Effects of prosodic boundary on /aC/ sequences: articulatory results

This study presents EMA (electromagnetic articulography) data on articulation of the vowel /a/ at different prosodic boundaries in French. Three speakers of metropolitan French produced utterances containing the vowel /a/, preceded by /t/ and followed by one of six consonants /b d g f s S/ (three stops and three fricatives), with different prosodic boundaries intervening between the /a/ and the six different consonants. The prosodic boundaries investigated are the Utterance, the Intonational phrase, the Accentual phrase, and the Word. Data for the Tongue Tip, Tongue Body, and Jaw are presented. The articulatory data presented here were recorded at the same time as the acoustic data presented in Tabain [J. Acoust. Soc. Am. 113, 516-531 (2003)]. Analyses show that there is a strong effect on peak displacement of the vowel according to the prosodic hierarchy, with the stronger prosodic boundaries inducing a much lower Tongue Body and Jaw position than the weaker prosodic boundaries. Durations of both the opening movement into and the closing movement out of the vowel are also affected. Peak velocity of the articulatory movements is also examined, and, contrary to results for phrase-final lengthening, it is found that peak velocity of the opening movement into the vowel tends to increase with the higher prosodic boundaries, together with the increased magnitude of the movement between the consonant and the vowel. Results for the closing movement out of the vowel and into the consonant are not so clear. Since one speaker shows evidence of utterance-level articulatory declension, it is suggested that the competing constraints of articulatory declension and prosodic effects might explain some previous results on phrase-final lengthening.     

Effects of prosodic boundary on /aC/ sequences: acoustic results

This study presents various acoustic measures used to examine the sequence /a # C/, where "#" represents different prosodic boundaries in French. The 6 consonants studied are /b d g f s S/ (3 stops and 3 fricatives). The prosodic units investigated are the utterance, the intonational phrase, the accentual phrase, and the word. It is found that vowel target values, formant transitions into the stop consonant, and the rate of change in spectral tilt into the fricative, are affected by the strength of the prosodic boundary. F1 becomes higher for /a/ the stronger the prosodic boundary, with the exception of one speaker's utterance data, which show the effects of articulatory declension at the utterance level. Various effects of the stop consonant context are observed, the most notable being a tendency for the vowel /a/ to be displaced in the direction of the F2 consonant "locus" for /d/ (the F2 consonant values for which remain relatively stable across prosodic boundaries) and for /g/ (the F2 consonant values for which are displaced in the direction of the velar locus in weaker prosodic boundaries, together with those of the vowel). Velocity of formant transition may be affected by prosodic boundary (with greater velocity at weaker boundaries), though results are not consistent across speakers. There is also a tendency for the rate of change in spectral tilt moving from the vowel to the fricative to be affected by the presence of a prosodic boundary, with a greater rate of change at the weaker prosodic boundaries. It is suggested that spectral cues, in addition to duration, amplitude, and F0 cues, may alert listeners to the presence of a prosodic boundary.     

The articulatory kinematics of final lengthening

In order to understand better the phonetic control of final lengthening, the articulation of phrase-final syllables was compared with that of two other contexts known to increase syllable duration: accent and slow tempo. The kinematics of jaw movements in [pap] sequences and of lower lip movements in [pe] sequences for four subjects were interpreted in terms of a task-dynamic model. There was evidence of two different control strategies: decreasing intragestural stiffness to slow down some part of the syllable, and changing intergestural phasing to decrease overlap of the vowel gesture by the consonant. The first was used in slowing down tempo, whereas the second was used to increase the duration of accented syllables over unaccented syllables. Both strategies were implicated in phrase-final lengthening. In accented syllables, final closing gestures generally were longer and slower, but not more displaced. The two slowest subjects, however, used the other strategy in their slow-tempo final syllables. Final lengthening in reduced syllables was more difficult to interpret. The relationship between peak velocity and displacement suggested that a lesser stiffness is obscured by an increased gestural amplitude. Thus, by comparison to lengthening for accent, final lengthening is like a localized change in speaking tempo, although it cannot be equated directly with the specification of stiffness.     

Variability in production of the vowels /i/ and /a/

A hypothesis on the nature of articulatory targets for the vowels /i/ and /a/ is proposed, based on acoustic considerations and vowel articulations. The conjecture is that positioning of points on the tongue surface in a repetition experiment should be most accurate in the direction perpendicular to the vocal-tract midline, at the acoustically critical point of maximal constriction for each vowel. The hypothesis was tested by: examining x-ray microbeam data for three speakers, conducting a partial acoustical analysis, and performing a modeling study. Distributions were plotted of the midsagittal locations of three tongue points at the time of maximal excursion toward the vowel target for numbers of examples of the vowels, embedded in a variety of phonetic contexts. More variation was found along a direction parallel to the vocal tract midline than perpendicular to the midline, supporting the hypothesis. Statistics on formant values for one subject have been calculated, and pairwise regressions of displacement and formant data have been run. An articulatory synthesizer [Rubin et al., J. Acoust. Soc. Am. 70, 321-328 (1981)] has been manipulated through displacements similar to the subject's articulatory variation. Although articulatory synthesis showed systematic relationships between articulatory relationships and formant frequencies, there were no significant correlations between the subject's measured articulatory displacements and his formant data. These additional results raise questions about the methodology and point to the need for additional work for an adequate test of the hypothesis.     

The effect of emphatic stress on consonant vowel coarticulation

This study assessed the acoustic coarticulatory effects of phrasal accent on [V1.CV2] sequences, when separately applied to V1 or V2, surrounding the voiced stops [b], [d], and [g]. Three adult speakers each produced 360 tokens (six V1 contexts x ten V2 contexts x three stops x two emphasis conditions). Realizing that anticipatory coarticulation of V2 onto the intervocalic C can be influenced by prosodic effects, as well as by vowel context effects, a modified locus equation regression metric was used to isolate the effect of phrasal accent on consonantal F2 onsets, independently of prosodically induced vowel expansion effects. The analyses revealed two main emphasis-dependent effects: systematic differences in F2 onset values and the expected expansion of vowel space. By accounting for the confounding variable of stress-induced vowel space expansion, a small but consistent coarticulatory effect of emphatic stress on the consonant was uncovered in lingually produced stops, but absent in labial stops. Formant calculations based on tube models indicated similarly increased F2 onsets when stressed /d/ and /g/ were simulated with deeper occlusions resulting from more forceful closure movements during phrasal accented speech.     

Different phase-stable relationships of the upper lip and jaw for production of vowels and diphthongs.

Relational invariants have been reported in the timing of articulatory gestures across suprasegmental changes, such as rate and stress. In the current study, the relative timing of the upper lip and jaw was investigated across changes in both suprasegmental and segmental characteristics of speech. The onset of upper lip movement relative to the vowel-to-vowel jaw cycle during intervocalic bilabial production was represented as a phase angle, and analyzed across changes in stress, vowel height, and vowel/diphthong identity. Results indicated that the relative timing of the upper lip and jaw varied systematically with changes in stress and vowel/diphthong identity, while remaining constant across changes in vowel height. It appears that modifications in relative timing may be due to adjustments in the jaw cycle as a result of the compound nature of jaw movement for diphthongs as compared to vowels, with further modifications due to the effect of stress on these compound movements.     

Lip-jaw and tongue-jaw coordination during rate-controlled syllable repetitions

The present study investigated the relationship between functionally relevant compound gestures and single-articulator component movements of the jaw and the constrictors lower lip and tongue tip during rate-controlled syllable repetitions. In nine healthy speakers, the effects of speaking rate (3 vs 5 Hz), place of articulation, and vowel type during stop consonant-vowel repetitions (/pa/, /pi/, /ta/, /ti/) on the amplitude and peak velocity of differential jaw and constrictor opening-closing movements were measured by means of electromagnetic articulography. Rather than homogeneously scaled compound gestures, the results suggest distinct control mechanisms for the jaw and the constrictors. In particular, jaw amplitude was closely linked to vowel height during bilabial articulation, whereas the lower lip component amplitude turned out to be predominantly rate sensitive. However, the observed variability across subjects and conditions does not support the assumption that single-articulator gestures directly correspond to basic phonological units. The nonhomogeneous effects of speech rate on articulatory subsystem parameters indicate that single structures are differentially rate sensitive. On average, an increase in speech rate resulted in a more or less proportional increase of the steepness of peak velocity/amplitude scaling for jaw movements, whereas the constrictors were less rate sensitive in this respect. Negative covariation across repetitions between jaw and constrictor amplitudes has been considered an indicator of motor equivalence. Although significant in some cases, such a relationship was not consistently observed across subjects. Considering systematic sources of variability such as vowel height, speech rate, and subjects, jaw-constrictor amplitude correlations showed a nonhomogeneous pattern strongly depending on place of articulation.     

Patterns of interarticulator phasing and their relation to linguistic structure

Work by Tuller and Kelso [J. Acoust. Soc. Am. 76, 1030-1036 (1984)] and Kelso et al. [J. Phon. 14, 29-59 (1986)] has demonstrated stable relations between jaw and lip movements in (bV#CVb) utterances across rate and stress conditions. Specifically, the onset of lip movement toward the intervocalic consonant was found to be constant with respect to the vowel-to-vowel jaw cycle in both time and relative phasing. An attempt was made to replicate and extend this work by investigating interarticulator phase relations for utterances having a broader range of linguistic organization: In addition to rate and stress, syllable structure (open versus closed syllables) and identity of the intervocalic consonant (/p/ vs /m/) were manipulated. Results showed that the upper lip's lowering onset varied systematically with respect to the jaw vowel cycle as a function of both rate and stress. In addition, syllable structure and consonant identity influenced the relation of lip and jaw gestures. There was a general tendency for any condition that shortened the first vowel to produce earlier onsets of the upper lip relative to the jaw. However, the within-condition jaw cycle duration variability did not correlate with the within-condition variability in phase. Thus it seems that stable interarticulator phase relations maintain not only the integrity of phonological structure, as suggested by Kelso et al., but structural integrity at other levels of linguistic organization as well.     

A study of jaw coarticulatory resistance and aggressiveness for Catalan consonants and vowels

The goal of this study is to investigate coarticulatory resistance and aggressiveness for the jaw in Catalan consonants and vowels and, more specifically, for the alveolopalatal nasal //[symbol see text]/ and for dark /l/ for which there is little or no data on jaw position and coarticulation. Jaw movement data for symmetrical vowel-consonant-vowel sequences with the consonants /p, n, l, s, ∫, [ symbol see text], k/ and the vowels /i, a, u/ were recorded by three Catalan speakers with a midsagittal magnetometer. Data reveal that jaw height is greater for /s, ∫/ than for /p, [see text]/, which is greater than for /n, l, k/ during the consonant, and for /i, u/ than for /a/ during the vowel. Differences in coarticulatory variability among consonants and vowels are inversely related to differences in jaw height, i.e., fricatives and high vowels are most resistant, and /n, l, k/ and the low vowel are least resistant. Moreover, coarticulation resistant phonetic segments exert more prominent effects and, thus, are more aggressive than segments specified for a lower degree of coarticulatory resistance. Data are discussed in the light of the degree of articulatory constraint model of coarticulation.     

A modeling investigation of articulatory variability and acoustic stability during American English /r/ production

This paper investigates the functional relationship between articulatory variability and stability of acoustic cues during American English /r/ production. The analysis of articulatory movement data on seven subjects shows that the extent of intrasubject articulatory variability along any given articulatory direction is strongly and inversely related to a measure of acoustic stability (the extent of acoustic variation that displacing the articulators in this direction would produce). The presence and direction of this relationship is consistent with a speech motor control mechanism that uses a third formant frequency (F3) target; i.e., the final articulatory variability is lower for those articulatory directions most relevant to determining the F3 value. In contrast, no consistent relationship across speakers and phonetic contexts was found between hypothesized vocal-tract target variables and articulatory variability. Furthermore, simulations of two speakers' productions using the DIVA model of speech production, in conjunction with a novel speaker-specific vocal-tract model derived from magnetic resonance imaging data, mimic the observed range of articulatory gestures for each subject, while exhibiting the same articulatory/acoustic relations as those observed experimentally. Overall these results provide evidence for a common control scheme that utilizes an acoustic, rather than articulatory, target specification for American English /r/.     

Effects of deafness on acoustic characteristics of American English tense/lax vowels in maternal speech to infants

Recent studies have demonstrated that mothers exaggerate phonetic properties of infant-directed (ID) speech. However, these studies focused on a single acoustic dimension (frequency), whereas speech sounds are composed of multiple acoustic cues. Moreover, little is known about how mothers adjust phonetic properties of speech to children with hearing loss. This study examined mothers' production of frequency and duration cues to the American English tense/lax vowel contrast in speech to profoundly deaf (N?=?14) and normal-hearing (N?=?14) infants, and to an adult experimenter. First and second formant frequencies and vowel duration of tense (/i/,?/u/) and lax (/I/,?/?/) vowels were measured. Results demonstrated that for both infant groups mothers hyperarticulated the acoustic vowel space and increased vowel duration in ID speech relative to adult-directed speech. Mean F2 values were decreased for the /u/ vowel and increased for the /I/ vowel, and vowel duration was longer for the /i/, /u/, and /I/ vowels in ID speech. However, neither acoustic cue differed in speech to hearing-impaired or normal-hearing infants. These results suggest that both formant frequencies and vowel duration that differentiate American English tense/lx vowel contrasts are modified in ID speech regardless of the hearing status of the addressee.     

Articulatory tradeoffs reduce acoustic variability during American English /r/ production.

The American English phoneme /r/ has long been associated with large amounts of articulatory variability during production. This paper investigates the hypothesis that the articulatory variations used by a speaker to produce /r/ in different contexts exhibit systematic tradeoffs, or articulatory trading relations, that act to maintain a relatively stable acoustic signal despite the large variations in vocal tract shape. Acoustic and articulatory recordings were collected from seven speakers producing /r/ in five phonetic contexts. For every speaker, the different articulator configurations used to produce /r/ in the different phonetic contexts showed systematic tradeoffs, as evidenced by significant correlations between the positions of transducers mounted on the tongue. Analysis of acoustic and articulatory variabilities revealed that these tradeoffs act to reduce acoustic variability, thus allowing relatively large contextual variations in vocal tract shape for /r/ without seriously degrading the primary acoustic cue. Furthermore, some subjects appeared to use completely different articulatory gestures to produce /r/ in different phonetic contexts. When viewed in light of current models of speech movement control, these results appear to favor models that utilize an acoustic or auditory target for each phoneme over models that utilize a vocal tract shape target for each phoneme.     

Effects and modeling of phonetic and acoustic confusions in accented speech

Accented speech recognition is more challenging than standard speech recognition due to the effects of phonetic and acoustic confusions. Phonetic confusion in accented speech occurs when an expected phone is pronounced as a different one, which leads to erroneous recognition. Acoustic confusion occurs when the pronounced phone is found to lie acoustically between two baseform models and can be equally recognized as either one. We propose that it is necessary to analyze and model these confusions separately in order to improve accented speech recognition without degrading standard speech recognition. Since low phonetic confusion units in accented speech do not give rise to automatic speech recognition errors, we focus on analyzing and reducing phonetic and acoustic confusability under high phonetic confusion conditions. We propose using likelihood ratio test to measure phonetic confusion, and asymmetric acoustic distance to measure acoustic confusion. Only accent-specific phonetic units with low acoustic confusion are used in an augmented pronunciation dictionary, while phonetic units with high acoustic confusion are reconstructed using decision tree merging. Experimental results show that our approach is effective and superior to methods modeling phonetic confusion or acoustic confusion alone in accented speech, with a significant 5.7% absolute WER reduction, without degrading standard speech recognition.     

A parametric model of the vocal tract area function for vowel and consonant simulation

A model of the vocal-tract area function is described that consists of four tiers. The first tier is a vowel substrate defined by a system of spatial eigenmodes and a neutral area function determined from MRI-based vocal-tract data. The input parameters to the first tier are coefficient values that, when multiplied by the appropriate eigenmode and added to the neutral area function, construct a desired vowel. The second tier consists of a consonant shaping function defined along the length of the vocal tract that can be used to modify the vowel substrate such that a constriction is formed. Input parameters consist of the location, area, and range of the constriction. Location and area roughly correspond to the standard phonetic specifications of place and degree of constriction, whereas the range defines the amount of vocal-tract length over which the constriction will influence the tract shape. The third tier allows length modifications for articulatory maneuvers such as lip rounding/spreading and larynx lowering/raising. Finally, the fourth tier provides control of the level of acoustic coupling of the vocal tract to the nasal tract. All parameters can be specified either as static or time varying, which allows for multiple levels of coarticulation or coproduction.     

American and Swedish children's acquisition of vowel duration: effects of vowel identity and final stop voicing

Vowel durations typically vary according to both intrinsic (segment-specific) and extrinsic (contextual) specifications. It can be argued that such variations are due to both predisposition and cognitive learning. The present report utilizes acoustic phonetic measurements from Swedish and American children aged 24 and 30 months to investigate the hypothesis that default behaviors may precede language-specific learning effects. The predicted pattern is the presence of final consonant voicing effects in both languages as a default, and subsequent learning of intrinsic effects most notably in the Swedish children. The data, from 443 monosyllabic tokens containing high-front vowels and final stop consonants, are analyzed in statistical frameworks at group and individual levels. The results confirm that Swedish children show an early tendency to vary vowel durations according to final consonant voicing, followed only six months later by a stage at which the intrinsic influence of vowel identity grows relatively more robust. Measures of vowel formant structure from selected 30-month-old children also revealed a tendency for children of this age to focus on particular acoustic contrasts. In conclusion, the results indicate that early acquisition of vowel specifications involves an interaction between language-specific features and articulatory predispositions associated with phonetic context.     

The relationship of vocal tract shape to three voice qualities

Three-dimensional vocal tract shapes and consequent area functions representing the vowels [i, ae, a, u] have been obtained from one male and one female speaker using magnetic resonance imaging (MRI). The two speakers were trained vocal performers and both were adept at manipulation of vocal tract shape to alter voice quality. Each vowel was performed three times, each with one of the three voice qualities: normal, yawny, and twangy. The purpose of the study was to determine some ways in which the vocal tract shape can be manipulated to alter voice quality while retaining a desired phonetic quality. To summarize any overall tract shaping tendencies mean area functions were subsequently computed across the four vowels produced within each specific voice quality. Relative to normal speech, both the vowel area functions and mean area functions showed, in general, that the oral cavity is widened and tract length increased for the yawny productions. The twangy vowels were characterized by shortened tract length, widened lip opening, and a slightly constricted oral cavity. The resulting acoustic characteristics of these articulatory alterations consisted of the first two formants (F1 and F2) being close together for all yawny vowels and far apart for all the twangy vowels.     

How far, how long: on the temporal scope of prosodic boundary effects

Acoustic lengthening at prosodic boundaries is well explored, and the articulatory bases for this lengthening are becoming better understood. However, the temporal scope of prosodic boundary effects has not been examined in the articulatory domain. The few acoustic studies examining the distribution of lengthening indicate that boundary effects extend from one to three syllables before the boundary, and that effects diminish as distance from the boundary increases. This diminishment is consistent with the pi-gesture model of prosodic influence [Byrd and Saltzman, J. Phonetics 31, 149-180 (2003)]. The present experiment tests the preboundary and postboundary scope of articulatory lengthening at an intonational phrase boundary. Movement-tracking data are used to evaluate durations of consonant closing and opening movements, acceleration durations, and consonant spatial magnitude. Results indicate that prosodic boundary effects exist locally near the phrase boundary in both directions, diminishing in magnitude more remotely for those subjects who exhibit extended effects. Small postboundary effects that are compensatory in direction are also observed.     

Effects of stress and final-consonant voicing on vowel production: articulatory and acoustic analyses

Durations of the vocalic portions of speech are influenced by a large number of linguistic and nonlinguistic factors (e.g., stress and speaking rate). However, each factor affecting vowel duration may influence articulation in a unique manner. The present study examined the effects of stress and final-consonant voicing on the detailed structure of articulatory and acoustic patterns in consonant-vowel-consonant (CVC) utterances. Jaw movement trajectories and F 1 trajectories were examined for a corpus of utterances differing in stress and final-consonant voicing. Jaw lowering and raising gestures were more rapid, longer in duration, and spatially more extensive for stressed versus unstressed utterances. At the acoustic level, stressed utterances showed more rapid initial F 1 transitions and more extreme F 1 steady-state frequencies than unstressed utterances. In contrast to the results obtained in the analysis of stress, decreases in vowel duration due to devoicing did not result in a reduction in the velocity or spatial extent of the articulatory gestures. Similarly, at the acoustic level, the reductions in formant transition slopes and steady-state frequencies demonstrated by the shorter, unstressed utterances did not occur for the shorter, voiceless utterances. The results demonstrate that stress-related and voicing-related changes in vowel duration are accomplished by separate and distinct changes in speech production with observable consequences at both the articulatory and acoustic levels.     

Acoustic and perceptual cues for compound-phrasal contrasts in Vietnamese

This paper reports two series of experiments that examined the phonetic correlates of lexical stress in Vietnamese compounds in comparison to their phrasal constructions. In the first series of experiments, acoustic and perceptual characteristics of Vietnamese compound words and their phrasal counterparts were investigated on five likely acoustic correlates of stress or prominence (f0 range and contour, duration, intensity and spectral slope, vowel reduction), elicited under two distinct speaking conditions: a "normal speaking" condition and a "maximum contrast" condition which encouraged speakers to employ prosodic strategies for disambiguation. The results suggested that Vietnamese lacks phonetic resources for distinguishing compounds from phrases lexically and that native speakers may employ a phrase-level prosodic disambiguation strategy (juncture marking), when required to do so. However, in a second series of experiments, minimal pairs of bisyllabic coordinative compounds with reversible syllable positions were examined for acoustic evidence of asymmetrical prominence relations. Clear evidence of asymmetric prominences in coordinative compounds was found, supporting independent results obtained from an analysis of reduplicative compounds and tone sandhi in Vietnamese [Nguye;n and Ingram, 2006]. A reconciliation of these apparently conflicting findings on word stress in Vietnamese is presented and discussed.     

Developmental and cross-linguistic variation in the infant vowel space: the case of Canadian English and Canadian French

This article describes the results of two experiments. Experiment 1 was a cross-sectional study designed to explore developmental and cross-linguistic variation in the vowel space of 10- to 18-month-old infants, exposed to either Canadian English or Canadian French. Acoustic parameters of the infant vowel space were described (specifically the mean and standard deviation of the first and second formant frequencies) and then used to derive the grave, acute, compact, and diffuse features of the vowel space across age. A decline in mean F1 with age for French-learning infants and a decline in mean F2 with age for English-learning infants was observed. A developmental expansion of the vowel space into the high-front and high-back regions was also evident. In experiment 2, the Variable Linear Articulatory Model was used to model the infant vowel space taking into consideration vocal tract size and morphology. Two simulations were performed, one with full range of movement for all articulatory paramenters, and the other for movement of jaw and lip parameters only. These simulated vowel spaces were used to aid in the interpretation of the developmental changes and cross-linguistic influences on vowel production in experiment 1.