Source-filter interaction in the opposite direction: subglottal coupling and the influence of vocal fold mechanics on vowel spectra during the closed phase

Studies of speech source-filter interaction usually investigate the effect of the speech transfer function (loading) on vocal fold vibration and the voice source. In this study we explore how vocal fold mechanics affect the transfer function throughout the glottal cycle, with emphasis on the closed phase. Coupling between the subglottal and supraglottal airways is modulated by the laryngeal impedance. Although coupling is generally thought to occur only during the open phase of vocal fold vibration, a posterior glottal opening and the vocal fold tissue itself can allow sound transmission, thereby introducing coupling during the closed phase as well. The impedance of the vocal fold tissue at closure is shown to be small enough to permit coupling throughout the phonatory cycle, even in the absence of a posterior glottal opening. Open- and closed-phase coupling is characterized using mathematical models of the subglottal and supraglottal airways, and the parallel laryngeal impedances of the membranous glottis, posterior glottal opening, and vocal fold tissue. Examples from sustained vowels are presented, using synchronous recordings of neck skin acceleration, laryngeal high-speed videoendoscopy, electroglottography, and radiated acoustic pressure.