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I've encountered some weird rooms too - one had such a strong response that if you stood on the centreline and spoke it felt like you were mic'ed up, and another with a corner that resonated for 20-odd seconds if you whistled at about 900Hz. But you only need to flatten for where the mics are, feedback-wise.
Exactly - the room acoustics are fixed properly, which is a much better approach. I doubt many pubs or small clubs bother with acoustic treatment so a bit of EQ is useful.
Needed Acoustic Gain and Potential Acoustic Gain.
As Danny says, most commonly applied to speech systems but the formulae are relevant for any PA system.
NAG is how much you gain you need to amplify the voice in order that people can hear and understand what's being said/sung.
PAG is how much gain you can apply given the number of open mics and the relative positions of the mics, loudspeakers and audience. The reason I mention it is that one factor in PAG is whether the system is EQ'd or not - if it is you can reasonably assume that you can get another 6dB of gain, which can easily make the difference between being loud enough without feedback and not.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Even without big boys toys and fancy systems, the correct placement and system EQ is still the first thing to attend to. Different mics makes and models will each have their own peaks and so present more points to excite feedback than a single type/model for everyone. A single type means you can cut the two worst offenders for feedback and still have a toneful system with good feedback resistance. After cutting more than three peaks you are hacking the life out of the tone and onto a loosing thing. a 31 band Graphic EQ is a bit like using an axe to take out a splinter compared to some of the digital filters available today, but if it's all you've got it can be used.
Try to keep the system sounding natural and it will cut through most things better. For Metal rock and screamers then that may not be true so much but still is a great rule of thumb.
So understand your microphones pickup pattern and response, then use that to your advantage.
Pink noise is good if you have an analyser of some sort, otherwise a slow swept tone from the lowest to the highest audible range tells you where frequencies are reduced or enhanced, the trick is knowing what is causing that peak or trough and taking appropriate action. Just using a graphic can exaserbate the problem sometimes by generating nodes around the source problem.