From what I understand, polyrhythms are basicly the same as '
beats', which is the effect produced when two
frequencies that are close, but not the same are played together. The result is a
beating sound, as the frequencies match up and
drift apart.
A real life exaple is
helicopter rotors:
As the blades rotate, the blade moving away from you is
dopler-shifted to a slightly lower frequency, and the blade moving towards you is dopler-shifted to a slightly higher frequency (just like a police siren passing you - first it is
high pitched, then low once it passes you). The combination of the slightly seperated frequencies produces the beating sound that we
recognise as a helicopter.