Q. How do you know what EQ frequencies to cut?

Published in SOS March 2008
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Thanks for featuring my band Imprint in the Mix Rescue article of SOS January 2008; we were really happy with the results that Mike Senior achieved. But whenever I read such articles, there are always mentions of very specific EQ cuts or boosts at very specific frequencies. In Mike's write-up on our mix, he says "drum harmonics were dominating at the low end — cuts of 3-4dB at 47Hz, 59Hz, 96Hz and 174Hz were all that were needed". How are these frequencies arrived at? Is it just a case of knowing the problem frequencies, is it with some sort of spectrum analyser, or it is trial and error? Also, how are the cuts done — is it just using a multi-band EQ?

I think this is the weakest part of my mixing side, and is probably the part I want to work on next, but I am not really sure where to start.

Blink (Imprint)

SOS contributor Mike Senior replies:

Drum samples, such as the ones in use in your track, don't usually change pitch, so you can use very narrow EQ cuts to home in on individual harmonics of the sound, and this is great for dealing with unwanted resonances in particular: a ringing sound at a particular frequency, for example. The problem with over-prominent individual harmonics is that they stop you fading up all the other harmonics of the sound far enough in the mix. By the time the sound as a whole is audible enough, those little resonant frequencies poke out too far. Very narrow EQ cuts make very little difference to the rest of the sound (the narrower the better really), so they're one of my favourite tactics when I can get away with them. They won't work on melodic instruments, though, because the harmonics move around as the pitch changes.

Q. How do you know what EQ frequencies to cut?In terms of which EQ to use, it doesn't matter at all, as long as you have a Q/resonance/bandwidth control and you can accurately enter frequencies — particularly at the low end, where 1Hz can mean a noticeable difference in pitch.

From memory, in your particular case I seem to remember that the kick part had two pitches to it, and I felt that both had low harmonics to them which were a little too prominent. This made them boom and hang on too long for my liking, especially as I knew that the effects and everything else would need space to move in. One of the pitches was more problematic than the other, hence the rough harmonic relations between some of the cuts. Still, the cuts weren't particularly severe. I normally find myself cutting more severely when using this technique. Here these notches were effectively just 'pulling down the fader a bit' for those individual kick harmonics without affecting the level of the kick as a whole.

The way I found those particular frequencies is by using a fairly well-known trick. Take a peaking filter and ratchet up the resonance (or Q value) to maximum. If you then apply a big gain boost with that filter, it acts as a kind of audio magnifying glass, picking out very narrow frequency bands and even individual harmonics of a pitched sound. If you sweep it around the frequency range for a while you'll soon get an idea of where the problem frequencies lie, at which point you can reverse the gain control setting to cut instead of boost.

You do need to be careful with this approach, though, because it's easy to be tricked into hearing problems that aren't actually there. What I do as a reality check is, once I've finished setting up the cut in question, I bypass it for a few seconds, and listen for the thing that I don't like. Only when I have it pinned down in my head do I re-engage the filter. That way I know for sure whether my EQ setting is doing the job I initially wanted it to. Funnily enough, I have a vague recollection that I tricked myself a couple of times with that very kick sound (maybe by initially setting the harmonic relations by eye rather than by ear), and I did come back to it and tweak it a couple of times as things progressed. The exact cuts at the two lowest frequencies in particular had to be quite finely judged — if you follow the fader analogy I made above, it makes sense that you'd need to adjust them as finely as you would any other fader.

My bottom line with EQ is that if I can't find a main fader level that I'm happy with for a given instrument, it's often because I actually need more than one fader for different frequency regions of that instrument. Using EQ makes a lot more sense if you think of it as just giving you these extra faders. In some senses, a graphic equaliser makes this easiest to visualise, but a parametric EQ will probably sound better, and it will offer more accurate control, especially for things such as killing pitched resonances. So I usually use a parametric. 

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