Vowel duration as a perceptual cue to postvocalic consonant voicing in young children and adults

Vowel duration as a perceptual cue to postvocalic consonant voicing in young children and adults

Sue Ellen Krause

Rush-Presbyterian-St. Luke • Medical Center, 1753 West Congress Parkway, Chicago, Illinois 60612 {Received 20 February 1980; accepted for publication 30 November 1981)

Vowel duration is a powerful acoustic cue for adults' perception of postvocalic consonant voicing, but it has not been studied sufficiently in children. The purpose of the present work was to study the development of the use of this duration cue in 3-year-olds, 6-year-olds, and adults. The duration of the vowel was varied to construct three stimulus continua {BIP-BIB, POT-POD, BACK-BAG). The subjects, who had normal language, articulation skills, and hearing sensitivity, identified all stimuli from each of the three continua ten times. Significant developmental differences in the perceptual judgments of voicing were demonstrated. These differences were reflected in both the locus of the category boundary and the slope of the identification function.

PACS numbers: 43.70.Dn, 43.70.Ve, 43.70.Yg

INTRODUCTION

Vowel duration is a powerful acoustic cue in Ameri- can English for adults' perception of the voicing contrast in postvocalic consonants. x Specifically, the probability of a voiced response increases as preceding vowel duration increases (Denes, 1955; Raphael, 1972). Vowel duration plays a similar role in the realization of voicing contrasts of postvocalic consonants in speech production. Typically, vowels are longer before voiced than before voiceless consonants (Peterson and Lehiste, 1960; House and Fairbanks, 1953; House, 1961). Because this duration cue appears to be learned (Zimmerman and Sapon, 1958; House, 1961; Lehiste, 1970), it is of interest to examine the use of this cue in children developing speech.

Data pertaining to children's use of vowel duration as a voicing cue are provided primarily from measures of speech production based on imitation tasks. Naeser (1970) has suggested that a child as young as 22 months differentiates vowel length as a function of the voicing feature of the following target consonant, although the consonant itself is occasionally produced as voiceless. It is apparent, however, that children's use of this timing parameter is different from that of adults. The absolute vowel duration difference between the two

voicing contexts, as well as variability of segment duration, appears to decrease with age, suggesting the development of articulatory precision (Smith, 1978). Conflicting evidence regarding the direction of change in vowel duration as a function of age is provided by DiSimoni (1974), though this discrepancy may in part be attributable to the phonetic contexts of the actual test items.

The development of production of these durational differences may be related to gradual refinement of the motor control systems, to refinement of the perception of voicing, or to some combination of these two factors. However, far less is known about children's use of vowel duration as a cue to voicing of final consonants in speech perception than in speech production. In an attempt to address this problem the present study examined the value of precedingvowel duration in the perception of postvoealic consonant voicing for

children who had established phonological use of the voicing contrast but were at different stages of development.

I. METHOD

Subjects. Ten 3-year-olds (2'11-3:3 yrs., •=3:1), ten 6-year-olds (5'11-6:2, •=6'1), and ten adults (18-28, X-20) served as subjects. Each of the 3- and 6-year-old subjects had a normal medical history, demonstrated no unusual behavior problems, and came from a monolingual home in which general American English was spoken. During pretest screening, each child was tested to assure normal hearing sensitivity bilaterally for the octave frequencies between 250 and 4000 Hz; age level performance on the Peabody Picture Vocabulary Test (Dunn, 1959) and on the receptive and expressive portions of the Northwestern Syntax Screen- ing Test (Lee, 1971); and misarticulation of no more than three sounds on the Goldman-Fristoe Test of Articulation (Goldman and Fristoe, 1969) that were correctly articulated by 75% of his/her age group (Templin, 1957). Each subject was also required to discriminate between voiced and voiceless postvocalic stop consonants in natural speech versions of the test words.

The adults who participated in the study had no for- real training in phonetic transcription or acoustic phonetics. Subjects had no history of a speech, language, or hearing problem, and they used a general American dialect. The screening procedures for adults included reading the Sentence Test of the Fisher-Logemann Test of Articulation Competence (Fisher and Logemann, 1971) without articulation er- rors. Auditory sensitivity was assessed as for the children. Subjects also discriminated between the voiced and voiceless postvocalic stop consonants in natural speech.

Stimuli: The test stimuli were constructed to form

three continua of synthetic syllables, BIP-BIB, POT-POD, and BACK-BAG, 2 by temporally adjusting the steady state portion of the vowel. The stimuli were generated at the Massachusetts Institute of Technology using a digital simulation of a terminal

990 J. Acoust. Soc. Am. 71(4), April 1982 00014966/82/040990-06500.80 ¸ 1982 Acoustical Society of America 990

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analog speech synthesizer (Klatt, 1972). The synthesis values were chosen from measurements of natural

speech made by Klatt and from my own measurements from spectrograms of a male talker producing the stimulus words. Both the POT-POD and the BACK-

BAG continua were synthesized with a falling F1 final transition. No final F1 transition was included in the

BIP-BIB continuum, as is typical for the/I/vowel preceding a labial stop in natural speech. No final stop burst was used in any of the continua. The values of vowel-formant steady states, the starting frequency of the initial transitions, and the ending frequency of the final transitions are listed in Table I.

Within the approximate range of stimuli over which perceptual judgments change from 80% to 20% voice- lessness (Raphael, 1972), duration adjustments were made in 20-ms steps. Outside this range, the stimuli were varied in 40-ms steps to cover the full range of vowel durations that have been previously studied (Raphael, 1972; Peterson and Lehiste, 1960; House, 1961; DiSimoni, 1974; Krause, 1 976). This method of varying the step size reduced the potential number of stimuli, which was an important consideration in testing young children.

In the BIP-BIB continuum, each stimulus was synthesized with a 6-ms initial burst and a 4-ms voicing lag, followed by the vowel. The fundamental frequency, F0, fell linearly from 1'48 to 104 Hz. For both F2 and F3, the initial transition duration was 28 ms, and the final transition duration was 22 ms. Total vowel

duration, including initial transition, steady state, and final transition, variedfrom 50to 270 ms in 20-ms steps.

In the POT-POD continuum, each stimulus was synthesized with a 48-ms period of aspiration followed by the vowel. F0 fell linearly from 138 to 95 Hz. The final transition of both F1 and F2 was 50 ms in dura-

tion. Total vowel duration varied from 92 to 232 ms in 20-

ms steps and from 232 to 392 ms in 40-ms steps.

In the BACK-BAG continuum, each stimulus was

TABLE I. Formant values (in Hz)for starting frequency of initial transitions (I. T. ), vowel steady states (S.S. ), and ending frequency of final transitions (F. T. ) for the three synthetic stimulus continua.

Vowel formant

Stimulus Vowel

continuum segment F1 F2 F3

BIP-BIB I.T. 360 1560 2260

S.S. 360 1770 2440

F.T. 360 1570 2250

POT-POD I.T. 675 l160 2460

S.S. 675 l160 2460

F.T. 455 1425 2460

BACK-BAG I.T. 520 1305 1935

S.S. 655 1710 2305

F.T. 500 1860 2205

synthesized with a 6-ms burst and a 4-ms voicing lag followed by the vowel. F0 fell linearly from 149 to 102 Hz. F1, F2, and F3 all had a 50-ms initial transition and a 47-ms final transition. Total vowel duration

varied from 110 to 250 ms in 20-ms steps and from 250 to 410 ms in 40-ms steps.

Test procedures: The original synthesized stimuli were recorded on audiotape. They were then digitized via A/D conversion and stored on computer. For each continuum, ten trials of each stimulus (120 stimuli per continuum) were randomizod, output through D/A conversion, low-pass filtered, and then recorded on audiotape for playback to subjects. Natural speech versions of the six test stimuli were also digitized, randomized, and output in the same manner. The test stimuli were presented over a loudspeaker in a sound-attenuated room. For any one subject, seated in a relatively stable position, the stimuli varied ap- proximately 5 dB, between 75-80 dB (SPL).

A simple, colorful representation of each of the six test words was painted on a semi-opaque Plexiglas square. The word BIP was designated as the name of a clown. The pictures representing the two words for a particular stimulus continuum were inserted in two slots on the front panel of a response box during testing of that continuum. The subject indicated his/ her responses by pressing a button directly under the picture that represented the test word heard. When- ever the button was pressed, the corresponding picture was backlighted by a bulb built into the box.

Initially, subjects were presented practice trials of nontest words, CUP and BALL, to establish the asso- ciation between the auditory stimulus and the visual display. A period of special training was carried out for the one nonsense word (BIP) and for the word ex- pected to be unfamiliar to children (POD). Presenta- tion of tape-recorded natural speech versions of all test pairs served as a means for screening. The criterion for successful screening was eight out of ten consecutive trials from a block of 40 trials. During the screening and actual testing of synthetic stimuli, no verbal feedback to indicate correctness of response was provided. In order to promote maximal coopera- tion of the young listeners, there were breaks in testing as well as intermediate tangible rewards, and a prize at the end of testing sessions. One session was needed to complete the series of tests for the adults; three sessions for the 6-year-olds (one continuum per session); and an average of six sessions for the 3-year- olds (1/2 of one continuum per session). The children were also required to pass the screening criterion at the start of each session for the continuum to be tested

on a particular day. Order of presentation of the three continua and the left/right location of the picture referents were counterbalanced across subjects.

Data analysis: The percentage of responses judged as voiced for a stimulus value was calculated separately for each subject and each phonemic context (i.e., each continuum). The logistic form of the psychometric function was fit to each set of identification data

by maximum likelihood techniques (Bush, 1963). s

991 J. Acoust. Soc. Am., Vol. 71, No. 4, April 1982 Sue Ellen Krause: Development of voicing perception 991


Locus of the category boundary (hereafter referred to as boundary) was defined as the point along the vowel duration continuum at which 50% of the judgments were voiced and 50% were voiceless. The slope of the curve represents the rate of change of perceptual judgments from voiceless to voiced. The data from four 3-year- olds and one 6-year-old were excluded from statistical analysis because of failure' to meet the criterion that the boundary value fall within the range of vowel durations studied. 4

TABLE III. Means and standard deviations of slope according to age level and phonemic context.

Phonemic context

BIP-BIB POT-POD BACK-BAG

X SD X SD X SD

3 years 0.91 0.66 0.79 0.78 0.43 0.27 6 years 0.94 0.70 0.57 0.32 0.39 0.40 Adult 1.59 0.49 1.61 0.61 1.27 0.57

II. RESULTS

The group means and standard deviations for the boundary values are provided in Table II and for the slope values are provided in Table III. s Averaged group data and psychometric functions based on these data are plotted separately for each of the three phonemic contexts and the three groups of subjects in Fig. 1.

The perceptual data were examined with a 3 x 3 (age x phonemic context) analysis of variance for repeated measures, using the multivariate procedure (MANOVA); this procedure involves an orthogonal polynomial trans- formation of the individual boundary and slope data prior to analysis of variance (McCall and Applebaum, 1973). A separate analysis of variance was conducted for each dependent measure, boundary, and slope, using the 0.05 level of confidence. 6 Since the age x phonemic context interaction was not significant either for boundary or for slope, only the main effects for each factor will be addressed. When significant F ratios were obtained from these analyses, multiple-comparison procedures were performed to further delineate sources of significant effects using an adjusted significance level (c•-0.016).

The analysis of variance on boundary revealed a significant main effect for age [F(2, 22)-4.43, p <0.0242]. Progressively shorter values of the boundary were found with increasing age, although only the contrast between 3-year-olds and adults reached significance (t=2.97, p <0.0071). Thus 3-year-olds demonstrated a shift in voicing judgment at significantly longer vowel durations than adults.

The analysis of variance on the slope measure also revealed a significant effect for age [F(2, 22)= 14.99, p <0. 0001]. Multiple comparisons indicated significant differences in slope between 6-year-olds and adults (t- 5.03, p <0. 0001) and between 3-year-olds

TABLE II. Means and standard deviations of locus of the

category boundary (in ms) according to age level and phonemic context.

Phonemic context

BIP- BIB POT-POD BACK-BAG

Age X SD X SD X SD

3years 198.75 53.90 251.88 74.33 220.32 57.85 6years 168.22 23.98 227.67 29.61 211.63 36.28 Adult 153.32 19.11 200.34 24.46 224.34 33.01

and adults (t - 4.07, p < 0.0005). Inspection of the means indicates larger values of slope for the adults than for the children and similar values of slope in the two groups of children. Since slope values are directly related to the steepness of the psychometric function, the values presented here indicate that adults' judgments of "voicedness" shifted more rapidly along the vowel duration continuum. The shallower slopes in the children represent either a more gradual change or less of a change in judgment of postvocalic consonant voicing.

Not surprisingly, a significant effect for the phonemic context factor on boundary IF(2,44)= 13.24, p <0.0001] was found. The boundary for the BIP-BIB continuum occurred at a significantly shorter value than for the POT-POD continuum (t=4.30, p <0.0003) or the BACK-BAG continuum (t= 4.95, p <0.0001), while the boundary for the POT-POD and BACK-BAG contexts was essentially the same. The vowel in the BIP-BIB context is the lax vowel /I/, which is intrinsically short in duration, whereas the vowels in the POT-POD and BACK-BAG contexts, /Q/and /•e/, respectively, are intrinsically long in duration. The results from the multiple-comparison procedures strongly suggest that it is the effect of intrinsic vowel length rather than the place of articulation of the final consonant that accounts for the main effect.

A significant effect for the phonemic context factor was also obtained on the slope measure [F(2,44)=5.01, p <0. 0109]. A flatter slope was observed in the BACK- BAG context than in either the BIP-BIB or the POT-

POD contexts, although only the contrast between

BACK-B AG and BIP-BIB was significant (t-3.90, p < o. ooo8).

In summary, the data on boundary suggest that as the age of the listener increased, progressively shorter vowel durations were required to shift a listener's judgment of a postvocalic stop from voiceless to voiced. This pattern was essentially uniform across the three phonemic contexts tested. In addition, adults' response functions were steeper than those of the 3- or 6-year-olds, although the age effect for slope was virtually the same across phonemic contexts. Differences among boundary values in the different phonemic contexts were attributed primarily to vowel length and not to postvocalic consonantal place.

III. DISCUSSION

One possible explanation for the age differences on measures of boundary and slope may lie in the absence



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of multiple acoustic cues normally available for the postvocalic voicing distinction in natural speech. That is, when young listeners are forced to rely on preceding vowel duration, they require greater vowel duration to judge a consonant voiced. Moreover, the boundary for the 6-year-olds fell between that of the youngest and oldest subject groups, which suggests a developmental progression in the-use of this single acoustic parameter of voicing. Hogan and Rozsypal (1980) have recently suggested that the relative duration of syllabic segments, or possibly overall syllabic duration, has more important cue value than single acoustic parameters for adults' perception of final consonant voicing.

The group data show that "endpoint" stimuli were generally categorized as voiceless or voiced, respectively (Fig. 1). However, the response functions of the young listeners were shallower in slope than the adults'. This indicates that children made the voicing distinction best when the preceding vowel had an ex- , treme duration and that a larger range of stimuli carried more ambiguous cue value for the children than the adults. The flatter slopes in the children's response functions may be partly related to the fact that

these listeners did not always identify endpoint stimuli with 100% "accuracy."This, too, may be indicative of a developmental trend. Specifically, the ability to make use of an isolated acoustic parameter, even one which has an extreme cue value, for a phonemic distinction may develop with age.

The findings on the slope measure from the present study are comparable to the perceptual findings reported earlier by Zlatin (Zlatin, 1972; Zlatin and Koenigsknecht, 1975) on boundary width, in relation to developmental use of voice-onset time (VOT). Bound- ary width, which best differentiated the age groups Zlatin studied, decreased with age. The authors con- cluded that there was 4... a systematic reduction in the amount of difference required for making the distinction between prevocalic cognates on the basis of variations in voice onset time" (Zlatin and Koenigs- knecht, p. 548).

Considering the age groups studied, one might ex- pect more intersubject variability in the children's data (and likewise, more in the 3-year-olds' than the 6-year-olds') than in the adults'. Most of the children in the present study categorized stimuli in an essen-



tially "typical" way. In certain instances, children did not meet the preset criterion for boundary. The four subjects who were excluded on the basis of the BACK-BAG context responded to essentially all stimuli with BAG (i.e., voiced), which may indicate that they were attending to the presence of the falling F1 transition characteristic of the stimuli in the con-

tinuum. Support for this account is provided by spon- taneous comments from certain listeners. For exam-

ple, on a trial of the shortest stimulus, one subject stated, '½ He went BAG so fast," indicating that although she perceived and attended to the absolute duration of the stimulus as short, she did not use vowel duration as a voicing cue; rather, she was attending to another set of acoustic attributes of the signal to cue voicing. A similar argument can be proposed for those subjects who failed to meet the boundary criterion on the BIP-BIB series, which did not employ an F1 transition, by responding to essentially all stimuli with BIP. For some children the presence of a n F1 offset transition may always cue a voiced stop, and absence of an F1 transition may always cue a voiceless stop, independent of vowel duration (Liberman et al., 1958; cf. Raphael, 1972). It is possible that the relative cue value differs with respect to phonemic context. How- ever, since the continua in the present study did not covary systematically the different acoustic parameters for postvocalic consonant voicing, this remains an open issue for future research.

The boundary differences between phonemic contexts were explained on the basis of the inherent length of the vowels. The boundary was, in fact, a bit longer in the alveolar context than in the velar context (both of which contained intrinsically long vowels). This finding is consistent with previous findings reported in the production literature (House and Fairbanks, 1953; Peterson and Lehiste, 1960; House, 1961; Lehiste, 1970). How- ever, considering the small vowel duration differences as a function of place of articulation of the postvocalic consonant in speech production, it is certainly not sur- prising that significant differences in boundary were not demonstrated between the alveolar and velar con-

texts in the present study. The possibility of differences in such perceptual judgments is not ruled out, but further interpretation of these boundary and slope measures will require additional research specifically designed to assess the place contrast in a variety of phonemic contexts.

Finally, it is of interest to consider the developmental changes demonstrated in this study in relation to the relevant data on speech production. The present findings for the locus of the category boundary are most consistent with those of Smith (1978), who found progressively shorter vowels before voiced and voiceless alveolar stops with an increase in age. Smith also reported decreased variability in segment duration with increasing age, which was indicated by the boundary data from the present study as well. It appears that a refinement of vowel duration with an increase in age is demonstrated for both speech perception and production. Also noteworthy is that perceptual research on infant subjects in which vowel duration cued the voicing contrast has

994 J. Acoust. Soc. Am., Vol. 71, No. 4, April 1982

revealed noncategorical-like discriminations for postvocalic consonant voicing (Eilers, 1977; Eilers et al., 1977). This may indicate that a voicing contrast that depends on the vowel duration cue is more "difficult" to learn than other acoustic cues for postvocalic voicing contrasts. Research is now needed to further examine

use of this acoustic parameter by infants and by children progressing through prelinguistic and linguistic stages of development.

In summary, results from the present investigation indicate that when limited to the vowel duration parameter, children as young as three years of age are able to make postvocalic voicing judgments. Develop- mental differences in these judgments are reflected in two aspects of the response functions: (1) progressively shorter vowel durations can be used to shift a listener's judgment from a voiceless to a voiced consonant with increasing age; and (2) steeper slope values are demonstrated for adults than for children. Fur-

ther developmental research directed specifically to- ward covarying multiple acoustic parameters will be enlightening with respect to possible changes in the relative or combined strength of acoustic parameters as perceptual cues for voicing. Such studies applied to both normal and clinical populations will define proces- ses for normal perceptual development and may have potential for direct diagnostic and therapeutic applica- tions with speech- and language-impaired children.

ACKNOWLEDGMENTS

This article is based on my Ph. D. dissertation, Northwestern University. The work was supported in part by a dissertation research grant awarded through the Graduate School, Northwestern University. I am extremely grateful to Hilda B. Fisher, my dissertation committee chairperson, for her support and input during all phases of the project. My sincere appreciation is also extended to Dennis H. Klatt for providing his laboratory at the Massachusetts Institute of Technology and his expertise in synthesizing the stimuli for this study; Frederic L. Wightman for making available the Psycho- acoustics Research Laboratory at Northwestern Univer- sity and for his guidance on both theoretical and prac- tical issues I considered; Doris J. Kistler for her statistical advice; and David B. Pisoni, Robert E. Remez, and Linda B. Smith for their advice and comments on earlier versions of this manuscript.

1Vowel duration can serve as a cue to other segmental distinc- t-ions in speech such as postvocalic consonantal manner (e. g., Umeda, 1975) or suprasegmental distinctions such as stress (e.g., Fry, 1955), aithough the focus of this work is on its cue value to postvocalic voicing.

2Other than including a sample of each possible place of articulation for the stop plosives (i.e., bilabial, alveolar, and velar) in the postvocalic position, every attempt was made to stabilize the phonemic environment between the continua tested. To insure picturability of the test words and to maximize the number of words that would be familiar to the children, it was necessary to include one minimal pair (BIP-BIB) with an intrinsic vowel length different than the other two pairs.

3This procedure estimates the slope and intercept (i.e., locus of the category boundary) of the psychometric function by taking account of each trial as an independent binomial process.

Sue Ellen Krause: Development of voicing perception 994


4Three-year-old subject ,6 (T6) was excluded on the basis of boundary for the BIP-BIB continuum; T10 and six-year-old subject #4 (S4) were excluded on the basis of boundary for the BIP-BIB and BACK-BAG continua.

5All data for individual subjects are available in the original work (Krause, 1978).

SSince the correlation between the dependent measures, boundary and slope, of-0.2686, with df= 23, did not reach significance at the 0.05 level of confidence, separate analyses for boundary and slope were performed.

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stops in initial position," Lang. Speech 1, 153-167. McCall, R. B., and Applebaum, M. I. (1973). "Bias in the

analysis of repeated measures designs: Some alternative approaches," Child Dev. 44, 401-415.

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Vowel duration as a perceptual cue to postvocalic consonant voicing in young children and adults