PA System Tuning 101
It never ceases to amaze me just how many engineers out there don’t understand the basic principles of tuning a PA. Twice this year already I have seen an engineer walk into a gig & destroy my afternoon’s work by drastically changing the graphic EQ settings before even taking a moment to listen. They think “hey mate, this is a JBL rig… JBL speakers should always be EQ’d like this…” Not necessarily.
Inevitably, their set is plagued with feedback & odd hums going off left, right & center. Yes, JBL Speakers do have a certain frequency response like any other brand, but that combined with the mics on stage, the shape/size of the outdoor arena, my speaker management, cross-over points, etc.. all contribute to the unique sound on any given day. I would like to remedy this by explaining how I go about manually EQing my rig…
Practical Use of a Graphic EQ.
So let’s say you’ve got your rig setup the way you like it, and there are 8 microphones on stage. Bring up your Master fader to zero & give the first touch of graphic EQ relevant to the sound of your well listened reference CD… Nice. Apply the Channel EQ to each mic according to its relevant instrument or voice sound check.
Next we need to find the feedback frequency hot-spots & eliminate them. One by one, bring each microphone level to zero to the point just before the microphone feeds back. We now end up with 8 open microphones that are really hot. Now go to the Graphic EQ and one by one test each frequency slider for feedback by slowly boosting to 100% & listening carefully. Some frequencies will feedback as soon as you move that slider a millimeter. If that is the case then cut that frequency 100%. Some frequencies will take up to a few seconds before you can hear the feedback creeping in, in which case only slight reduction is required. Some frequencies won’t feedback at all, so leave the relevant filter at zero. Basically, judge how much to cut a frequency by how quickly you can hear feedback creep in when a given frequency is boosted on the Graphic EQ.
Now go back to the reference CD. Sound like crap? Don’t panic yet. If you have found yourself with a pretty drastic graphic EQ setting then it is probably time go back to the individual microphone channels & find any hotspots using the parametric EQ. Using the same principle as before, open only the first microphone & push the high EQ knob to max. Hear any feedback creeping in? If so, cut accordingly with the Channel EQ. Now boost the mid filter and gradually sweep the frequency until you find any hotspots & again cut accordingly using the same EQ. Repeat with the lower mid & bass frequencies. Repeat this process on each microphone until they are all tuned accordingly. Now open all the microphone channels & go back to the overall Graphic EQ, testing each individual frequency again by boosting as before, and cutting accordingly.
You should now have a less drastic looking set of curves, and shouldn’t need to tweak too much more to get that reference track sounding near perfect. You should also find it difficult to get those microphones to feedback on stage, even if you do get excited and push that guitar solo a bit hotter than you should. You can now enjoy the mix with piece of mind. That is until the next bands mix engineer comes in and carves a smiley face into your Graphic EQ…