The term 'orchestration' refers to the idea of taking a part and re arranging the sounds that are used to play it. For example, if you took a Single Stroke Roll pattern that was intended to be played on the snare and played it in the floor tom, you will have orchestrated it. This same idea can apply to any element of drumming. So fills can be orchestrated, as can grooves. A second example of applying orchestration could be taking This Groove and playing the right hand on the ride and the left on the high tom. The basic part is still the same, but the 'voices' used are different.
In our rudiments lessons you will find many different orchestrations of the basic sticking and this has many different purposes. First of all if will increase the difficulty of the pattern, meaning more elements of your playing will improve more by learning the exercise. Specific orchestrations may be designed to highlight one area of your playing, in these cases a note will be included. It will also give you more realistic options for using the rudiment practically as most orchestration exercises can be used as fills very easily.
Generally, rudiment orchestration exercises will be based on a 'concept', meaning that each exercise has some kind of logical idea behind its creation. You will be able to build several different exercises from one concept, so it is worth understanding them as it will allow you to come up with infinitely more of your own ideas. An example of a concept would be to play a single stroke roll but change the voice on every numbered count. So with this idea you could play 4 snares, 4 high toms, 4 mid toms and 4 floor toms. Or you could mix the order up giving you many different orchestrations and fills from one simple concept. It is worth using some Manuscript Paper to expand on these concepts further with your own ideas.
Orchestration exercises are usually intended to be purely mechanical, meaning that they aren't necessarily designed to be used as a musical part. However, it is often the case that they just so happen to sound cool.