Volume 104, Issue 1, July 1998
Index of content:
- SPEECH PRODUCTION 
104(1998); http://dx.doi.org/10.1121/1.423250View Description Hide Description
Phonation onset is discussed in the framework of dynamical systems as a Hopf bifurcation, i.e., as a transition from damped to sustained vocal fold oscillations due to changes of parameters defining the underlying laryngeal configuration (e.g., adduction, subglottal pressure, muscular activity). An analytic envelope curve of the oscillation onset is deduced by analyzing the Hopf bifurcation in mathematical models of the vocal folds. It is governed by a single time constant which can be identified with the physiological parameter phonation onset time. This parameter reflects the laryngeal state prior to phonation and can be used as a quantitative classification criterion in order to assess the phonation onset in clinical diagnosis. The extraction of the phonation onset time from simulated time series using a simplified two-mass model and from digital high-speed videos is described in detail. It shows a good agreement between theory and measurement.
104(1998); http://dx.doi.org/10.1121/1.423298View Description Hide Description
Magnetic resonance imaging(MRI) was used to acquire vocal tract shapes of ten vowels /i, ɪ, ɛ, æ, ʌ, ɑ, ɔ, o, ʊ, u/ and two liquid approximants /ɝ, l/ for a 27-year-old adult female. These images were complemented with additional images acquired with electron beamcomputed tomography(CT) of /i/ and /ɑ/. Each 3-D shape was condensed into a set of cross-sectional areas of oblique sections perpendicular to the centerline of the vocal tract’s long axis, resulting in an “area function.” Formant frequencies computed for each area function showed reasonable similarity to those determined from the natural (recorded) speech of the imaged subject, but differences suggest that some of the imagedvocal tract shapes were articulated differently during imaging than during recording of natural speech, and also that imaging procedures may have compromised some accuracy for a few shapes. The formant calculations also confirmed the significant effect that the piriform sinus can have on lowering the formant frequencies. A comparison is made between area functions derived using both MRI and CT methods for the vowels /i/ and /ɑ/. Additionally, the area functions reported in this study are compared with those from two previous studies and demonstrate general similarities in shape but also obvious differences that can be attributed to anatomical differences of the imaged subjects and to differences in imaging techniques and image processing methods.