The vocals are undoubtedly the most important element of your mix. You may have captured the greatest guitar tone known to man but if the vocal sucks then no one is even going to notice.
In broad terms vocal eq or any eq for that matter falls into two categories, creative and corrective. The creative side is where you ensure the vocal fits with the tone and vibe of your song while corrective issues may involve high passing the vocal, notching out any annoying frequencies or dealing with sibilance.
The image below shows some typical areas of interest when eqing a vocal. I usually start out by high passing the vocal in the 80 – 100Hz region a free plugin like Brainworx’s bx_cleansweep makes for a great dedicated high/low pass filter.
The first question you should ask yourself before reaching for a compressor is, does this track really need to be compressed at all and if so how do i want it to affect the sound. If you find yourself constantly adjusting the vocal level on different sections and vocal lines then that would be a strong indication that compression is going to be necessary.
Setting the compressor is done on a case by case basis no two tracks are the same but common problems do surface so these can be addressed with common solutions.
Threshold: For example ( Starting with a mild 2:1 ratio) if I am only interested in taming the peaks of my vocal and feel the majority of the track is sitting well in the mix then I will set the compressors threshold to only catch the loudest parts leaving the rest of the vocal untouched. If I was interested in a more in your face sound then I would set the threshold lower so the compressor is always acting on the signal and not just the peaks.
Ratio: Once I have the threshold set I now turn to the ratio. I may be happy with what part of the signal the compressor is acting on but I might want more compression, in that case I will increase the ratio while listening to the effect it is having on the track. It is important to recognize how the threshold and ratio settings affect each, experiment with different ratios while watching the gain reduction meter.
Attack: When I am dialing in the attack time on my vocal I like to think about it in a musical way rather than just numbers on a dial. For example a fast attack time on the vocal may “blunt” or soften the impact of the words and remove some of the presence. This may be perfect if I want to sit the vocal back in the mix but in in other situations I might prefer a slower attack so my vocal transients are not getting squashed by the compressor and are popping out a little more.
Release: With respect to the vocal, different release times can either keep you vocal steady and upfront or it may undo any settings you have made to the other parameters. For example, you set the threshold and ratio to keep the vocal sitting just right, all the lyrics are perfectly clear and no words are too loud or too quiet. But then you dial in a really fast release time, what then starts to happen is the compressor effectively starts engaging on and off too fast or unmusically resulting in the vocal dynamic becoming unsteady again.
What’s important to understand is how the different controls interact with each other and how that are not independent of one another.