hear the guitar louder, but when you turn it up the lower mids become overpowering. In these cases, subtractive EQ is going to be more effective.
I hope that clears up the reality of EQ. I have another article that dives deep into equalization here.