Art by Katsura
Overtone singing is a type of singing in which the singer manipulates the resonances (or formants) created as air travels from the lungs, past the vocal folds, and out the lips to produce a melody. The partials (fundamental and overtones) of a sound wave made by the human voice can be selectively amplified by changing the shape of the resonant cavities of the mouth, larynx and pharynx. This resonant tuning allows the singer to create apparently more than one pitch at the same time (the fundamental and a selected overtone), while in effect still generating a single fundamental frequency with his/her vocal folds. (Source)
Rainbow Voice reminded me of the film Baraka I mentioned recently.
Here is another work by David Hykes.
This reminded me of this scene from Cowboy Bebop…