A technique which has served me well in composing, when I’m layering one part upon another, is to think in terms of call and response. In most situations, a new part takes shape as I listen to what I already have and try to figure out how to complement that. Keeping in mind the possibility that I won’t ever need the part I’m figuring out (I often mute or delete vast swaths of parts I make), I nevertheless sit there and try to come up with something that fits.
I’m learning to key in on little things in the music that I think of as “calls” that I can respond to. Calls are almost always rhythmic in nature—events that reoccur in regular ways. For example, maybe a bass part plays around the downbeat of every two bars (or once every 8 beats). If I’m layering another melodic part, instead of playing over the bass part, I can respond to it on the downbeat of every second bar (and every 8 beats thereafter). This creates a slow-moving call and response between the bass and the melody part. Recording a second part in this responding way to a first part requires a bit of restraint. My tendency is to want to play each part continuously, so I have to hold back, waiting for the part’s turn to respond to the bass part’s call.
Things get compositionally more interesting as you layer more and more parts, because your musical puzzle is becoming increasingly complex and thus you need to listen more closely to where the available spaces are and where the potential calls in need of responses are hiding. For example, let’s say that my second melodic part is a set of chords and my next (third) part is some kind of bell sound. (I like bells.) As I try to figure out where to place the bell, I now relate it to both the bass part on the downbeats and the responding chords every second measure. I can use the bell to cut the distance between the bass calls and chord responses by half, or I can go the other way and play the bell just once every two or four cycles of the bass.
Layering parts using this call and response method is one way to get the music listening to itself at the various layers of its rhythmic action. No matter how I choose to relate a new part to what is already recorded, the goal is to make the groove as lively and tensile as possible. In essence, I don’t just want a piece to merely have a beat; I want every part of the piece to be it’s own kind of beat. Adding parts in call and response thickens the texture of the piece and animates the music into a more unified and pulsating whole.