Many people are too concerned with the main PA and put the monitors as secondary in importance. Depending on the music and the budget the monitor system can be quite extensive. Most major concert artists use in-ear monitoring systems now days. A solo artist or a duo can often get by with a single monitor mix. Bands do also get by with a single mix, but it requires experience and co-operation.
It is best to have separate monitor mixes so that the musicians can hear what they need to hear. For most situations I run three or four monitor mixes. Four mixes would typically be a stage right mix, a center mix, a stage left mix, and a drum mix. A main console usually will allow for two to eight mixes. Concert systems and more complex band systems use a separate monitor console
What sounds should be in the monitors?
The answer to this question will depend on various things. Vocals and any acoustic instruments will need to be in the monitors. Then, those things needed for timing may need to be added. For lead players the timing often comes from the snare drum or high hat. For rhythm section players the kick drum may be important.
It will be easiest to hear what is needed if only those things that are absolutely necessary are in the monitors. If there is too much in the monitors or the things in the monitors are too loud it raises the overall volume on the stage and ultimately makes everything harder to hear. This is part of the reason that multiple monitor mixes can be of help. If a player on the right side of the stage needs to hear what a player on the left side of the stage is doing that can be turned up only in the right side mix, saving the volume that would be added by the center and left side monitors, and vise versa.
Each monitor mix should consist of an equalizer for the mix, a channel of an amplifier for the mix (two if bi-amped) and the monitor speaker or speakers. These components should be repeated for each mix that is run. The equalizer is primarily for feedback control. Good monitor speakers should sound pretty good without too much sound shaping but the room can cause some problems as far as the sound. If the stage is not built as well as it could be there may be some problems in the low-mids that may need some correction.
Reflections from walls and ceilings may cause feed back problems. The frequencies that are causing problems need to be isolated and cut on the eq. There is no specific way that the eq should look. The settings on the eq will depend on the speakers used and the environment of the stage. Be careful to cut only as much as needed to clear up the sound and prevent feedback. Cutting too much can damage the sound and make things harder to hear.
In-Ear Monitor Systems
Caution: When using in ear monitoring systems volume control is very important. The monitors are like ear buds that fit within the ear canal. Since there is no place else for the sound to go it is possible to damage your hearing if they are played too loud. There should be some form of limiting on the in ear mix to prevent excessive volume.
In ear monitors have several advantages, but the systems will need to be a bit more complex. The ears can be either wired or wireless. Most people use wired in ears for drums and keys and any players that have a station on stage. Players that move around need wireless systems. Most players will need and individual monitor mix for their ears as well. Background singers may be able to share a mix.
In ear monitors can allow each musician to hear exactly what they need to hear without adding to the stage volume as is the case with conventional monitors. Most people use some sort of ambiance mikes with the in ear system so that there will be some sound of the room and crowd heard.
The monitor system is very important to the performance. In most cases, good monitors will give opportunity to have a good show. Run only the things that need to be heard in the monitors to help contain the volume. Multiple mixes can help because the musician can hear the things that are important to them without blasting the other musicians. With good equipment and experience everybody will be able to hear on stage and the band can truly play as a team.