Even the deepest bass voice doesn't dip much below 100 hertz (Hz). Any frequencies on the vocal track below this level are likely to be artifacts from the microphone or another part of the recording process. Leaving these frequencies in the vocal track can cause it to conflict with the other elements of the song, especially the bass and drums, and create a "muddy" sound. Whenever you're equalizing vocals, set the EQ to sharply reduce all of the frequencies below approximately 80 to 100 Hz.
The frequencies around 200 Hz add fullness to the vocal track. If the vocal recording sounds weak or nasal, or if the rest of the tracks are overpowering the vocal, add a moderate boost to this frequency band. If, however, the singer's recording sounds muddy or indistinct, reducing these frequencies can help the vocals stand out from the rest of the tracks.
If the vocalist sounds like he or she's fading into the background, use the EQ to boost the vocal's presence. Raising the level of the frequencies between 4,000 and 6,000 Hz can bring the vocals to the foreground of the mix. These frequencies can also, however, cause harshness. If, after boosting this area of the frequency spectrum, the vocals sound harsh, apply a band-reject filter -- which cuts out a narrow range of the frequency spectrum -- and move it back and forth until you eliminate the harshness.
Boost the frequencies between 7,000 and 15,000 Hz to add brightness to a dull-sounding vocal track. Be cautious when increasing this frequency range, however: the "ess" sound falls in the lower section of this portion of the frequency spectrum, and magnifying the "ess" can create sibilance, which makes a vocal sound unpleasantly harsh. If this occurs, use a notch filter on the EQ to reduce the "ess" sound. The highest part of this frequency spectrum affect the breathiness, or "air," of the vocal; if you think the vocal needs to float more above the rest of the tracks, increase these frequencies.