Out of the three main tools for mixing (Compression, Reverb and EQ), I feel EQ is the most important and also difficult to master. If given no other tools, I think the best results could be achieved with only having access to EQ. That being said, I think this is the tool with which I have the most yet to learn and master. While EQ can do so much to alter the timbre of an element to make it sound exactly the way you want, I get the most enjoyment when I can use it to create clarity and separate elements in the mix.
One of the biggest mistakes made by beginners when starting to mix is to EQ everything so that it sounds exacly the way they want it to sound. Only problem is they usually do this for each element individually. As soon as they listen to everything together, it turns into an audio soup. Every element becomes unfocused and muddy with a lack of definition.
Instead, the focus should be on what the important qualities of an element in the mix are. What makes that timbre stand out and sound unique from the rest? What do you want to stand out about it? Decide what elements live where in the mix with the entire mix and relationship of all elements in mind.
A great example of this is in dance music. There has to be a conscious decision made on whether the kick drum or bass is going to occupy the low frequencies. If they both occupy 100hz and lower, you won’t get much definition in either and neither will sound good. Dig deeper and really think about what you like about each sound. Do you really like a low boomy kick drum or is the snap of the mallet hitting the head on the hi end really important? Does the deepness of the bass sound pleasing or is there a distorted aspect to the timbre of the bass that you like? Where in the spectrum is that?
For the track below, I wanted the bass synth to have a low end body and consciously chose to make the kick drum have a higher snappier quality to it. The kick drum still sounds big and heavy since an instrument is still playing the low end frequencies I took out of the kick drum. In addition, I also put a hi shelf on the kick drum around 8k to add snap. I also put a cut in the bass synth from 90-110hz with a narrow Q. I then boosted the kick drum with a narrow Q to occupy the space that the bass synth was now not occupying. Now, the entire are of the spectrum has an instrument occupying it but they don’t overlap creating clarity as well as the timbre I wanted for both instruments.
There are tricks such as automating EQ to “trade” which instrument is occupying what space so you can create the illusion that multiple instruments are occupying the same space. Or, taking a very narrow cut and sneaking only that sliver sound in the void from another instrument like I did in the example above.
Ultimately, when you start prioritizing elements, analyzing timbral qualities and making conscious decisions on where each sound is occupying the frequency spectrum, you can start to make some fantastic sounding mixes.