This is a drum kit demonstration. A polyrhythm is when you can discern while listening more than one main beat tempos and that these do not sub divide into each other by multiples of 2. So a beat which contains tempos like 50, 100 and 200 is not a polyrhythm, but if you can hear tempos like 50, 75 and 80 all at once, then what you have there may very well be described as a polyrhythm. Mathematically speaking the tempo numbers make up the odd order fractions: thirds, fifths, sixths, sevenths, but not eighths or twelves if that makes sense. The even order ones are debatable in my opinion.
However polyrhythms are not tuples, which in this case means to say a “three against four” polyrhythm does not involve any triplets. Or that a “five against four” rhythm does not involve any pentuplets. This video shows a polyrhythm of 3/4 which means for each 4 quarter notes in the 4/4 measure, there is going to be exactly 3 snares or exactly an average of 0.75 snares per beat (which makes no sense) but you get the point. Hopefully. 🙂
Thanks to: http://f.unkster.tripod.com/polyrhythm.html