Jorge C. Lucero

Speech production variability in fricatives of children and adults: Results of functional data analysis

This study investigates token-to-token variability in fricative production of 5 year olds, 10 year olds, and adults. Previous studies have reported higher intrasubject variability in children than adults, in speech as well as nonspeech tasks, but authors have disagreed on the causes and implications of this finding. The current work assessed the characteristics of age-related variability across articulators (larynx and tongue) as well as in temporal versus spatial domains. Oral airflow signals, which reflect changes in both laryngeal and supralaryngeal apertures, were obtained for multiple productions of /h s z/. The data were processed using functional data analysis, which provides a means of obtaining relatively independent indices of amplitude and temporal (phasing) variability. Consistent with past work, both temporal and amplitude variabilities were higher in children than adults, but the temporal indices were generally less adultlike than the amplitude indices for both groups of children. Quantitative and qualitative analyses showed considerable speaker- and consonant-specific patterns of variability. The data indicate that variability in /s/ may represent laryngeal as well as supralaryngeal control and further that a simple random noise factor, higher in children than in adults, is insufficient to explain developmental differences in speech production variability.

A model of facial biomechanics for speech production

Modeling the peripheral speech motor system can advance the understanding of speech motor control and audiovisual speech perception. A 3-D physical model of the human face is presented. The model represents the soft tissue biomechanics with a multilayer deformable mesh. The mesh is controlled by a set of modeled facial muscles which uses a standard Hill-type representation of muscle dynamics. In a test of the model, recorded intramuscular electromyography (EMG) was used to activate the modeled muscles and the kinematics of the mesh was compared with 3-D kinematics recorded with OPTOTRAK. Overall, there was a good match between the recorded data and the models movements. Animations of the model are provided as MPEG movies.

Optimal glottal configuration for ease of phonation

Recent experimental studies have shown the existence of optimal values of the glottal width and convergence angle, at which the phonation threshold pressure is minimum. These results indicate the existence of an optimal glottal configuration for ease of phonation, not predicted by the previous theory. In this paper, the origin of the optimal configuration is investigated using a low dimensional mathematical model of the vocal fold. Two phenomena of glottal aerodynamics are examined: pressure losses due to air viscosity, and air flow separation from a divergent glottis. The optimal glottal configuration seems to be a consequence of the combined effect of both factors. The results agree with the experimental data, showing that the phonation threshold pressure is minimum when the vocal folds are slightly separated in a near rectangular glottis.

Dynamics of the two‐mass model of the vocal folds: Equilibria, bifurcations, and oscillation region

The dynamics of the large‐amplitude oscillation of the vocal folds is analyzed using the two‐mass model. First, the equilibrium positions are determined in the case of a rectangular prephonatory glottis, and the existence of two equilibrium positions besides the rest position is shown. Their stability is examined and a bifurcation diagram is derived with a normalized subglottal pressure and a coupling coefficient as control parameters. Phase plane plots are shown to illustrate the results. The cases of convergent and divergent prephonatory glottis are then briefly considered. The main results are finally discussed relative to previous analytical works; it is shown that they disprove the previous oscillation theory based on the existence of a glottal negative differential resistance.

Simulations of temporal patterns of oral airflow in men and women using a two-mass model of the vocal folds under dynamic control

In this study we use a low-dimensional laryngeal model to reproduce temporal variations in oral airflow produced by speakers in the vicinity of an abduction gesture. It attempts to characterize these temporal patterns in terms of biomechanical parameters such as glottal area, vocal fold stiffness, subglottal pressure, and gender differences in laryngeal dimensions. A two-mass model of the vocal folds coupled to a two-tube approximation of the vocal tract is fitted to oral airflow records measured in men and women during the production of /aha/ utterances, using the subglottal pressure, glottal width, and Q factor as control parameters. The results show that the model is capable of reproducing the airflow records with good approximation. A nonlinear damping characteristics is needed, to reproduce the flow variation at glottal abduction. Devoicing is achieved by the combined action of vocal fold abduction, the decrease of subglottal pressure, and the increase of vocal fold tension. In general, the female larynx has a more restricted region of vocal fold oscillation than the male one. This would explain the more frequent devoicing in glottal abduction-adduction gestures for /h/ in running speech by women, compared to men.

Time normalization of voice signals using functional data analysis

The harmonics-to-noise ratio (HNR) has been used to quantify the waveform irregularity of voice signals [Yumoto et al., J. Acoust. Soc. Am. 71, 1544-1550 (1982)]. This measure assumes that the signal consists of two components: a harmonic component, which is the common pattern that repeats from cycle-to-cycle, and an additive noise component, which produces the cycle-to-cycle irregularity. It has been shown [J. Qi, J. Acoust. Soc. Am. 92, 2569-2576 (1992)] that a valid computation of the HNR requires a nonlinear time normalization of the cycle wavelets to remove phase differences between them. This paper shows the application of functional data analysis to perform an optimal nonlinear normalization and compute the HNR of voice signals. Results obtained for the same signals using zero-padding, linear normalization, and dynamic programming algorithms are presented for comparison. Functional data analysis offers certain advantages over other approaches: it preserves meaningful features of signal shape, produces differentiable results, and allows flexibility in selecting the optimization criteria for the wavelet alignment. An extension of the technique for the time normalization of simultaneous voice signals (such as acoustic, EGG, and airflow signals) is also shown. The general purpose of this article is to illustrate the potential of functional data analysis as a powerful analytical tool for studying aspects of the voice production process.

THE MINIMUM LUNG PRESSURE TO SUSTAIN VOCAL FOLD OSCILLATION

In previous experimental studies it has been observed that the minimum lung pressure to sustain vocal fold oscillation after its onset is lower than the threshold pressure needed to initiate it. This phenomenon is studied analytically using a previous body-cover model of the vocal folds and applying the describing function method to the general case of large amplitude oscillations. It is shown that the phenomenon is a consequence of the nonlinear characteristic of the effective aerodynamic damping introduced by the air pressure acting on the vocal folds. The results predict a value for minimum sustaining pressure equal to half the threshold pressure for a rectangular prephonatory glottis, which is in the order of experimental results.

Phonation thresholds as a function of laryngeal size in a two-mass model of the vocal folds

This letter analyzes the oscillation onset-offset conditions of the vocal folds as a function of laryngeal size. A version of the two-mass model of the vocal folds is used, coupled to a two-tube approximation of the vocal tract in configuration for the vowel /a/. The standard male configurations of the laryngeal and vocal tract models are used as reference, and their dimensions are scaled using a single factor. Simulations of the vocal fold oscillation and oral output are produced for varying values of the scaling factor. The results show that the oscillation threshold conditions become more restricted for smaller laryngeal sizes, such as those appropriate for females and children.

On the relation between the phonation threshold lung pressure and the oscillation frequency of the vocal folds.

This Letter presents an extension of a previous equation for the phonation threshold pressure by Titze [I. R. Titze, J. Acoust. Soc. Am. 83, 1536-1552 (1988)]. The extended equation contains the vocal-fold oscillation frequency as an explicit factor. It is derived from the mucosal wave model of the vocal folds by considering the general case of an arbitrary time delay for the mucosal wave to travel the glottal height. The results are illustrated with a numerical example, which shows good qualitative agreement with experimental measures.

Relation between the phonation threshold pressure and the prephonatory glottal width in a rectangular glottis

Recent experimental measurements have shown a nonlinear relation between the phonation threshold pressure and the prephonatory glottal width, with a minimum for the threshold pressure which would indicate the existence of an optimal glottal width for ease of phonation [I. R. Titze et al., J. Acoust. Soc. Am. 97, 3080–3084 (1995)]. This relation is studied analytically using a simplified vocal fold model which includes an explicit term for the air pressure losses due to glottal viscous resistance. It is shown that the observed nonlinearity may be a consequence of the viscous pressure losses, which cause an increase of the threshold pressure at small values of the prephonatory glottal width.