Because monitors are on stage, close to the microphones, they can be more troublesome as regards feedback than the main PA speakers and often some equalisation is desirable to 'notch out' problem frequencies that can be identified by turning up the system gain during the soundtrack until the system starts to 'ring'. Graphic equalisers are routinely employed to cut frequency bands where feedback is becoming an issue. Using a model of graphic EQ with metering LEDs on each band will help identify problem frequencies more quickly if your ears are not yet attuned to such things. After the frequency of the first 'ring' has been identified, you can turn up the system gain a little more until the next feedback 'hot spot' is discovered, then this too can be pulled down in gain. Soon there comes a point when you can not EQ any more out of the sound without messing it up, simply because even a large graphic equaliser attenuates sections of the spectrum a third of an Octave or more wide, while the feedback peaks are only fractions of a semitone wide. Even time you apply EQ cut, you remove some of the sound you want to keep, though this is less serious on monitors than on the front of house speakers.
The alternative to a graphic EQ is, of course, a dedicated feedback suppressor . These are worth having if feedback is a particular problem, as they automatically lock onto the feedback frequencies and deploy a number of very narrow but deep notch filters that stabilise the system without significantly affecting the sound.
Level Setting
Having done the best you can with PA speaker, microphone and stage monitor placement, the next thing you will want to do is set up initial mix levels of the band members. Different engineers use different methods for setting these levels. After getting the DI (Direction injection) feeds working and checking that you are getting plenty of signal from close-microphone amplifiers and drums, set the vocal microphone levels, after which everything else has to be balanced to them, even if that means the overall level is not as high as you would have liked. The quick way to setting vocal microphone levels is as follows:
* Set the master level fader at around two-thirds maximum and the microphone faders at maximum. Turn the input gain trims right down.
* Gradually turn up the microphone gain trims one at a time until the microphone just starts to 'ring', then, back it off just a hint.
* When you have done this for all the vocal microphones, pull their faders back down to the unity gain position (full up is usually +10dB). This leaves you with around 10dB of headroom before feedback again becomes a problem.
* Now simply balance the instruments to this vocal level by doing a quick run-through of a song or two during the sound check.
If the guitar amplifiers are always too loud, and it is your band that you are engineering (rather than a client), consider using smaller combos or power soaks at smaller gigs, so that they can be played at a lower level without sacrificing tone. If you can run the backline at a level where the PA can be used to boost is slightly, you will have far more control when it comes to turning up for solos and so forth. If you are lucky, the presence of an audience will soak up some of the unwanted reflections and make your job easier during the show.
Usually you would set up monitors in a similar way to the main PA, by turning off the main PA then seeing how much monitor gain you can set on each microphone before you hear ringing. Again, back off from this maximum level by a few dBs to give some operating headroom. If the monitors are positioned properly and EQ used to tame the worst peaks, that is about the best you can do without using an automatic feedback eliminator.
