Tuning the PA:
It is essential to achieve a “flat” eq, and therefore maximum vocal volume. This is my favoured method. Have the kick drum, and all vocal mics in position and turned up, (kick drum micbecause it will be in line with the centre of the room and at the very rear, therefore sensitive to standing wave anti-nodes.
Regarding standing waves; these are sound waves that inconveniently have multiples of their wavelength match the width, depth & height of a room. Therefore they resonate. To eq a room effectively you need to find these and attenuate them. The “worst” parts of these standing waves are the anti-nodes which generally occur near boundaries (walls, rear of stage) and also any centre line through the room. Chances are that your kick drum mic (the one mic that you want to pick up good clear acoustic bottom end) is going to be right there!
Take another mic & set it up on the dance floor if this is a caberet or multi-cultural gig (yes, way forward of the stage, this where the singer might spend most of the evening). Make sure that the eq is flat on all these channels but with the high pass filter (50 or 80 Hz cut) in, except for the kick drum mic, this is the one that will pick up bottom end standing waves. Next push the masters up on the desk, forcing the PA into resonance, when you hear a frequency start to feed, locate it on the graphic and pull it all the way down. OK, like me you haven’t got perfect pitch, so pull the masters back a fraction and then, one at a time, push up likely frequencies on the graphic until you hear a match. Bingo, that’s the one, pull it all the way out.
Now, repeat the process; push up the masters until the second frequency (standing wave) starts to resonate, pull back just a fraction and play with the graphic till you find this one. however this time only pull it maybe two thirds (probably 9dB). Repeat for a third time, but pull it by an appropriate amount (probably 6dB). Hopefully, you have now found and dealt with all the standing waves, but you may have also discovered that some of these resonant frequencies occur in between those offered by the graphic. If so choose the lower frequency as that’s the way the acoustics will change when the air in the room warms up with people/audience.
Now we start over to find the feed-back frequencies. Push up the mic channels and masters again. Put on a cassette or CD at a moderate volume to get the speakers and therefore air in the room moving. Go back to the graphic and, one by one, push up each fader.Those that feedback need to be pulled by 3 to 6dB depending on their severity. With luck, no more than half of the frequencies require attenuation, the less is best rule definitely applies here.Be warned though that if you colour the eq with boosted hi & low end or reduced mids (the rock n’ roll smile), the PA may sound nicer but you will lose the maximum achievable vocal volume.
Finally, to double check that the PA is eq-ed to suit the room, with the above mic.s still live, let the cassette or CD rip with a short burst of music at high volume. Any resonant frequencies that hang in the air need to be found on the graphic and attenuated a little bit more.