I'll take a
swing at explaining parametric EQ for ya the best I can.
I'll start with a few definition of terms
Center Frequency - center/apex of the affected frequencies you are adjusting
Q(bandwith) - how wide the adjustment is made across the frequency band, so you can adjust many or just a narrow selection of frequencies in either direction of the center frequency
Frequency Selection (sweep) - selects the frequency you are adjusting
Gain - (in application of EQ) adjusts the selected frequency based on Q and Sweep Selection in either a boost(+more of that frequency) or a cut(-less of that frequency,) commonly with max/min of 15 dB
now that you know a little more about the common terms involved in Parametric EQ, we can go on to how you use these to your advantage
Most boards based on what it seems you're going to be encountering have the highs, mids, and lows available to adjust, commonly with the mids available attached to a sweep control.
before you start "turning knobs" you have to think about what exactly the source is lacking or has too much of while you hear it going through the
system.
now based on what you hear, you can use the sweep to find the "sweet spot" that you like. To find this, you can boost the
gain on the sweepable frequency band, and sweep the frequency selection knob until you find the sound that fits the
instrument/vocal the best allowing for clarity and intellegibility. Once you've found that "sweet spot" you can then (if needed) bring the frequency adjustment back down into perspective (rather than blasting your audience with that frequency, unless that's what you're going for:wink
Now if it has a troublesome frequency, apply the same thing by boosting until you find the peak of where it is at, and then cut it until it sounds desirable.
One thing I find that can increase a lot of clarity is reducing a lot of the extreme low mids and lows to get rid of some of that "mud"
Now, the highs and lows are typically shelves, so they
cover typically anything above and beyond what the sweeps will
cover so just adjust these up and down depending on what you hear
Now, with EQ'in you typically are inclined to boost a lot of frequencies, but by taking away frequencies you don't need you get a lot more potential for getting over
feedback and having a more natural sound to your EQ job than placing random spikes where you can when you have limited control available on the board. so keep this in mind while you're setting it. Also, this frees up room in the mix for other things.
Now if its acoustic guitar, if the
system sounds good playing just about anything with everything set
flat, the acoustic should be relatively the same as far as EQ adjustment, just depending on the guitar only minor things should be done, given that its a good guitar.
For vocals, I've come across so many different voices on both systems good and bad, so to tell you anything particularly solid would be assuming a voice that I hear often, and isn't as likely to work for you at your
venue, with the voice you're dealing with.
What I can tell you is that a lot of voices can benefit from a hi-mid boost, and possibly a lesser amount of boost on the highs, with a low cut, to get rid of a lot of mud present in those troublesome lower frequencies.
right now I've gotta go, and I really hope this has some help and clears things up for you ;D