A Resonator ukulele, whose sound is produced by one or more spun aluminum cones instead of the wooden soundboard, is classified as a plucked string instrument with 321.322 HornbostelSachs classification.
A resonator ukulele or "resophonic ukulele" is a ukulele whose sound is produced by one or more spun aluminum cones (resonators) instead of the wooden soundboard (ukulele top/face). String instrument (plucked) 321.322 (plucked) 
