A cross-rhythm is a combination of two or more time signatures, playing together over the same period of time – ie, sharing the same barlines – and thus at two or more different tempos.
The most rudimentary of cross-rhythms is the hemiola, or ‘two against three’, which you can easily demonstrate yourself. Tap a repeating three-beat pattern on a desk with your right hand, then tap with the left hand on beat one and in between beats two and three – and then, voila, your left hand is in 3/4 time, your right is in 2/4 at a lower tempo.
In this walkthrough, we’ll be taking things to a rather deeper level than that as we build up a polyrhythmic drum and percussion groove. The process, as illustrated below, is simple – make an audio or MIDI clip of a certain number of beats in length, then timestretch it to fit within a bar length set by a regular 4/4 drum beat. You’ll find, however, that the sonic results you get from it are anything but simple…
Step 1: The first thing to do is set out our 4/4 base, which comes courtesy of a Looptone kick-and-hi-hats loop, and a snare/clap backbeat. Rolling along at 125bpm, this is the beat against which the following cross-rhythmic parts will be juxtaposed.
Step 2: For our first cross-rhythm, let’s bring a 3/4 percussion part into play. We start by importing a 4/4 percussion loop, then simply dragging its right hand edge back to get rid of everything after the first three beats. The new clip is now effectively in 3/4 time, as is apparent when it’s looped.
Step 3: Now, with Logic Pro’s Flex Time timestretching system activated for the track, we stretch the three-beat clip out to the length of the full four-beat bar – as defined by the main drums – and loop it to fill the phrase. Playing this alongside the drum loop totally transforms the groove.
Step 4: 4/4 and 3/4 are the most conventional of time signatures, but things start to get temporally weird with 5/4, as demonstrated by this agogo bells line – hear how the extra beat interrupts the expected flow. While 3/4 crossed with 4/4 usually works well, getting 5/4 involved is always an experiment…
Step 5: In Logic Pro, MIDI is timestretched in the same way as audio, by Option-dragging either bottom corner of the MIDI clip. Pulling the five-beat agogo bells clip back to fit the bar speeds it up so that it sits awkwardly over the 4/4 beat. At first it sounds bizarre, but after a few cycles, the brain starts to make sense of it.
Step 6: In four-to-the-floor dance music, it’s best to keep your polyrhythmic activity to secondary percussion parts, rather than the kick or snare. Here, for example, we've shortened and stretched the snare/clap part to put it in 7/8 time, which breaks the backbeat. A cross-rhythm too far? We'll leave you to decide…
