Acoustics and perception of overtone singing.

Overtone singing, a technique of Asian origin, is a special type of voice production resulting in a very pronounced, high and separate tone that can be heard over a more or less constant drone. An acoustic analysis is presented of the phenomenon and the results are described in terms of the classical theory of speech production. The overtone sound may be interpreted as the result of an interaction of closely spaced formants. For the lower overtones, these may be the first and second formant, separated from the lower harmonics by a nasal pole-zero pair, as the result of a nasalized articulation shifting from /c/ to /a/, or, as an alternative, the second formant alone, separated from the first formant by the nasal pole-zero pair, again as the result of a nasalized articulation around /c/. For overtones with a frequency higher than 800 Hz, the overtone sound can be explained as a combination of the second and third formant as the result of a careful, retroflex, and rounded articulation from /c/, via schwa /e/ to /y/ and /i/ for the highest overtones. The results indicate a firm and relatively long closure of the glottis during overtone phonation. The corresponding short open duration of the glottis introduces a glottal formant that may enhance the amplitude of the intended overtone. Perception experiments showed that listeners categorized the overtone sounds differently from normally sung vowels, which possibly has its basis in an independent perception of the small bandwidth of the resonance underlying the overtone. Their verbal judgments were in agreement with the presented phonetic-acoustic explanation.