One of the most commonly used and powerful tools in the mastering engineer’s arsenal is equalization or EQ. Using EQ, you can pick out specific frequencies in a mix and boost or diminish their prominence in the overall mix to achieve the best sound from your mix.
EQ breaks the sound spectrum into typically 3 or 4 “bands” or spectrums, typically low, mid, and high. The low end is where you’ll find the instruments and sounds which make up that lower end of the sound spectrum. This most notably includes the bass and kick drum. If you find that your mix is lacking in a low end, you can boost it using the EQ to improve the mix to give it more presence there.
The high end on the other end of the spectrum can be boosted to give the mix a shinier, cleaner sound. While this makes the mix sound cleaner and more professional overall, going overboard with this spectrum will make the mix sound grating or “tinny”.
This is why it’s always recommended that you use the EQ sparingly. EQ has other uses where you can dampen specific ranges in the mix to remove offensive tones, pops or clicks in the mix, or anything else which you’d like, assuming you can find it in the mix.
Remember though that if something is lacking in the final mix, a great mastering job is no replacement for another and better mix. Fix everything you can in the mixing stage then simply look for the mastering stage to improve the audio and professional quality of your final mix.