This is one of those seemingly simple questions that could turn into a very very long thread.
First a slightly different perspective than the two earlier posts.
There are two different uses of eq, one is for monitors, were you are to get the most
GBF (
gain before
feedback). Here an
rta hardware of software helps, very experienced folks learn to be able to detect the frequency by ear and make the slight adjustment. The
RTA helps to identify the frequency. All the work here should be done with the mic being used on
stage, NOT a
rta dedicated mic and you need to check for mic positioning, cupping your
hand around the mic
etc, all those actions that the mic user is likely to use. Typically this is called ringing out the
system. You can have several options of how to do this
There are automatic systems, like Sabine Behringer
etc, which detect the offending frequency and have a very narrow filter that they use. In a pinch these work, but you loose control, and you can very well wind up with a sound in the monitoring
system that has good
GBF, BUT poor audio quality.
You can use a parametric eq or a 1/3 octave graphic. Graphics are typical, but you need to realize that the marked band is the center of the frequency area being affected, that there is quite a
bit of overlap and the tool is not as fine as you might suspect. A 1/3 octave does NOT mean that only a 1/3 octave is affected, but rather that the center
point is at a third octave
point, Subtle but major
point. A parametric gives you more precise control, and with a
rta can be quite effective. Most of the auto systems use parametric technicques, and they argue for their effectiveness based on this.
IMO it is NOT a good idea to simply
drop all the 1/3 octave faders and then boost. First, as I mentioned each
fader has quite a
bit of influence on the adjacent frequencies, and for
feedback elimination you should be reducing frequencies, not boosting the ones around them. Sometimes this idea comes from the technique that
IMO you rarely ever want to boost a frequency about the zero
line, but simply reduce the frequencies, SO in an extreme this could be interpreted as supporting the bring all the faders down to the minimal setting.
House eq is a different matter. Unfortunately a lot of people thing you just get a good ref mic, set the mic up and use the analysis to "
level out" the room. The joke is the way to
level out a room is with a bulldozer ;-)
Problem with the set the
rta mic up is that you are doing it in a room where the reflections from the floor
etc will have a dramatic
effect on performance, and you typically have really extreme effects. In most cases these days people now look at really not using
Rta systems to alter the room, but rather too correct the
speaker system for its non linearities.
SO you are looking for smooth transitions at the
crossover points, you are looking at time alignment combing
etc. Problem is that these are usually a combination of some graphic but typically a
DSP system like Driverack or BSS . IN addition a lot of the settings should have been properly set during installetc.
So graphic eq on
FOH is something to be approached carefully, listening to music you know is very helpful, moving around in the room
etc. making small changes and then going back and checking from the previous location
etc.
An experienced person typically when the see a graphic set up on mains that has dramatic all over the lot adjustments knows that it has been done incorrectly mainly based on a simple
rta mic with an auto or semi auto set up.
The other thing to realize is that the frequency performance in a audience space is related to humidity and temperature, and also absorption. I know people selling
auditorium seats talk about same absorption characteristics as people but that is just a minor part of all of it.
So
imo look at eq on the main
system as correcting the
speaker system NOT THE ROOM. There are some cases where you have a horrible room with massive reflections, or an apparent standing wave problem. IN most cases you are better correcting these with physical techniques if at all possible rarely will the graphic fix this.
One technique is to set the graphic for
foh flat, and
play some music that you are familiar with, and then go
thru and carefully boost the various frequence bands one at a time, and hear the increase of NEGATIVE
effect, and then use
the eq to reduce these areas.
Another reason not to use the boost is that here again is where you can cause
distortion or over driving in the
system with expensive results (blowing the
driver/
speaker). For instance you have a multi way
system, with a cross over
system if you boost the signal around the
crossover point you could overide effectively the cross over frequency and for instance put too low a frequence into a high freq
driver, remember crossovers typically have 12-18 db
roll offs, and you could off set this easily with the graphic.
There are some cases when you need to look at
feedback elimination on main speakers, but if the
system was installed properly this should be less of an issue as the speakers should not be placed so that the output feeds into the mics. Improper install or placement can lead to problems, most of the time you should first try to move the
speaker or the mic.
The other eq is for the individual signals to get the correct tonal balance you want based on the mic performer
etc etc. That is where you tend to do more individual tweaking.
So in general, monitors get rung out for feed back elimination\
Mains get eq'ed to smooth out the response of the
speaker system
mics get eq'ed for tonal support
Just some ideas
Sharyn