EQ it is one of the most well-known forms of audio processing. Often, when saying "make it sound brighter," "fatter," "more transparent," "add some air," etc., producers and artists attribute these functions to equalization. But what is EQ really?
Essentially, it is just another "volume knob" - one that works in a specific frequency range. And considering EQ as a "local volume knob" can change the way you use it. Instead of thinking about adding "brightness", "meatiness", or "transparency", remember that you are simply making a particular part of the spectrum louder or quieter. This mindset can help you to avoid relying on EQ to solve issues where it's incapable.
For example, if you want more bass, you need to make sure that the source material already contains some low-end information. You can't add bass that is not there.
Another important point to remember is that it is not always necessary to boost something that is not enough. In many situations, it is more effective and natural to decrease something that's too much.
Since we perceive a louder sound as "better", and equalization changes volume, it is important to match levels of the processed and unprocessed material to find out if equalization has actually improved the sound.
EQ is not only about changing the tone. It can also be used to affect psychoacoustic characteristics. For example, reducing high frequencies can make a sound seem more distant. Or boosting lows can make a sound seem larger.
In addition to amplitude changes, many equalizers (such as IIR-based filters and all analog devices) affect phase shift. More aggressive equalization can result in more phase anomalies.
On the other hand, linear-phase equalizers don't affect the phase of the signal, but they do add pre-ringing artifacts. That sounds like a reversed tail before the original sound. This can be harmful for sources carrying important transient information, such as drums and percussion.
Finally, it's useful to remember that mixing with EQ is not just about shaping sounds and cutting resonances. It's also about working with musical ranges, notes, and octaves. The tasks we solve with EQ mainly depend on the musical objectives we are facing. Therefore, equalization is best done in the context of the music, not on solo tracks.