So, a 4/4 bar at 120bpm would create a polyrhythm with a 3/4 bar at 90bpm (120 x 3 ÷ 4). Or, in reverse, you would have to multiply the tempo of the 3/4 bar by four and divide by three to get the tempo of the 4/4 bar. To form a 3/4 over 4/4 cross-rhythm, for example, you must multiply the tempo of the 4/4 bar by three and divide by four to get the tempo for the 3/4 bar. To properly create a cross-rhythm, you must also multiply the original tempo by the ratio between to the two meters. Similarly, while layering these two rhythms will result in three bars of 4/4 over four bars of 3/4, an interesting syncopation, this isn’t a cross-rhythm. Instead, the tempo will result in the 3/4 bar lasting only three quarters of the length of the 4/4 bar. However, simply writing a pattern in 4/4 and switching the time signature to 3/4, for example, won’t produce a cross-rhythm. Most DAWs allow you to alter the meter of your track, Pro Tools included.
The Nine Inch Nails song La Mer uses a three-over-four rhythm, with the piano holding down a pattern in 3/4, while the drums remain in 4/4.
The track’s rhythm does not have to be created via a single voice, nor does it have to be created solely by percussive instruments. Utilising one rhythm for the voice and applying another with the beat is an important and powerful facet of polyrhythms and cross-rhythms. Polyrhythms and cross-rhythms, while uncommon in straightforward radio-friendly fare, are customary in many forms of contemporary music, from modern classical, improvised jazz and madcap progressive rock to almost all forms of sub-Saharan African music, as well as electronic music and hip-hop, in which many talented rappers and MCs enunciate polyrhythmically to the beat. These concepts are relatively easy to implement and can be incredibly rewarding in practice.
The theories at play here might seem off-putting to those lacking in conventional training but don’t despair. This will help ground things for the listener, giving them a simple hook to engage with alongside some more complex rhythms. But this can be taken further, by playing off various combinations of meters – not just three against four but two against three or four against five – all while retaining the simplicity of a more conventional rhythmic foundation. Triplets indicate a 3/4 rhythm atop the standard 4/4 meter. If you’ve ever employed a triplet rhythm inside a 4/4 meter, for example, then you already have some experience.
There’s a chance you’ve played with polyrhythms and cross-rhythms before, without even realising it. With cross-rhythms, the two different meters overlap throughout the song, while polyrhythms are used to accent bars or phrases. Polyrhythms and cross-rhythms see the use of two different overlapped time signatures.
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