On the Acoustic Sensitivity of a Symmetrical Two-Mass Model of the Vocal Folds to the Variation of Control Parameters

The acoustic properties of a recently proposed two-mass model for vocal-fold oscillations are analysed in terms of a set of acoustic parameters borrowed from phenomenological glottal-flow signal models. The analysed vocal-fold model includes a novel description of flow separation within the glottal channel at a point whose position may vary in time when the channel adopts a divergent configuration. It also assumes a vertically symmetrical glottal structure, a hypothesis that does not hinder reproduction of glottal-flow signals and that reduces the number of control parameters of the dynamical system governing vocal-fold oscillations. Measuring the sensitivity of acoustic parameters to the variation of the model control parameters is essential to describe the actions that the modelled glottis employs to produce voiced sounds of different characteristics. In order to classify these actions, we applied an algorithmic procedure in which the implementation of the vocal-fold model is followed by a numerical measurement of the acoustic parameters describing the generated glottal-flow signal. We use this algorithm to generate a large database with the variation of acoustic parameters in terms of the model control parameters. We present results concerning fundamental frequency, intensity and pulse shape control in terms of subglottal pressure, muscular tension, and the effective mass of the folds participating in vocal-fold vibration. We also produce evidence for the identification of vocal-fold oscillation regimes with the first and second laryngeal mechanisms, which are the most common phonation modes used in voiced-sound production. In terms of the model, the distinction between these mechanisms is closely related to the detection of glottal leakage, i.e. to an incomplete glottal closure during vocal-fold vibration. The algorithm is set to detect glottal leakage when transglottal air flow does not reach zero during the quasi-closed phase. It is also designed to simulate electroglottographic signals with the vocal-fold model. Numerical results are compared with experimental electroglottograms. In particular, a strong correspondence is found between the features of experimental and numerical electroglottograms during the transition between different laryngeal mechanisms.