Hornbostel - Sachs
Sets of flutes with internal duct
The air itself is the vibrator in the primary sense. The standing waves are significantly confined within each flute, where a narrow duct directs the air stream against the sharp edge of a lateral orifice. Two or more flutes of more than one kind (open, partly-stopped or stopped) are combined to form a set.
Sets of dissimilar flutes with internal duct