Christine H. Shadle

A parametric study of the spectral characteristics of European Portuguese fricatives

Studies of Portuguese phonetics and phonology indicate that fricatives are central to some interesting features of the language, yet studies of Portuguese fricatives have been few and limited. In this study, Portuguese fricatives were analyzed in ways designed to enhance our description of the language and to increase our understanding of the production of fricatives. Corpora of Portuguese words containing /f, v, s, z, P, W/, nonsense words of the pattern =V1FV2= that follow Portuguese phonological rules, and sustained fricatives were recorded by four native speakers of European Portuguese (two men, two women). Results of analysis show that more than half of the voiced fricatives devoice; devoicing occurs more often in word-final fricatives. Averaged power spectra were computed for all fricatives and parameterized in order to aid comparisons across speaker and across corpus, and to gain insight into the production mechanisms underlying the language-specific variations. Substantial differences were found between spectra of voiced and unvoiced, same-place fricatives. The parameters spectral slope, frequency of maximum amplitude, and dynamic amplitude, derived from previous studies, behaved as predicted for changes in effort level, voicing, and location within the fricative. Changes in syllable stress, however, did not affect the fricatives in a manner consistent with effort level variation. Some combinations were also useful for separating the fricatives by place or by sibilance. r 2002 Elsevier Science Ltd. All rights reserved.

Pitch-scaled estimation of simultaneous voiced and turbulence-noise components in speech

Almost all speech contains simultaneous contributions from more than one acoustic source within the speakers vocal tract. In this paper, we propose a method-the pitch-scaled harmonic filter (PSHF)-which aims to separate the voiced and turbulence-noise components of the speech signal during phonation, based on a maximum likelihood approach. The PSHF outputs periodic and aperiodic components that are estimates of the respective contributions of the different types of acoustic source. It produces four reconstructed time series signals by decomposing the original speech signal, first, according to amplitude, and then according to power of the Fourier coefficients. Thus, one pair of periodic and aperiodic signals is optimized for subsequent time-series analysis, and another pair for spectral analysis. The performance of the PSHF algorithm is tested on synthetic signals, using three forms of disturbance (jitter, shimmer and additive noise), and the results were used to predict the performance on real speech. Processing recorded speech examples elicited latent features from the signals, demonstrating the PSHFs potential for analysis of mixed-source speech.

Fluid flow in a dynamic mechanical model of the vocal folds and tract. I. Measurements and theory

In this study, aerodynamic and acoustic measurements were obtained in a dynamic mechanical model of the larynx and vocal tract. The model consisted of a uniform duct, intersected by a pair of sinusoidally oscillating shutters. A controlled airflow along the duct was periodically disturbed by the action of the shutters and pressure, and flow velocity measurements were obtained in the region downstream. The velocity field in the duct could be decomposed into three distinct components: a mean flow, a fluctuating acoustic particle velocity, and a fluctuating nonacoustic velocity associated with the transport of vortices along the duct at the local mean flow velocity. Two theoretical models for sound radiation from the duct exit were investigated. The first was based on the in-duct acoustic field alone and was unable to provide a realistic prediction of the measured, radiated sound field except at the first formant of the duct. In the second a simple description of sound generation due to the interaction of vo...

Articulatory-Acoustic Relationships in Fricative Consonants

The work described in this paper is concerned with improving fricative models by investigating the acoustic mechanisms involved in their production more closely. Previous studies reviewed fall into four categories: general aeroacoustics, mechanical models of speech, analysis of speech, and theoretical models, including synthesis. The work reported here combines these various approaches. Three levels of experiments with mechanical models of increasing realism are described. As a result, two source mechanisms are identified: the obstacle source, in which sound is generated at an abrupt obstacle to the airflow, such as the teeth, downstream of the constriction, corresponding to /s, ∫/; and the wall source, in which sound is generated by striking a wall at an oblique angle to the flow, corresponding to /c, x/. Source spectra differ, and are given in each case. Comparison to speech data from which the models were derived supports and extends the conclusions.

Quantifying spectral characteristics of fricatives

In a search for spectral parameters that can be used to distinguish and to model fricatives, spectral moments, dynamic amplitude, and slope above maximum amplitude were computed for a fricative corpus including sustained fricatives at different effort levels, and fricatives in vowel context. Moments varied significantly by frequency range used in computation. M3 appeared to vary the least across fricative, contrasting with Forrest et al.s 1988 study. Dynamic amplitude separated sibilants and non-sibilants, as predicted; slope above the maximum amplitude varied significantly with effort level.

Fluid flow in a dynamic mechanical model of the vocal folds and tract. II. Implications for speech production studies

A companion paper [Barney et al., J. Acoust. Soc. Am. 105, 444–455 (1999)] presents measurements in a dynamic mechanical model (the DMM) of the vocal folds and vocal tract: It was shown that closer prediction of the radiated sound pressure was possible when nonacoustic (vortical) as well as acoustic components of the velocity in the duct were included. In this paper, using such a simple geometry to model the vocal tract is justified by comparing acoustic and aerodynamic measurements in the DMM to those made in vivo: sub- and supraglottal pressures, radiated pressure, and hot wire velocities. The DMM produces sound equivalent to weak, low-frequency falsetto. A Rothenberg mask was then placed on the end of the DMM, and two estimates of the glottal waveform were compared to velocities measured near the “glottis.” The results show that the glottal waveform does not resemble any hot wire velocities measured near the shutters; travel times for acoustic and nonacoustic components of the velocity field differ sig...

Articulatory–acoustic kinematics: The production of American English /s/

Due to its aerodynamic, articulatory, and acoustic complexities, the fricative /s/ is known to require high precision in its control, and to be highly resistant to coarticulation. This study documents in detail how jaw, tongue front, tongue back, lips, and the first spectral moment covary during the production of /s/, to establish how coarticulation affects this segment. Data were obtained from 24 speakers in the Wisconsin x-ray microbeam database producing /s/ in prevocalic and pre-obstruent sequences. Analysis of the data showed that certain aspects of jaw and tongue motion had specific kinematic trajectories, regardless of context, and the first spectral moment trajectory corresponded to these in some aspects. In particular contexts, variability due to jaw motion is compensated for by tongue-tip motion and bracing against the palate, to maintain an invariant articulatory-aerodynamic goal, constriction degree. The change in the first spectral moment, which rises to a peak at the midpoint of the fricative, primarily reflects the motion of the jaw. Implications of the results for theories of speech motor control and acoustic-articulatory relations are discussed.

An articulatory-acoustic-aerodynamic analysisof [s] in VCV sequences

Previous studies of the effect of vowel context on fricatives show seeming contradictions in the case of /s/: acoustic analysis shows the greatest context effect, while aerodynamic analysis shows relatively little effect, for the same subject. In this study, aerodynamic, acoustic, and articulatory data for the same subject producing /s, z/ in a variety of contexts were compared systematically. The strong acoustic effect of the /u - u/ context exists with /z/ as well as /s/, and appears to arise from a whistle-like source mechanism caused by lip rounding; the main tongue constriction does not appear to be immune to vowel context. Our interpretation of aerodynamic data as constrictions in series can be generalized to include the influence of lip rounding, thus: for this speaker and for these speaker-like sequences, the area of the vocal tract constriction for /s/ is independent of the vowel context but the overall aerodynamic effect does vary with lip rounding. Our aerodynamic and acoustic data seem to be consistent; both support the view that some rounding extends into the /s/ fricative.

Frication noise modulated by voicing, as revealed by pitch-scaled decomposition

A decomposition algorithm that uses a pitch-scaled harmonic filter was evaluated using synthetic signals and applied to mixed-source speech, spoken by three subjects, to separate the voiced and unvoiced parts. Pulsing of the noise component was observed in voiced frication, which was analyzed by complex demodulation of the signal envelope. The timing of the pulsation, represented by the phase of the anharmonic modulation coefficient, showed a step change during a vowel-fricative transition corresponding to the change in location of the noise source within the vocal tract. Analysis of fricatives [see text] demonstrated a relationship between steady-state phase and place, and f0 glides confirmed that the main cause was a place-dependent delay.

Pharyngeal articulation in the production of voiced and voiceless fricatives

A structural magnetic resonance imaging study has revealed that pharyngeal articulation varies considerably with voicing during the production of English fricatives. In a study of four speakers of American English, pharyngeal volume was generally found to be greater during the production of sustained voiced fricatives, compared to voiceless equivalents. Though pharyngeal expansion is expected for voiced stops, it is more surprising for voiced fricatives. For three speakers, all four voiced oral fricatives were produced with a larger pharynx than that used during the production of the voiceless fricative at the same place of articulation. For one speaker, pharyngeal volume during the production of voiceless labial fricatives was found to be greater, and sibilant pharyngeal volume varied with vocalic context as well as voicing. Pharyngeal expansion was primarily achieved through forward displacement of the anterior and lateral walls of the upper pharynx, but some displacement of the rear pharyngeal wall was also observed. These results suggest that the production of voiced fricatives involves the complex interaction of articulatory constraints from three separate goals: the formation of the appropriate oral constriction, the control of airflow through the constriction so as to achieve frication, and the maintenance of glottal oscillation by attending to transglottal pressure.