Acoustic Reinforcement of Vocal Fold Vibratory Behavior in Singing

In the model for the production of voice that has dominated the literature on speech and singing until relatively recently, the glottis acts as a source of volume velocity that is independent of the time-varying supraglottal acoustic load imposed by the maneuvers of the various articulators. This view was a valuable one in the effort to identify the primary acoustic parameters that convey the linguistic and emotional message intended by the speaker or singer and has been extremely useful in such practical applications as synthesis and analysis of speech and singing. However, much recent research has been focused on a reconsideration of the independent source-tract model. In such a reconsideration, the supraglottal system can be viewed as producing pressure variations above the glottis that may have two possible effects: (1) these pressures can affect the pattern of air flow within the glottis, with the motions of the vocal folds relatively unaffected, and (2) the resulting changes in glottal airflow and intraglottal pressure can alter the vibratory pattern of the vocal folds. The success of the independent source-tract model was based primarily on the high acoustic impedance of the glottal orifice compared to the impedance of the supraglottal vocal tract during the central or target segments of most vocalic speech sounds. Thus the supraglottal pressure in those cases was always small compared to the subglottal pressure. (We ignore in this paper the periodic variations in subglottal pressure that occur during voicing. Although the subglottal acoustic system is, of course, also of interest in some situations, it is not under active control and does not vary much during the act of speech or singing.) However, the independent source-tract model was also successful because of the relatively high impedance of the vibrating vocal cords or, more precisely, of the mechanical-aerodynamic system that is responsible for the generation of the periodic variations in glottal dimensions that are responsible for voice production, compared to the typical supraglottal acoustic impedance. Thus, even when a supraglottal articulatory constriction tended to cause enough oscillatory back pressure to affect the pattern of glottal air flow, the vibratory pattern of the vocal folds tended to remain the same, apparently until the constriction was enough to raise the average supraglottal pressure to an appreciable fraction of the subglottal pressure. The hedge "apparently" in the previous sentence is necessary, since this latter hypothesis has been documented only sparsely. This lack of empirical verification is probably due to the difficulty of recording laryngeal function during transient constrictions and the difficulty of ensuring that any change (or