I had occasion to visit a church recently for the purposes of giving some advice on elements of a sound system. Some of the complaints were mics feeding back at low volume and poor sound quality. On inspecting the system I was horrified to find that the pair of cheap speaker cabinets used for stage monitors were behind the musicians, mounted on the wall at more-or-less the same level as the microphones, pointing directly at them.

So I thought that this month I would talk about monitor systems, what they are, what to buy and how to set them up correctly.

What is a Monitor System?
A Monitor System is a separate (but connected) PA system which which allows musicians (and other stage performers) to hear themselves and their fellow musicians.

Why do You Need a Monitor System?
Musicians play better when they can hear themselves and their fellow musicians well.

Some instruments - voice being a prime example - are relatively quiet in comparison to a lot of modern amplified instruments and some acoustic instruments. While these quieter sources may be amplified by the PA system, the PA speakers are in front of the musicians, pointing away from them (well... they should be!). Some other instruments - particularly horns - are directional and throw the vast majority of their sound forwards, and very little backwards to any musicians behind the horns. A monitor system allows these instruments to be amplified and the sound sent to the onstage musicians so that everyone can hear everyone else.

Another problem that a monitor system improves on is the problem of delayed sound. Sound takes approximately 3 milliseconds (3ms) to travel one meter (about one foot per millisecond). While this seems fast for most purposes, consider that two musicians on opposite sides of a large stage of say, 10M wide will be hearing each other 30ms late. This is an appreciable delay and causes problems when the two musicians are trying to play in time with each other. Also, in our example of a vocalist singing with a band in our large church (with about 25m from the stage to the back wall), the delay is the time it takes for the amplified sound to travel to the rear wall and back to the stage again - 50m total and 150ms late.

The Components of a Monitor System
A monitor system is a simple one for most purposes. The first thing you need is for your mixing desk to have at least one extra output to send a signal to the monitors. Most mixing desks designed for live sound have a dedicated monitor output and a send control on each input channel. This allows you to send as much or as little of each input that you need to the monitors - effectively sending a different mix to the musicians. An Auxiliary (aux) send does exactly the same thing, but will often give you the option to send the signal 'pre-fade' or 'post-fade' (before or after the main faders). Monitor systems should be pre-fade, so that changes made to the main mix will not affect the monitor mix. Pre-fade is usually used as an effect send. If your mixer does not have monitor or aux sends, there is often a second output for the main mix and this can be used in a pinch, although this will not allow you to have a separate monitor mix.

The next stage in your monitor system is an amplifier - usually a single channel for each monitor mix. Two channel amps are often used to send two different monitor mixes. Your drummer, for instance, may want a monitor mix which is predominantly bass guitar, while your vocalist may want a mix which is mostly themselves (I speak from experience). As many free aux sends, amps and speakers as you have, you can have separate monitor mixes. Most PA amps are 4 Ohm, which means you can slave two 8 Ohm monitor speakers from each channel. This means that a single twin channel amp can drive four monitor cabinets with two different monitor mixes.

The third and final piece of the puzzle is the monitor cabinet (speakers and box) itself. Monitor cabinets are those wedge-shaped speaker boxes at the musician's feet. They are made this way for two reasons: to be unobtrusive from the audience perspective; and to project the sound in such a way that the onstage mics are less likely to pick up the sound from them and thus avoid feedback. Some monitor cabinets are "active" - they have an amplifier built into them. Active cabs usually have an output for one "slave" cab - allowing you to slave (connect) one "passive" (unamplified) monitor to them.

Placement & Tips
Monitors should, where possible, be on the floor in front of microphones, facing the back of the stage. This helps avoid feedback as their sound is projecting along the axis of the mic where the mic is least sensitive.

Get the best monitor cabs you can afford, as a bad quality sound will affect the performance of the musicians. Regardless of how good your main mix is, if the musicians are not happy with their monitor sound, they will think of you as a bad sound man.

If you have a musician who keeps on raising the volume on their amp or instrument, it is a sure sign that they cannot hear themselves properly. Raise their level in the monitor mix and they will be happier and make your job easier.

Drummers usually want to hear the bass guitar. Bassists need to hear the kick drum. You will find that on a concrete floor the musicians will need more kick in the monitor mix than they do on a wooden stage (as they cannot feel the pulse as well). Many vocalists and guitarists want mostly themselves in the monitors.