Equalizers, by their nature and name, are supposed to redress imbalances in a sound system.
Unfortunately, equalizers, whether graphic, parametric, or shelving in nature, are only as good as the person using them. Knowing where to turn the knob or push the fader is usually a dark science, but there is a light at the end of the tunnel.
The first step in proper EQ technique is to realize the knobs go to the left better than they go to the right (or in the case of faders, down rather than up).
In other words, reduction of energy is more beneficial than an increase in energy, due to the nature of live sound. Live music in particular is awash in competing frequencies from melody and rhythm instruments in struggle with cascading voices emanating from the stage and the audience.
Since human hearing is most sensitive to frequencies in the mid-band and most instruments produce a majority of their fundamental and initial order harmonics in the same region, there is too much energy localized between 250 Hz and 2.5 kHz.
Again, though, without knowing where to cut the energy levels, the cure will be worse than the disease. Here, then, are several descriptive words and the corresponding frequency most attuned to the problem area:
Tubby – 125 Hz is a common resonant frequency in church auditoriums. Stage rumble can be reduced by eliminating (-9 dB) the frequencies below 125 Hz on the graphic EQ assigned to the main stage monitors.
Boomy – 160 Hz is “fake bass,” a tone associated with a wimpy boom box trying to reproduce the solid fundamental tone of kick drum and bass guitar that occur at 80 Hz. Because an octave is a doubling or halving of frequency, boom-bass is an octave above real bass. Pulling out 5 dB at 160 Hz will clear up the rhythm section.
Muddy – 250 Hz is the most effective region to grab and reduce “masked” energy from the sound system. If you can do one thing, reduce 250 Hz by 3 dB.