The Ngbaka use an unusual instrument called a mbela, which is made with an arched branch and a string strung between the two ends and held in front of the musician's mouth.
Banda folk music includes ongo, a kind of trumpet made from wood or antelope horn.
Ongos are used in ceremonies and rituals, including adolescent initiation rites, in polyphonic ensembles of eighteen trumpets.
In fact it is based on repetition of periods of equal length, which each singer divides using different rhythmic figures specific to different repertoires and songs.
This interesting case of ethnomusicology and ethnomathematics creates a detailed surface and endless variations of not only the same period repeated, but the same piece of music.