Mixing audio involves many different tools and techniques, all designed to help tracks complement each other nicely in the mix, mainly by "making room for one another", either on the frequencies spectrum, or in power level. However, EQ have another very common and often overlooked use and that is detecting and shaving off nasty frequencies. Two very common frequency areas that I always keep an eye on are 380-420Hz and 2500-3200Hz. The 350-400Hz range is common to most instruments and is probably the most common problem generator. If I had only one EQ move to do in a mix, my first try would be to notch 400Hz off a few DB and see what happens. Anyways, in this post I'm going to show you an example of a real problem I encountered in my latest session. In this case, we're talking about whistling 2500-3200Hz frequencies. These tend to come from distorted electric guitars more often that anything else, especially when there are several of them mixed together (I think that SM57s contribute to that phenomenon a lot).
Whistling frequencies are something you need to learn how to intentionally look for
Whistling frequencies are something you need to learn how to intentionally look for, because it's hard to notice their presence until you hear your mix without them. Here is a very clear A/B example of a soloed guitars bus in one of my recent mixing sessions.
How To Detect Such Problems
The most efficient technique for finding offensive frequencies is pretty simple and requires any single parametric EQ, I use the one built into Cubase's channels for most EQ tasks. So here is what you do:
[recommended] Bypass any plugins from the channel you work on
Set the frequency, gain and "Q" of your EQ plugin to max
Choose a time where your track has representative audio in it and set your DAW to loop over it
Solo your track
Start playback
As the track plays, start sweeping the frequency knob of your EQ slowly all the way through the spectrum and listen for unpleasant or whistling frequencies
Once an offensive frequency is detected, invert the gain to take it off and see if things sound clearer, keep adjusting the gain and listen.
Cancel solo mode on your track and listen in the context of the whole mix. Take the EQ on/off and listen to the effect in context.
The same concept applies to virtually all instruments, each with its own frequency range of course. And mastering that technique is critical on your journey to becoming a better mixer.
Happy mixing!