sound
The engineer should be mixing the house so that it sounds right no matter what the band is doing (within reason, of course). He should also understand what changes need to be made in the monitor mix when, for instance, the acoustic guitar player who's just played finger style picks up his bagpipes and puts them in front of his guitar mic. This is why you have (in order of effectiveness) rehearsals with a sound system and engineer, a sound check before the show, a set list with cues where changes to the mix will be required, and an engineer who's paying attention. When something is blasting out in the mix, the engineer shouldn't need to be told by a musician on stage to turn it down.
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answer:
In general the mix in the front-of-house should not affect the feedback situation if the mics are behind the PA speakers. Ringing out the feedback in the monitors can be done with the mains muted. For monitors, give the performers only minimum of aux signals they ask for to keep stage volumes and feedback at bay. Good, flat response monitors and and mics are key to feedback reduction. Also, proper aiming of mics and monitors assists in feedback reduction.
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