A perceptual model of vowel recognition based on the auditory representation of American English vowels

A quantitative perceptual model of human vowel recognition based upon psychoacoustic and speech perception data is described. At an intermediate auditory stage of processing, the specific bark difference level of the model represents the pattern of peripheral auditory excitation as the distance in critical bands (barks) between neighboring formants and between the fundamental frequency (F0) and first formant (F1). At a higher, phonetic stage of processing, represented by the critical bark difference level of the model, the transformed vowels may be dichotomously classified based on whether the difference between formants in each dimension falls within or exceeds the critical distance of 3 bark for the spectral center of gravity effect [Chistovich et al., Hear. Res. 1, 185-195 (1979)]. Vowel transformations and classifications correspond well to several major phonetic dimensions and features by which vowels are perceived and traditionally classified. The F1-F0 dimension represents vowel height, and high vowels have F1-F0 differences within 3 bark. The F3-F2 dimension corresponds to vowel place of articulation, and front vowels have F3-F2 differences of less than 3 bark. As an inherent, speaker-independent normalization procedure, the model provides excellent vowel clustering while it greatly reduces between-speaker variability. It offers robust normalization through feature classification because gross binary categorization allows for considerable acoustic variability. There was generally less formant and bark difference variability for closely spaced formants than for widely spaced formants. These findings agree with independently observed perceptual results and support Stevens' quantal theory of vowel production and perceptual constraints on production predicted from the critical bark difference level of the model.     

Formant frequencies in Estonian folk singing.

Formant frequencies in an old Estonian folk song performed by two female voices were estimated for two back vowels /a/ and /u/, and for two front vowels /e/ and /i/. Comparison of these estimates with formant frequencies in spoken Estonian vowels indicates a trend of the vowels to be clustered into two sets of front and back ones in the F1/F2 plane. Similar clustering has previously been shown to occur in opera and choir singing, especially with increasing fundamental frequency. The clustering in the present song, however, may also be due to a tendency for a mid vowel to be realized as a higher-beginning diphthong, which is characteristic of the North-Estonian coastal dialect area where the singers come from. No evidence of a "singer's formant" was found.     

Detection and discrimination of synthetic English vowels by Old World monkeys (Cercopithecus, Macaca) and humans

Abilities to detect and discriminate ten synthetic steady-state English vowels were compared in Old World monkeys (Cercopithecus, Macaca) and humans using standard animal psychophysical procedures and positive-reinforcement operant conditioning techniques. Monkeys' detection thresholds were close to humans' for the front vowels /i-I-E-ae-E), but 10-20 dB higher for the back vowels /V-D-C-U-u/. Subjects were subsequently presented with groups of vowels to discriminate. All monkeys experienced difficulty with spectrally similar pairs such as /V-D/, /E-ae/, and /U-u/, but macaques were superior to Cercopithecus monkeys. Humans discriminated all vowels at 100% correct levels, but their increased response latencies reflected spectral similarity and correlated with higher error rates by monkeys. Varying the intensity level of the vowel stimuli had little effect on either monkey or human discrimination, except at the lowest levels tested. These qualitative similarities in monkey and human vowel discrimination suggest that some monkey species may provide useful models of human vowel processing at the sensory level.     

Vowel perception by adults and children with normal language and specific language impairment: based on steady states or transitions?

The current investigation studied whether adults, children with normally developing language aged 4-5 years, and children with specific language impairment, aged 5-6 years identified vowels on the basis of steady-state or transitional formant frequencies. Four types of synthetic tokens, created with a female voice, served as stimuli: (1) steady-state centers for the vowels [i] and [ae]; (2) voweless tokens with transitions appropriate for [bib] and [baeb]; (3) "congruent" tokens that combined the first two types of stimuli into [bib] and [baeb]; and (4) "conflicting" tokens that combined the transitions from [bib] with the vowel from [baeb] and vice versa. Results showed that children with language impairment identified the [i] vowel more poorly than other subjects for both the voweless and congruent tokens. Overall, children identified vowels most accurately in steady-state centers and congruent stimuli (ranging between 94%-96%). They identified the vowels on the basis of transitions only from "voweless" tokens with 89% and 83.5% accuracy for the normally developing and language impaired groups, respectively. Children with normally developing language used steady-state cues to identify vowels in 87% of the conflicting stimuli, whereas children with language impairment did so for 79% of the stimuli. Adults were equally accurate for voweless, steady-state, and congruent tokens (ranging between 99% to 100% accuracy) and used both steady-state and transition cues for vowel identification. Results suggest that most listeners prefer the steady state for vowel identification but are capable of using the onglide/offglide transitions for vowel identification. Results were discussed with regard to Nittrouer's developmental weighting shift hypothesis and Strange and Jenkin's dynamic specification theory.     

Piecewise--planar representation of vowel formant frequencies

The first three formant frequencies for 778 steady-state tokens of 30 nonretroflex vowel types uttered by a female speaker are found to lie close to a piecewise-planar surface (expressed numerically as 0.634F1 +0.603F2 -- 0.485F3 -- 366 = 0, for F2 greater than 0.027F1 +1692 and 0.686F1 -- 0.528F2 -- 0.501F3 +1569 = 0, otherwise). The rms distance of the vowels from this surface is only 86 Hz. The intersection between the two planes is a line of nearly constant F2, corresponding closely to the F2 of a uniform vocal tract of the same length as our speaker's. The piecewise-planar representation also suggests a way to test the hypotheses of uniform and nonuniform formant-frequency scaling between speakers.     

Cross-language acoustic similarity predicts perceptual assimilation of Canadian English and Canadian French vowels

Monolingual Peruvian Spanish listeners identified natural tokens of the Canadian French (CF) and Canadian English (CE) /?/ and /?/, produced in five consonantal contexts. The results demonstrate that while the CF vowels were mapped to two different native vowels, /e/ and /a/, in all consonantal contexts, the CE contrast was mapped to the single native vowel /a/ in four out of five contexts. Linear discriminant analysis revealed that acoustic similarity between native and target language vowels was a very good predictor of context-specific perceptual mappings. Predictions are made for Spanish learners of the /?/-/?/ contrast in CF and CE.     

The effect of reduced vowel working space on speech intelligibility in Mandarin-speaking young adults with cerebral palsy

The purpose of this study was to examine the effect of reduced vowel working space on dysarthric talkers' speech intelligibility using both acoustic and perceptual approaches. In experiment 1, the acoustic-perceptual relationship between vowel working space area and speech intelligibility was examined in Mandarin-speaking young adults with cerebral palsy. Subjects read aloud 18 bisyllabic words containing the vowels /i/, /a/, and /u/ using their normal speaking rate. Each talker's words were identified by three normal listeners. The percentage of correct vowel and word identification were calculated as vowel intelligibility and word intelligibility, respectively. Results revealed that talkers with cerebral palsy exhibited smaller vowel working space areas compared to ten age-matched controls. The vowel working space area was significantly correlated with vowel intelligibility (r=0.632, p<0.005) and with word intelligibility (r=0.684, p<0.005). Experiment 2 examined whether tokens of expanded vowel working spaces were perceived as better vowel exemplars and represented with greater perceptual spaces than tokens of reduced vowel working spaces. The results of the perceptual experiment support this prediction. The distorted vowels of talkers with cerebral palsy compose a smaller acoustic space that results in shrunken intervowel perceptual distances for listeners.     

Intelligibility of vowels sung by a countertenor

Ten American English vowels were sung in a /b/-vowel-/d/ consonantal context by a professional countertenor in full voice (at F0 = 130, 165, 220, 260, and 330 Hz) and in head voice (at F0 = 220, 260, 330, 440, and 520 Hz). Four identification tests were prepared using the entire syllable or the center 200-ms portion of either the full-voice tokens or the head-voice tokens. Listeners attempted to identify each vowel by circling the appropriate word on their answer sheets. Errors were more frequent when the vowels were sung at higher F0. In addition, removal of the consonantal context markedly increased identification errors for both the head-voice and full-voice conditions. Back vowels were misidentified significantly more often than front vowels. For equal F0 values, listeners were significantly more accurate in identifying the head-voice stimuli. Acoustical analysis suggests that the difference of intelligibility between head and full voice may have been due to the head voice having more energy in the first harmonic than the full voice.     

Acoustic and perceptual similarity of North German and American English vowels

Current theories of cross-language speech perception claim that patterns of perceptual assimilation of non-native segments to native categories predict relative difficulties in learning to perceive (and produce) non-native phones. Cross-language spectral similarity of North German (NG) and American English (AE) vowels produced in isolated hVC(a) (di)syllables (study 1) and in hVC syllables embedded in a short sentence (study 2) was determined by discriminant analyses, to examine the extent to which acoustic similarity was predictive of perceptual similarity patterns. The perceptual assimilation of NG vowels to native AE vowel categories by AE listeners with no German language experience was then assessed directly. Both studies showed that acoustic similarity of AE and NG vowels did not always predict perceptual similarity, especially for "new" NG front rounded vowels and for "similar" NG front and back mid and mid-low vowels. Both acoustic and perceptual similarity of NG and AE vowels varied as a function of the prosodic context, although vowel duration differences did not affect perceptual assimilation patterns. When duration and spectral similarity were in conflict, AE listeners assimilated vowels on the basis of spectral similarity in both prosodic contexts.     

The Relative Contributions of Speaking Fundamental Frequency and Formant Frequencies to Gender Identification Based on Isolated Vowels

The purpose of this study was to determine the accuracy with which listeners could identify the gender of a speaker from a synthesized isolated vowel based on the natural production of that speaker when (1) the fundamental frequency was consistent with the speaker's gender, (2) the fundamental frequency was inconsistent with the the speaker's gender, and (3) the speaker was transgendered. Ten male-to-female transgendered persons, 10 men and 10 women, served as subjects. Each speaker produced the vowels /i/, /u/, and //. These vowels were analyzed for fundamental frequency and the first three formant frequencies and bandwidths. Formant frequency and bandwidth information was used to synthesize two vowel tokens for each speaker, one at a fundamental frequency of 120 Hz and one at 240 Hz. Listeners were asked to listen to these tokens and determine whether the original speaker was male or female. Listeners were not aware of the use of transgendered speakers. Results showed that, in all cases, gender identifications were based on fundamental frequency, even when fundamental frequency and formant frequency information was contradictory.     

Acoustic variability within and across German, French, and American English vowels: phonetic context effects

Cross-language perception studies report influences of speech style and consonantal context on perceived similarity and discrimination of non-native vowels by inexperienced and experienced listeners. Detailed acoustic comparisons of distributions of vowels produced by native speakers of North German (NG), Parisian French (PF) and New York English (AE) in citation (di)syllables and in sentences (surrounded by labial and alveolar stops) are reported here. Results of within- and cross-language discriminant analyses reveal striking dissimilarities across languages in the spectral/temporal variation of coarticulated vowels. As expected, vocalic duration was most important in differentiating NG vowels; it did not contribute to PF vowel classification. Spectrally, NG long vowels showed little coarticulatory change, but back/low short vowels were fronted/raised in alveolar context. PF vowels showed greater coarticulatory effects overall; back and front rounded vowels were fronted, low and mid-low vowels were raised in both sentence contexts. AE mid to high back vowels were extremely fronted in alveolar contexts, with little change in mid-low and low long vowels. Cross-language discriminant analyses revealed varying patterns of spectral (dis)similarity across speech styles and consonantal contexts that could, in part, account for AE listeners' perception of German and French front rounded vowels, and "similar" mid-high to mid-low vowels.     

The effects of cross-generational and cross-dialectal variation on vowel identification and classification

Cross-generational and cross-dialectal variation in vowels among speakers of American English was examined in terms of vowel identification by listeners and vowel classification using pattern recognition. Listeners from Western North Carolina and Southeastern Wisconsin identified 12 vowel categories produced by 120 speakers stratified by age (old adults, young adults, and children), gender, and dialect. The vowels /?, o, ?, u/ were well identified by both groups of listeners. The majority of confusions were for the front /i, ?, e, ?, ?/, the low back /ɑ, ?/ and the monophthongal North Carolina /a?/. For selected vowels, generational differences in acoustic vowel characteristics were perceptually salient, suggesting listeners' responsiveness to sound change. Female exemplars and native-dialect variants produced higher identification rates. Linear discriminant analyses which examined dialect and generational classification accuracy showed that sampling the formant pattern at vowel midpoint only is insufficient to separate the vowels. Two sample points near onset and offset provided enough information for successful classification. The models trained on one dialect classified the vowels from the other dialect with much lower accuracy. The results strongly support the importance of dynamic information in accurate classification of cross-generational and cross-dialectal variations.     

A study of high front vowels with articulatory data and acoustic simulations

The purpose of this study is to test a methodology for describing the articulation of vowels. High front vowels are a test case because some theories suggest that high front vowels have little cross-linguistic variation. Acoustic studies appear to show counterexamples to these predictions, but purely acoustic studies are difficult to interpret because of the many-to-one relation between articulation and acoustics. In this study, vocal tract dimensions, including constriction degree and position, are measured from cinéradiographic and x-ray data on high front vowels from three different languages (North American English, French, and Mandarin Chinese). Statistical comparisons find several significant articulatory differences between North American English /i/ and Mandarin Chinese and French /i/. In particular, differences in constriction degree were found, but not constriction position. Articulatory synthesis is used to model the acoustic consequences of some of the significant articulatory differences, finding that the articulatory differences may have the acoustic consequences of making the latter languages' /i/ perceptually sharper by shifting the frequencies of F(2) and F(3) upwards. In addition, the vowel /y/ has specific articulations that differ from those for /i/, including a wider tongue constriction, and substantially different acoustic sensitivity functions for F(2) and F(3).     

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.     

Cross-linguistic studies of children's and adults' vowel spaces

This study examines cross-linguistic variation in the location of shared vowels in the vowel space across five languages (Cantonese, American English, Greek, Japanese, and Korean) and three age groups (2-year-olds, 5-year-olds, and adults). The vowels /a/, /i/, and /u/ were elicited in familiar words using a word repetition task. The productions of target words were recorded and transcribed by native speakers of each language. For correctly produced vowels, first and second formant frequencies were measured. In order to remove the effect of vocal tract size on these measurements, a normalization approach that calculates distance and angular displacement from the speaker centroid was adopted. Language-specific differences in the location of shared vowels in the formant values as well as the shape of the vowel spaces were observed for both adults and children.     

Dynamic spectral structure specifies vowels for children and adults

When it comes to making decisions regarding vowel quality, adults seem to weight dynamic syllable structure more strongly than static structure, although disagreement exists over the nature of the most relevant kind of dynamic structure: spectral change intrinsic to the vowel or structure arising from movements between consonant and vowel constrictions. Results have been even less clear regarding the signal components children use in making vowel judgments. In this experiment, listeners of four different ages (adults, and 3-, 5-, and 7-year-old children) were asked to label stimuli that sounded either like steady-state vowels or like CVC syllables which sometimes had middle sections masked by coughs. Four vowel contrasts were used, crossed for type (front/back or closed/open) and consonant context (strongly or only slightly constraining of vowel tongue position). All listeners recognized vowel quality with high levels of accuracy in all conditions, but children were disproportionately hampered by strong coarticulatory effects when only steady-state formants were available. Results clarified past studies, showing that dynamic structure is critical to vowel perception for all aged listeners, but particularly for young children, and that it is the dynamic structure arising from vocal-tract movement between consonant and vowel constrictions that is most important.     

Neighboring spectral content influences vowel identification

Four experiments explored the relative contributions of spectral content and phonetic labeling in effects of context on vowel perception. Two 10-step series of CVC syllables ([bVb] and [dVd]) varying acoustically in F2 midpoint frequency and varying perceptually in vowel height from [delta] to [epsilon] were synthesized. In a forced-choice identification task, listeners more often labeled vowels as [delta] in [dVd] context than in [bVb] context. To examine whether spectral content predicts this effect, nonspeech-speech hybrid series were created by appending 70-ms sine-wave glides following the trajectory of CVC F2's to 60-ms members of a steady-state vowel series varying in F2 frequency. In addition, a second hybrid series was created by appending constant-frequency sine-wave tones equivalent in frequency to CVC F2 onset/offset frequencies. Vowels flanked by frequency-modulated glides or steady-state tones modeling [dVd] were more often labeled as [delta] than were the same vowels surrounded by nonspeech modeling [bVb]. These results suggest that spectral content is important in understanding vowel context effects. A final experiment tested whether spectral content can modulate vowel perception when phonetic labeling remains intact. Voiceless consonants, with lower-amplitude more-diffuse spectra, were found to exert less of an influence on vowel perception than do their voiced counterparts. The data are discussed in terms of a general perceptual account of context effects in speech perception.     

Similarities of vowels in nonreverberant and reverberant fields

Perceptual distances among single tokens of American English vowels were established for nonreverberant and reverberant conditions. Fifteen vowels in the phonetic context (b-t), embedded in the sentence "Mark the (b-t) again" were recorded by a male talker. For the reverberant condition, the sentences were played through a room with a reverberation time of 1.2 s. The CVC syllables were removed from the sentences and presented in pairs to ten subjects with audiometrically normal hearing, who judged the similarity of the syllable pairs separately for the nonreverberant and reverberant conditions. The results were analyzed by multidimensional scaling procedures, which showed that the perceptual data were accounted for by a three-dimensional vowel space. Correlations were obtained between the coordinates of the vowels along each dimension and selected acoustic parameters. For both conditions, dimensions 1 and 2 were highly correlated with formant frequencies F2 and F1, respectively, and dimension 3 was correlated with the product of the duration of the vowels and the difference between F3 and F1 expressed on the Bark scale. These observations are discussed in terms of the influence of reverberation on speech perception.     

Thresholds for second formant transitions in front vowels

Formant dynamics in vowel nuclei contribute to vowel classification in English. This study examined listeners' ability to discriminate dynamic second formant transitions in synthetic high front vowels. Acoustic measurements were made from the nuclei (steady state and 20% and 80% of vowel duration) for the vowels /i, I, e, epsilon, ae/ spoken by a female in /bVd/ context. Three synthesis parameters were selected to yield twelve discrimination conditions: initial frequency value for F2 (2525, 2272, or 2068 Hz), slope direction (rising or falling), and duration (110 or 165 ms). F1 frequency was roved. In the standard stimuli, F0 and F1-F4 were steady state. In the comparison stimuli only F2 frequency varied linearly to reach a final frequency. Five listeners were tested under adaptive tracking to estimate the threshold for frequency extent, the minimal detectable difference in frequency between the initial and final F2 values, called deltaF extent. Analysis showed that initial F2 frequency and direction of movement for some F2 frequencies contributed to significant differences in deltaF extent. Results suggested that listeners attended to differences in the stimulus property of frequency extent (hertz), not formant slope (hertz/second). Formant extent thresholds were at least four times smaller than extents measured in the natural speech tokens, and 18 times smaller than for the diphthongized vowel /e/.