Have you ever heard the saying, "Ask 100 engineers that question, and you'll get 100 different answers?" That can be said about how stage monitors are selected and placed. Whether it be wide dispersion, narrow dispersion, horns in, horns out, wedge angles, horn throat sizes – the list goes on and on. There are hundreds of products out there and just as many ways to achieve the same goal.
There must be some kind of method to the madness, right? Well, you are in luck, because there is. Different situations can call for different types of monitors placed in a variety of ways.
The Box
Typically you will only see one type of monitor box design on a stage. There are a couple different things to call it, but it is most commonly known as a "bass reflex box." Sometimes it is also known as a front-loaded box, vented enclosure or a ported box. This describes the box as having the driver(s) mounted directly onto the baffle board with a "port" or "vent" that is tuned at a specific frequency to extend low frequency response.
Typically there are two ways in which drivers are oriented in these boxes. Both share a common characteristic. They both have at least two of the following drivers, low frequency and high frequency. The traditional design has at least one woofer (for low frequency) mounted onto a baffle board. Adjacent to the woofer(s) is a horn lens with a high frequency driver mounted to it. The other type is a coaxial driver. This is a driver that actually has two drivers within one. It is a woofer, with a high frequency driver mounted directly behind it. Each of these has their own unique benefits and characteristics.
The traditional way of driver orientation continues to be a very popular box design. Generally, these are bigger than coaxial boxes because of the need to house the two or more separate drivers. For louder and bigger concerts, some manufacturers even make double 12-inch and double 15-inch monitors.
Coaxial box designs have gotten more popular over the years, and for a couple different reasons. Since both low and high drivers are in one physical driver, the actual box can be smaller. This is a great solution for small stages as well as a more pleasant look for television and/or broadcast. In addition, a problem some traditional box designs have is being able to perceptually hear where the low frequency and high frequency drivers are. This is not an issue in a coaxial design because both low frequency and high frequency are originating sound from the same physical point.
The Drivers
There is a seemingly unlimited variety of drivers these days as well as new technologies creating higher power handling, lighter weight and more efficiency. Low frequency drivers in monitors are typically anywhere from 8-inch to 15-inch in diameter, with a voice coil diameter ranging anywhere from 2 inches and higher. High frequency drivers are usually seen as being a 1-inch, 1.4-inch, 1.5-inch or 2-inch. This refers to the circular exit diameter at which sound exits the driver and is mounted onto the horn lens. The voice coil of these drivers are anywhere from 1-inch to 4-inches. The horn lenses that these high frequency drivers mount too come in a plethora of dispersion angles too.
Selecting a size of low frequency driver can depend on what you want. Typically the smaller the driver, the better the "warmth" or "solid punch" in the low mid range. This is usually expressed when talking about the human voice. There is a reason for this: Simply put, bigger drivers just have bigger cones. A bigger cone can be a bit more flimsy because there is a bigger area of paper (usually) that is unsupported from one end to the other. This can lead to a false representation of what is supposed to be reproduced.
Different types of high frequency drivers have unique characteristics too. A general rule of thumb is that the smaller a high frequency driver is, the more sibilant it will sound. However, bigger drivers can produce more SPL and have more of that "in-your-face" sound. Bigger drivers typically have more distortion to them as well. Two-inch diameter drivers used to be primarily used for bigger and louder monitors, but because of high distortion factors, manufacturers started to go with the smaller 1.5-inch and 1.4-inch drivers. Due to technology advancements with these distortion factors among other things, some manufacturers have been going back to the 2-inch drivers.
Selection
Selection of monitors is very important. There are a few things to think about that will make your selection easy. Proper selection will enable the highest output performance out of your monitor system as well as more feedback stability and an overall better attitude from the talent on stage.
Dispersion and wedge angle are key. Is your stage deep or shallow? Are the performers going to be placed in close proximity to the wedges, or are they going to be more than a couple feet away? Are the performers going to be standing in one spot or do they like to hear themselves everywhere on the stage? These are just some of the things to consider when selecting monitors.
Most touring monitors have a pretty narrow dispersion pattern. Two benefits that this gives are an increased SPL output in the high frequency section and more control of where the sound is going on stage or "less spillage." The high frequency diaphragm will put out a certain amount of energy, and that same amount of energy will be focused out into the angle of the horn lens. So in turn the smaller the horn lens angle, the higher the SPL output. A wider dispersion angle will spread more sound over a broader area, but will sacrifice some of the maximum SPL output.
Placement
Placement is one of the most important, and neglected aspects of stage monitoring. I can't remember how many times I've seen a guest monitor engineer place monitors incorrectly. This can create a situation that is unfixable, and just sound plain bad. There are a couple of techniques that can make it easy to get the most out of your monitor rig, and equally as easy to spot when placement needs to be adjusted.
We are going to assume that two monitors are being used at each location. Another assumption will be that these monitors are properly time aligned with a speaker controller.
Knowing what kind of cabinet is being used is very important, especially if it is a traditional or coaxial box type, and what dispersion angle is in the high frequency section. This is important because high frequencies are most subject to comb-filtering. A comb filter is created when two identical versions of the same signal are perceived at one point with one signal delayed from the other, causing constructive and destructive interference. The resulting frequency response consists of a series of regularly spaced spikes, resembling a comb. In short, comb filtering comes from two sources (monitors) that are either placed too closely and/or are directed too much toward one another. When this happens, as you move side to side in front of the monitors, you are changing how far away you are from one monitor relative to the other. So in the end the frequency response is different anywhere you are standing, making it nearly impossible to get rid of feedback and have a consistent tone.
So how do we keep from creating this vile, vicious comb-filtering? It is easy, actually – just properly space your monitors. Wherever the center of the two stage monitors are, is where you want each monitors high frequency pattern to die off. This means the wider the dispersion angle on your high frequency section, the more distance needed in between monitors.
Shown on Fig. 1 are a couple of ways to do this, as well as how not to. Some engineers like to put both monitors facing straight at the performer (Fig. 1C), while others prefer to have a little "angle in" (Fig. 1B). Both ways work as long as they are not too close (Fig. 1A). In layman's terms, placing monitors too close will create a variance in the high frequency response (comb-filtering) as you move from side to side. If spaced too far apart you will hear an absence of high frequency reproduction in the middle of the two monitors. There is a lot of theory to high frequency dispersion, but in short, changing the spacing and just listening to what happens is the best way to find out what works best. Different frequencies disperse at different degrees. A horn lens may have a 60 degree dispersion pattern, but at what frequency?
If you are using a traditionally-designed box, also keep in mind what side the high frequency section is on. Typically you should have either both HF sections out or in. If one HF is in and one is out, it will more than likely sound perceptually out of balance.
Proper stage monitor selection is about what works best for what kind of show you are using them for. In actualization it's not about what about looks the best on stage or on a computer screen; it's about what makes you look the best by sounding the best.
