EQ is one of the most commonly used tools in mixing. This also makes it one of the most commonly misused tools, particularly when starting out. There is great temptation to apply massive EQ cuts or boosts to fix frequency imbalances, rather than making level adjustments, or even finding more suitable sounds in the first place. The vast majority of a great sounding mix comes from sounds which fit together well, set at appropriate levels relative to each other. if the mix isn't sounding decent before EQ and other tools have been applied, then the work of mixing is going to be much more difficult.
So we have a mix which sounds decent - how can EQ help? I picked up a very handy summary from Bobby Owsinski's "Mixing Engineer's Handbook" - cut to make things sound better, boost to make things sound different. This is how I apply it as well. Cuts clean out the parts of the sound we don't want, while leaving the overall character of the remaining parts of the sound relatively intact. Boosts introduce coloration which can spice up or change sounds quite a bit.
The first EQ I apply in a typical trance song is highpass filtering. Low frequencies take up a lot of headroom, due to the prominent kick and bass. So I remove the low frequencies from all the other sounds. This clears out the low end, leaving plenty of room for the kick and bass. Where I set the highpass cutoff frequency depends on the sounds in the song (including both the sounds being highpassed as well as the kick and bass). The filters are applied either directly to individual sounds, or to groupings of sounds. While the full mix is playing, I find the point where the filter begins to cut away part of the sound I want to retain, and move it back a bit from there. Sometimes I'll use a low shelf EQ instead, to use a much gentler slope - my goal is simply to significantly reduce the low end content which isn't essential to the sound within the context of the mix. When I started highpass filtering sounds I made some sounds too thin by setting the cutoff frequencies too high, in a misguided attempt to introduce separation and clarity into my mixes. As usual, experience helps to train our ears and get better at finding the sweet spot.
With the highpass filtering done, I turn to problematic or resonant frequencies, finding and taming them. Certain sounds (especially basses and pads, though sometimes kick drums) frequently have a lot of content around a particular frequency, in a way which isn't pleasing to the ear. To me, it makes the mix sound like it's under pressure. It can quickly cause muddiness and will mean the apparent loudness of the song is decreased, since the overpowering frequency will be hitting the ceiling of our levels. To locate these problem frequencies, a frequency analyzer can be used on the particular sound to see where there is excessive content. Another way is to set an EQ to maximum boost, with a very narrow Q, and sweep around the problem area to find the frequency where the resonance is greatest (be careful with your ears, don't do this at high levels). With problem areas, my first instinct is to try pulling the fader of the offending instrument down a bit and see whether that fixes the problem. If not, I notch out the problem frequency on that instrument. The depth and width of the notch are dependent on the sound. Sometimes it will only take around 4 dBs of cut with a very narrow Q to fix the problem. Sticking with the "under pressure" viewpoint, this feels like taking a small pin and deflating the pressure in the mix. Suddenly, the mix is cleaner, clearer, more open, and the offending sound's character hasn't changed at all. It's a great feeling.
At other times it's not so much a resonant frequency as a conflicting region - say, a pad which is overlapping with the bass sounds at 350 Hz, muddying things up. In this case I'll apply a much wider cut. I make it as deep as it needs to be - just to the point where the conflict is resolved. if the pad (or whichever sound) disappears, I've either chosen the wrong sound or I should move it up an octave. Conflicting regions extend all the way up the frequency spectrum - I'll sometimes cut back a bass sound's high end if it's getting in the way of the lead sounds (even if it makes the bass sound less defined when solo'd - it's the mix that counts, not the individual sound). I'll sometimes pull back the high end of a particularly hissy hihat for similar reasons.
Then I apply a few boosts. I generally don't boost the low end, there's rarely a need. Only when I feel there's a particularly flat spot do I add a bump of a dB or so, with a medium to wide Q, either to a particular sound or to the kick and bass grouping. I use boosts much more to bring out definition - 1 or 2 dBs added to the kick, centered between 3 to 8 kHz. Mid/high bass sounds often benefit from a 2 to 4 dB boost centered around 1 kHz. A large, wide boost of 2.5 to 5 dBs to lead sounds, centered around 3 to 6 kHz really helps to bring these sounds through clearly. Drum sounds are sometimes treated to a boost around 1.5 to 2.5 kHz. Sometimes a high shelf boost is also applied to the drums to bring out high end above 8 kHz. However, often with appropriate levels and highpass filtering applied the high end will already be where it should be. All these frequencies and amounts are ballpark figures. Actual usage in a particular song may be quite different.
My send channels (delays and reverbs) get EQ'd, sometimes quite extremely. When I was starting out I didn't think to EQ effects, not realizing they could sound a lot better if I did. Highpass filters are applied to most send effects (except maybe the "small room" reverb), since low end content can quickly build up and it can sound odd to have very low frequencies spread out to the sides of the stereo image. This may be a holdover from the days of vinyl records, where the bass frequencies needed to be centered. The high end is also often pulled down a bit, if I want to blend the effect into the mix. Conversely, if I want an effect to stick out, I lift the high end up. In between the extremes, the same principles apply as with regular EQing - I pull down certain frequencies to make things sound better (sometimes 300 to 500 Hz can get a bit boxy/ boomy on my large reverb, so I'll pull that down for example). I lift certain frequency ranges to give effects a different character and make them stand out, especially on my delays, which I don't feel need to sound "natural". If I have any flat spots in a mix, I like to try boosting one of my send effects in that region rather than altering the sound of a particular instrument. This can work well, filling in the flat spot without changing the sound of any of the instruments.
Finally, there's the master channel EQ. Very rarely this won't require any EQ, which is satisfying in a way, but I don't lose any sleep when a few bands of EQ are required to make a song sound better overall. Sometimes I'll apply a steep highpass filter to bring down excessive content under 40 Hz. Often there will be a broad boost of a dB or two, at frequencies where all the sounds seem to benefit from the treatment. I used to go quite wild with my master EQ, using large cuts and boosts to try to squeeze my mixes into the shape of my reference songs! Nowadays, as noted above, things sound much better through choosing sounds which actually want to fit together and setting appropriate levels, meaning I EQ a lot less.
I hope this article has been helpful, keep making great music!
Fabian