Placement of speakers within a venue is critical to the quality of sound reproduction. Most important is avoiding feedback from the stage. The house array should be at least five feet forward of the front line of stage mics. If the P.A. is hung (or stacked) too close to the front edge of the stage, then feedback is inevitable. Line arrays may let you shave this distance by a foot or so, thanks to their directional control, but placing conventional boxes close to the performers is a recipe for disaster. In club situations, you may be able to get the house crew to push the house stacks forward (i.e., toward the mix position) to help avoid this problem. If that's not practical, or if the P.A. is flown, then pull the stage monitors and vocal mics upstage a bit to get them farther behind the house stacks.
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Having addressed some of the physical aspects of loudspeaker placement, turn on the P.A. and focus your attention on ensuring that all drivers and amps are operating properly.
Many engineers use a favorite CD to tune P.A. systems. They become very familiar with a certain piece of music and they know how it should sound over a variety of different sources, such as headphones or studio monitors. Try listening to the piece of music through the P.A. and applying EQ until it “sounds right.” There's nothing wrong with this approach, but keep in mind that a CD — with its processing, compression and mastering — is not representative of the transients encountered with amplified live musicians. An alternative method employs using a vocal microphone for equalizing a system. (Carry your own mic for consistency and hygiene.) Talk into the mic to excite the resonant frequencies of the P.A. and then use EQ to correct them. In this case, you're using the same tools that you use in your live mix: compressors, equalizers and mics (as opposed to a CD).
The majority of sound systems are set up with the L/R bus feeding signal to a drive rack (or crossover), which in turn divides the audio signal into various bands and routes signal to various amps in a multi-way P.A. In this type of system, it is possible to send a signal to the subwoofers that has no business being there, such as a hi-hat or lead vocal mic. Hopefully, the engineer has the smarts to use a highpass filter to remove whatever low-frequency crud might make its way into a hi-hat mic in the first place — thus preventing that signal from ever reaching the subwoofer.