For those who think the human voice can produce only one note at a time, the resonant harmonies of throat-singing are surprising. In throat-singing, a singer can produce two or more notes simultaneously through a specialized vocalization technique taking advantage of the throat's resonance characteristics. Singers use a form of circular breathing which allows them to sustain multiple notes for long periods of time. Young Tuvan singers are trained from childhood through a sort of apprentice system to use the folds of the throat as reverberation chambers. Mark van Tongeren, an ethnomusicologist specializing in khöömei throat-singing, teaches the technique.