Bandwidth of spectral resolution for two-formant synthetic vowels and two-tone complex signals

Spectral integration refers to the summation of activity beyond the bandwidth of the peripheral auditory filter. Several experimental lines have sought to determine the bandwidth of this "supracritical" band phenomenon. This paper reports on two experiments which tested the limit on spectral integration in the same listeners. Experiment I verified the critical separation of 3.5 bark in two-formant synthetic vowels as advocated by the center-of-gravity (COG) hypothesis. According to the COG effect, two formants are integrated into a single perceived peak if their separation does not exceed approximately 3.5 bark. With several modifications to the methods of a classic COG matching task, the present listeners responded to changes in pitch in two-formant synthetic vowels, not estimating their phonetic quality. By changing the amplitude ratio of the formants, the frequency of the perceived peak was closer to that of the stronger formant. This COG effect disappeared with larger formant separation. In a second experiment, auditory spectral resolution bandwidths were measured for the same listeners using common-envelope, two-tone complex signals. Results showed that the limits of spectral averaging in two-formant vowels and two-tone spectral resolution bandwidth were related for two of the three listeners. The third failed to perform the discrimination task. For the two subjects who completed both tasks, the results suggest that the critical region in vowel task and the complex-tone discriminability estimates are linked to a common mechanism, i.e., to an auditory spectral resolving power. A signal-processing model is proposed to predict the COG effect in two-formant synthetic vowels. The model introduces two modifications to Hermansky's [J. Acoust. Soc. Am. 87, 1738-1752 (1990)] perceptual linear predictive (PLP) model. The model predictions are generally compatible with the present experimental results and with the predictions of several earlier models accounting for the COG effect.     

Auditory normalization of French vowels synthesized by an articulatory model simulating growth from birth to adulthood

The present article aims at exploring the invariant parameters involved in the perceptual normalization of French vowels. A set of 490 stimuli, including the ten French vowels /i y u e ? o E oe (inverted c) a/ produced by an articulatory model, simulating seven growth stages and seven fundamental frequency values, has been submitted as a perceptual identification test to 43 subjects. The results confirm the important effect of the tonality distance between F1 and f0 in perceived height. It does not seem, however, that height perception involves a binary organization determined by the 3-3.5-Bark critical distance. Regarding place of articulation, the tonotopic distance between F1 and F2 appears to be the best predictor of the perceived front-back dimension. Nevertheless, the role of the difference between F2 and F3 remains important. Roundedness is also examined and correlated to the effective second formant, involving spectral integration of higher formants within the 3.5-Bark critical distance. The results shed light on the issue of perceptual invariance, and can be interpreted as perceptual constraints imposed on speech production.     

A comparison of vowel normalization procedures for language variation research

An evaluation of vowel normalization procedures for the purpose of studying language variation is presented. The procedures were compared on how effectively they (a) preserve phonemic information, (b) preserve information about the talker's regional background (or sociolinguistic information), and (c) minimize anatomical/physiological variation in acoustic representations of vowels. Recordings were made for 80 female talkers and 80 male talkers of Dutch. These talkers were stratified according to their gender and regional background. The normalization procedures were applied to measurements of the fundamental frequency and the first three formant frequencies for a large set of vowel tokens. The normalization procedures were evaluated through statistical pattern analysis. The results show that normalization procedures that use information across multiple vowels ("vowel-extrinsic" information) to normalize a single vowel token performed better than those that include only information contained in the vowel token itself ("vowel-intrinsic" information). Furthermore, the results show that normalization procedures that operate on individual formants performed better than those that use information across multiple formants (e.g., "formant-extrinsic" F2-F1).     

Acoustic correlates of the front/back vowel distinction: a comparison of transition onset versus "steady state"

This study investigated whether F2 and F3 transition onsets could encode the vowel place feature as well as F2 and F3 "steady-state" measures [Syrdal and Gopal, J. Acoust. Soc. Am. 79, 1086-1100 (1986)]. Multiple comparisons were made using (a) scatterplots in multidimensional space, (b) critical band differences, and (c) linear discriminant functional analyses. Four adult male speakers produced /b/(v)/t/, /d/(v)/t/, and /g/(v)/t/ tokens with medial vowel contexts /i,I, E, ey, ae, a, v, c, o, u/. Each token was repeated in a random order five times, yielding a total of 150 tokens per subject. Formant measurements were taken at four loci: F2 onset, F2 vowel, F3 onset, and F3 vowel. Onset points coincided with the first glottal pulse following the release burst and steady-state measures were taken approximately 60-70 ms post-onset. Graphic analyses revealed two distinct, minimally overlapping subsets grouped by front versus back. This dichotomous grouping was also seen in two-dimensional displays using only "onset" data as coordinates. Conversion to a critical band (bark) scale confirmed that front vowels were characterized by F3-F2 bark differences within a critical 3-bark distance, while back vowels exceeded the 3-bark critical distance. Using the critical distance metric onset values categorized front vowels as well as steady-state measures, but showed a 20% error rate for back vowels. Front vowels had less variability than back vowels. Statistical separability was quantified with linear discriminant function analysis. Percent correct classification into vowel place groups was 87.5% using F2 and F3 onsets as input variables, and 95.7% using F2 and F3 vowel. Acoustic correlates of the vowel place feature are already present at second and third formant transition onsets.     

Effects of spectral flattening on vowel identification

The identification of front vowels was studied in normal-hearing listeners using stimuli whose spectra had been altered to approximate the spectrum of vowels processed by auditory filters similar to those that might accompany sensorineural hearing loss. In the first experiment, front vowels were identified with greater than 95% accuracy when the first formant was specified in a normal manner and the higher frequency formants were represented by a broad, flat spectral plateau ranging from approximately 1600 to 3500 Hz. In the second experiment, the bandwidth of the first formant was systematically widened for stimuli with already flattened higher frequency formants. Normal vowel identification was preserved until the first formant was widened to six times its normal bandwidth. These results may account for the coexistence of abnormal vowel masking patterns (indicating flattened auditory spectra) and normal vowel recognition.     

Evolving theories of vowel perception

Research on the perception of vowels in the last several years has given rise to new conceptions of vowels as articulatory, acoustic, and perceptual events. Starting from a "simple" target model in which vowels were characterized articulatorily as static vocal tract shapes and acoustically as points in a first and second formant (F1/F2) vowel space, this paper briefly traces the evolution of vowel theory in the 1970s and 1980s in two directions. (1) Elaborated target models represent vowels as target zones in perceptual spaces whose dimensions are specified as formant ratios. These models have been developed primarily to account for perceivers' solution of the "speaker normalization" problem. (2) Dynamic specification models emphasize the importance of formant trajectory patterns in specifying vowel identity. These models deal primarily with the problem of "target undershoot" associated with the coarticulation of vowels with consonants in natural speech and with the issue of "vowel-inherent spectral change" or diphthongization of English vowels. Perceptual studies are summarized that motivate these theoretical developments.     

Developmental study of vowel formant frequencies in an imitation task.

Imitations of ten synthesized vowels were recorded from 33 speakers including men, women, and children. The first three formant frequencies of the imitations were estimated from spectrograms and considered with respect to developmental patterns in vowel formant structure, uniform scale factors for vowel normalization, and formant variability. Strong linear effects were observed in the group data for imitations of most of the English vowels studied, and straight lines passing through the origin provided a satisfactory fit to linear F1--F2 plots of the English vowel data. Logarithmic transformations of the formant frequencies helped substantially to equalize the dispersion of the group data for different vowels, but formant scale factors were observed to vary somewhat with both formant number and vowel identity. Variability of formant frequency was least for F1 (s.d. of 60 Hz or less for English vowels of adult males) and about equal for F2 and F3 (s.d. of 100 Hz or less for English vowels of adult males).     

Probing the independence of formant control using altered auditory feedback

Two auditory feedback perturbation experiments were conducted to examine the nature of control of the first two formants in vowels. In the first experiment, talkers heard their auditory feedback with either F1 or F2 shifted in frequency. Talkers altered production of the perturbed formant by changing its frequency in the opposite direction to the perturbation but did not produce a correlated alteration of the unperturbed formant. Thus, the motor control system is capable of fine-grained independent control of F1 and F2. In the second experiment, a large meta-analysis was conducted on data from talkers who received feedback where both F1 and F2 had been perturbed. A moderate correlation was found between individual compensations in F1 and F2 suggesting that the control of F1 and F2 is processed in a common manner at some level. While a wide range of individual compensation magnitudes were observed, no significant correlations were found between individuals' compensations and vowel space differences. Similarly, no significant correlations were found between individuals' compensations and variability in normal vowel production. Further, when receiving normal auditory feedback, most of the population exhibited no significant correlation between the natural variation in production of F1 and F2.     

Vowel recognition via cochlear implants and noise vocoders: effects of formant movement and duration

Previous work has demonstrated that normal-hearing individuals use fine-grained phonetic variation, such as formant movement and duration, when recognizing English vowels. The present study investigated whether these cues are used by adult postlingually deafened cochlear implant users, and normal-hearing individuals listening to noise-vocoder simulations of cochlear implant processing. In Experiment 1, subjects gave forced-choice identification judgments for recordings of vowels that were signal processed to remove formant movement and/or equate vowel duration. In Experiment 2, a goodness-optimization procedure was used to create perceptual vowel space maps (i.e., best exemplars within a vowel quadrilateral) that included F1, F2, formant movement, and duration. The results demonstrated that both cochlear implant users and normal-hearing individuals use formant movement and duration cues when recognizing English vowels. Moreover, both listener groups used these cues to the same extent, suggesting that postlingually deafened cochlear implant users have category representations for vowels that are similar to those of normal-hearing individuals.     

Perceptual differences between vowels located in a limited phonetic space

The perception of subphonemic differences between vowels was investigated using multidimensional scaling techniques. Three experiments were conducted with natural-sounding synthetic stimuli generated by linear predictive coding (LPC) formant synthesizers. In the first experiment, vowel sets near the pairs (i-I), (epsilon-ae), or (u-U) were synthesized containing 11 vowels each. Listeners judged the dissimilarities between all pairs of vowels within a set several times. These perceptual differences were mapped into distances between the vowels in an n-dimensional space using two-way multidimensional scaling. Results for each vowel set showed that the physical stimulus space, which was specified by the two parameters F1 and F2, was always mapped into a two-dimensional perceptual space. The best metric for modeling the perceptual distances was the Euclidean distance between F1 and F2 in barks. The second experiment investigated the perception of the same vowels from the first experiment, but embedded in a consonantal context. Following the same procedures as experiment 1, listeners' perception of the (bv) dissimilarities was not different from their perception of the isolated vowel dissimilarities. The third experiment investigated dissimilarity judgments for the three vowels (ae-alpha-lambda) located symmetrically in the F1 X F2 vowel space. While the perceptual space was again two dimensional, the influence of phonetic identity on vowel difference judgments was observed. Implications for determining metrics for subphonemic vowel differences using multidimensional scaling are discussed.     

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.     

Compensation following real-time manipulation of formants in isolated vowels

Auditory feedback influences human speech production, as demonstrated by studies using rapid pitch and loudness changes. Feedback has also been investigated using the gradual manipulation of formants in adaptation studies with whispered speech. In the work reported here, the first formant of steady-state isolated vowels was unexpectedly altered within trials for voiced speech. This was achieved using a real-time formant tracking and filtering system developed for this purpose. The first formant of vowel /epsilon/ was manipulated 100% toward either /ae/ or /I/, and participants responded by altering their production with average Fl compensation as large as 16.3% and 10.6% of the applied formant shift, respectively. Compensation was estimated to begin <460 ms after stimulus onset. The rapid formant compensations found here suggest that auditory feedback control is similar for both F0 and formants.     

Perceptuomotor bias in the imitation of steady-state vowels

Previous studies suggest that speakers are systematically inaccurate, or biased, when imitating self-produced vowels. The direction of these biases in formant space and their variation may offer clues about the organization of the vowel perceptual space. To examine these patterns, three male speakers were asked to imitate 45 self-produced vowels that were systematically distributed in F1/F2 space. All three speakers showed imitation bias, and the bias magnitudes were significantly larger than those predicted by a model of articulatory noise. Each speaker showed a different pattern of bias directions, but the pattern was unrelated to the locations of prototypical vowels produced by that speaker. However, there were substantial quantitative regularities: (1) The distribution of imitation variability and bias magnitudes were similar for all speakers, (2) the imitation variability was independent of the bias magnitudes, and (3) the imitation variability (a production measure) was commensurate with the formant discrimination limen (a perceptual measure). These results indicate that there is additive Gaussian noise in the imitation process that independently affects each formant and that there are speaker-dependent and potentially nonlinguistic biases in vowel perception and production.     

Vowel formant discrimination for high-fidelity speech

The goal of this study was to establish the ability of normal-hearing listeners to discriminate formant frequency in vowels in everyday speech. Vowel formant discrimination in syllables, phrases, and sentences was measured for high-fidelity (nearly natural) speech synthesized by STRAIGHT [Kawahara et al., Speech Commun. 27, 187-207 (1999)]. Thresholds were measured for changes in F1 and F2 for the vowels /I, epsilon, ae, lambda/ in /bVd/ syllables. Experimental factors manipulated included phonetic context (syllables, phrases, and sentences), sentence discrimination with the addition of an identification task, and word position. Results showed that neither longer phonetic context nor the addition of the identification task significantly affected thresholds, while thresholds for word final position showed significantly better performance than for either initial or middle position in sentences. Results suggest that an average of 0.37 barks is required for normal-hearing listeners to discriminate vowel formants in modest length sentences, elevated by 84% compared to isolated vowels. Vowel formant discrimination in several phonetic contexts was slightly elevated for STRAIGHT-synthesized speech compared to formant-synthesized speech stimuli reported in the study by Kewley-Port and Zheng [J. Acoust. Soc. Am. 106, 2945-2958 (1999)]. These elevated thresholds appeared related to greater spectral-temporal variability for high-fidelity speech produced by STRAIGHT than for formant-synthesized speech.     

The Acoustic Effects of Vowel Equalization Training in Singers

Vowel equalization is a technique that can be used by singers to achieve a more balanced vocal resonance, or chiaroscuro, by balancing corresponding front and back vowels, which share approximate tongue heights, and also high and low vowels by means of a more neutral or centralized lingual posture. The goal of this single group study was to quantify acoustic changes in vowels after a brief training session in vowel equalization. Fifteen young adults with amateur singing experience sang a passage and sustained isolated vowels both before and after a 15-minute training session in vowel equalization. The first two formants of the target vowels /e, i, ɑ, o, u/ were measured from microphone recordings. An analysis of variance was used to test for changes in formant values after the training session. These formant values mostly changed in a manner reflective of a more central tongue posture. For the sustained vowels, all formant changes suggested a more neutral tongue position after the training session. The vowels in the singing passage mostly changed in the expected direction, with exceptions possibly attributable to coarticulation. The changes in the vowel formants indicated that even a brief training session can result in significant changes in vowel acoustics. Further work to explore the perceptual consequences of vowel equalization is warranted.     

Level and Center Frequency of the Singer''s Formant

The "singer's formant" is a prominent spectrum envelope peak near 3 kHz, typically found in voiced sounds produced by classical operatic singers. According to previous research, it is mainly a resonatory phenomenon produced by a clustering of formants 3, 4, and 5. Its level relative to the first formant peak varies depending on vowel, vocal loudness, and other factors. Its dependence on vowel formant frequencies is examined. Applying the acoustic theory of voice production, the level difference between the first and third formant is calulated for some standard vowels. The difference between observed and calculated levels is determined for various voices. It is found to vary considerably more between vowels sung by professional singers than by untrained voices. The center frequency of the singer's formant as determined from long-term spectrum analysis of commercial recordings is found to increase slightly with the pitch range of the voice classification.     

Vowel spaces in Swedish adolescents with cochlear implants

This paper examines vowel production in Swedish adolescents with cochlear implants. Twelve adolescents with cochlear implants and 11 adolescents with normal hearing participated. Measurements were made of the first and second formants in all the nine long Swedish vowels. The values in hertz were bark-transformed, and two measures of the size of the vowel space were obtained. The first of them was the average Euclidean distance in the F1-F2 plane between the nine vowels and the mean F1 and F2 values of all the vowels. The second was the mean Euclidean distance in the F1-F2 plane between all the vowels. The results showed a significant difference for both vowel space measures between the two groups of adolescents. The cochlear implant users had a smaller space than the adolescents with normal hearing. In general, the size of the vowel space showed no correlations with measures of receptive and productive linguistic abilities. However, the results of an identification test showed that the listeners made more confusions of the vowels produced by speakers who had a small mean distance in the F1-F2 plane between all the vowels.     

Perceptual confusions of high-pitched sung vowels

Questions exist as to the intelligibility of vowels sung at extremely high fundamental frequencies and, especially, when the fundamental frequency (F0) produced is above the region where the first vowel formant (F1) would normally occur. Can such vowels be correctly identified and, if so, does context provide the necessary information or are acoustical elements also operative? To this end, 18 professional singers (5 males and 13 females) were recorded when singing 3 isolated vowels at high and low pitches at both loud and soft levels. Aural-perceptual studies employing four types of auditors were carried out to determine the identity of these vowels, and the nature of the confusions with other vowels. Subsequent acoustical analysis focused on the actual fundamental frequencies sung plus those defining the first 2 vowel formants. It was found that F0 change had a profound effect on vowel perception; one of the more important observations was that the target tended to shift toward vowels with an F1 just above the sung frequency.     

Can intrinsic vowel Fo be explained by source/tract coupling?

There is extensive evidence that in the same phonetic environment the voice fundamental frequency (Fo) of vowels varies directly with vowel "height." This Fo difference between vowels could be caused by acoustic interaction between the first vowel formant and the vibrating vocal folds. Since higher vowels have lower first formants than low vowels the acoustic interaction should be greatest for high vowels whose first formant frequencies are closer in frequency to Fo. Ten speakers were used to see if acoustic interaction could cause the Fo differences. The consonant [m] was recorded in the utterances [umu] and [ama]. Although the formant structure of [m] in [umu] and [ama] should not differ significantly, the Fo of each [m] allophone was significantly different. However, the Fo of each [m] allophone did not differ significantly from the Fo of the following vowel. These results did not support acoustic interaction. However, it is quite reasonable to conclude that the Fo variation of [m] was caused by coarticulatory anticipation of the tongue and jaw for the following vowel. Another experiment is offered in order to help explain the physical causes of intrinsic vowel Fo. In this experiment Fo lowering was found at the beginning of vowels following Arabic pharyngeal approximants. This finding indicates that the Fo of pharyngeal constricting vowels, e.g., [ae] and [a], might be lowered as a result of similar articulary movements, viz. tongue compression and active pharyngeal constriction.     

Multimodal Standardization of Voice Among Four Multicultural Populations Formant Structures

A stratified random sample of 20 males and 20 females matched for physiologic factors and cultural-linguistic markers was examined to determine differences in formant frequencies during prolongation of three vowels: [a], [i], and [u]. The ethnic and gender breakdown included four sets of 5 male and 5 female subjects comprised of Caucasian and African American speakers of Standard American English, native Hindi Indian speakers, and native Mandarin Chinese speakers. Acoustic measures were analyzed using the Computerized Speech Lab (4300B) from which formant histories were extracted from a 200-ms sample of each vowel token to obtain first formant (F1), second formant (F2), and third formant (F3) frequencies. Significant group differences for the main effect of culture and race were found. For the main effect gender, sexual dimorphism in vowel formants was evidenced for all cultures and races across all three vowels. The acoustic differences found are attributed to cultural-linguistic factors.