Temporal aspects of the voiced–voiceless distinction in speech development of young Dutch children

Journal of Phonetics (1993) 21, 313-327

Temporal aspects of the voiced-voiceless distinction in speech development of young Dutch children Cecile T. L. Kuijpers Institute of Phonetic Sciences , University of Amsterdam, Herengracht 338, 1016 CG Amsterdam, The Netherlands Received 3rd October 1990, and in revised form 5th October 1992

Several temporal phenomena have been examined in the speech of six four-year-old and six six-year-old Dutch children. Intervocalic closure and burst durations of voiced and voiceless stops, as well as preceding vowel durations, were compared to study developmental patterns. Although the you nger children produce longer segmental durations, relative differences in voiced and voiceless closure duration and burst duration seem to correspond between the two age groups. In the same way, relative durational differences between phonologically short and long vowels are produced in an ad ult-like way by both of these groups of children. However, the temporal adjustment between vowel and consonant in the VC sequence displays a developmental trend. Although adult-like co-ordination of vowel and closure duration in the VC sequence with voiced context has been acquired at the age of four, only the older children have a relative shortening of the vowel in the voiceless context. The durational differences can be interpreted as evidence of development from a "syllable-independent" mechanism towards a "syllableintegrated" mechanism with increase of consonantal influence across the syllable boundary .

1. Introduction

Research concerning voiced and voiceless plosives shows that many factors account for the distinction in word-final and in word-medial position. For adult speech, several acoustic parameters have been described extensively by Lisker (1953) and Sharf (1964). This paper will be concerned with the development of three factors that involve segmental timing: the closure duration of voiceless plosives is longer than that of voiced plosives and the release in voiceless plosives is characterized by a longer noise burst than is found in voiced plosives. In addition, vowels preceding voiced or voiceless consonants show just the inverse durational characteristics; vowel duration preceding voiced consonants is longer than that preceding voiceless consonants, and several articulatory and auditory hypotheses have been formulated to explain this phenomenon of temporal compensation between consonant and preceding vowel (Siis, 1971; Raphael, 1981; Port, 1981; Luce & Charles-Luce, 1985; Kluender, Diehl & Wright, 1988; Fowler, 1992) . 0095-4470/93/030313 + 15 $08.00/ 0

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One finding to note is that in intervocalic position the durational differences between vowels preceding voiced vs. voiceless consonants are known to be less important than in word-final position. In English the lengthening of the vowel as a cue to final voicing is very prominent, particularly when compared to other languages. In word-medial intervocalic position , however, the preceding vocalic behaviour is a much less salient acoustic cue to voicing. Moreover, a study by Davis & Summers (1989) shows that the differences in closure duration and preceding vowel duration, as well as the negative correlation between them , is produced less consistently in VCV sequences than in VCV sequences. Therefore , the authors argue that the stops after unstressed vowels can be considered as not word-final but initial to the following stressed syllable . These durational characteristics of the voicing contrast can be found in many languages, such as English (House & Fairbanks , 1953 ; Chen, 1970; Lehiste , 1970; Davis & Van Summers , 1989), German (van Dommelen, 1982) , and Dutch (Slis & Cohen , 1969a,b) . Apparently, within the voiced and voiceless context, adults make use of a set of rules which transforms phonemes into phonetic units containing the appropriate durational values. The temporal compensation between vowel and consonant in the VC-unit , as in the English words rabid vs. rapid (Lisker , 1978), demands a precise co-ordination of gestures and can be considered to be part of internal motor programming. However, it is unknown whether speakers make use of a look-ahead mechanism or whether the negative correlation between V and C is just inherent to the articulatory movements . From a developmental point of view, English-speaking children have been examined in their production of voiced and voiceless consonants in word-final position (DiSimoni, 1974a,b; Higgs & Hodson, 1978; Smith, 1978; Raphael , Dorman & Geffner, 1980). In word-final position , speech utterances of pre-school children manifest differences in closure duration and preceding vowel duration between voiced and voiceless consonants that are generally quite adult-like. However , some results from these studies contradict each other, possibly due to different experimental set-ups . Smith (1978), using a method of nonsense word repetition , for words such as bab, bdbab and babab, concludes that three-year-old children already behave in adult-like ways with respect to closure duration in word-final and in intervocalic plosives. Also, the preceding vowel duration does show the inverse durational relationship with respect to following consonant duration . In final position, the difference in durations for vowels preceding voiced vs. voiceless consonants was 106 ms in the speech of the four-year-old children and 62 ms in the speech of the adults, whereas in intervocalic position this difference was reduced to 58 ms for the four-year-olds and 28 ms for the adults. DiSimoni (1974a), using another imitation task involving monosyllabic CVC nonsense words, indicates that relative closure durations of children are similar to those of adults, although a slower and more variable performance in speech production is present; the speaking rate of three- to four-year-old children is approximately 15% slower compared to that of adults (see also Kent & Forner, 1980). On the other hand, in a second study using the same procedure, DiSimoni (1974b) presents results for the speech of three-year-old children indicating that vowel duration does not differ significantly preceding a voiced vs. a voiceless consonant.

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Raphael et al. (1980), eliciting meaningful monosyllabic CVC words from four-year-old children , show that the ratio of voiced closure duration to voiceless closure duration is even larger than the ratio found in adult speech. The ratio of the

preceding vowel durations does not indicate any significant difference from the ratio found in adult literature. All these studies refer to the voicing contrast in speech of English children. In this paper we report on a developmental study with Dutch children concerning temporal characteristics of the voiced and voiceless plosives as well as the preceding vowel duration . Speech of two groups of children is examined; one group consisted of four-year-old children , the other consisted of six-year-old children. We chose children in these age groups for several reasons: (i) The phonological acquisition of the voicing contrast had to be (almost) accomplished in spontaneous speech. (ii) In general , children between the ages of three and six years show a developmental progression toward attaining adult speech motor control and they are said to reach a period of relative stability (Hawkins , 1984). (iii) An appropriate comparison could be made with the English data. Contrary to most acoustic-phonetic studies of young children's utterances we did not choose an imitation task of nonsense words but a procedure of spontaneous word elicitation. In a previous study (Kuijpers , 1989) we described the role and the accurateness of the imitative mechanism of children up to three years old . Certainly, in an imitation task of nonsense syllables several disturbing factors, such as semantic influence, are ruled out. On the other hand, we do not know whether repetition of nonsense syllables distorts speech production that is characteristic of young children. Imitation could influence the child's speech production in such a way that segment durations become more like the adult model than they really would be. That is why we decided to concentrate on spontaneous but controlled speech. In Dutch the voiced-voiceless distinction in word-final position is neutralized because of a devoicing rule that modifies the voiced obstruents into their voiceless cognates (Booij, 1981). There is no clear evidence whether this phonological neutralization process of word-final voiced consonants is also a complete phonetic neutralization. Port & O ' Dell (1985) suggest that, in German, voiceless obstruents derived by the final devoicing rule are phonetically different from underlying voiceless obstruents, either in terms of closure duration, vowel duration or aspiration. However, Fourakis & Iverson ( 1984) argue that these very small differences may be an artifact of the reading task the authors used which causes a hypercorrect articulation. In their experiments with natural , non-orthographic, linguistic contexts, a complete phonetic neutralization of the final voiced consonant is found . In Dutch , the contextual neutralization process of the voicing contrast in word-final position is generally accepted (Cohen, Ebeling, Fokkema & van Holk , 1961; Booij, 1981). Taking into account this phonetic neutralization process, temporal characteristics of the voicing contrast , in the consonant as well as the preceding vowel , could only be examined word-medially. Although the classification is not without problems, Dutch monophthongal vowels are classified by length into the following categories (Nooteboom & Slis , 1972; Koopmans-van Beinum, 1980):

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(i) long vowels [a eo 0] (ii) short vowels [:J o re I c] (iii) half-long vowels [u y i] but in continuous speech behaving as short vowels (Koopmans-van Beinum, 1980) We have seen that temporal characteristics of the v01cmg contrast have been studied thoroughly in adult speech forms while all developmental studies examine the voicing contrast in utterances of English children , mainly in word-final position . In the present study several questions were posed: First, what are the temporal characteristics of voiced and voiceless plosives in medial position in words pronounced spontaneously by four- and six-year-old children? Second, are the durational characteristics of the short vs. long vowels already mastered by four- and six-year-old children? Third, do young children master the temporal coordination in VC sequences and do they exhibit already the adult-like negative correlation between vowel and consonant duration? During the developmental stage , speech production is assumed to reflect an increasing anticipation and an improving economy which finally results in a precise timing as found in adult speech (Bruner, 1973). Children's speech production can be described as developing from a so called " articulatory" model into an adult "timing" model (Kent , 1976; Hawkins , 1984) , which means that young children start to produce utterances segment by segment and the co-ordinative structuring of segments only develops with age. Taking into account these models, we would expect segmental durations per se, such as voiced and voiceless stop or short and long vowel, to be acquired accurately in both age groups; the compensatory effects between vowel and consonant duration, however , would only emerge in the older group.

2. Method 2.1. Participants

We report on data from four-year-old children and six-year-old children. Each age group consisted of six subjects and was equally divided into males and females . The age range was from 4;2 to 4;8 in the four-year-olds and from 6;4 to 7;0 in the six-year-olds. All of them were monolingual speakers of Dutch. Moreover, we had the opportunity to record four children in the older age group who were siblings of children in the younger age group. None of them was judged to have any hearing loss or speech disorder. In addition, all subjects lived in the same area in the south-east of The Netherlands and their parents belonged to the same socialeconomic status. 2.2. Material A set of 37 meaningful disyllabic words was selected (see Table I). The intervocalic plosives reported on are the voiceless plosives I p t k/ and the voiced cognates /b d/; the velar consonant /k/ has no voiced cognate in Dutch. In approximately 50% of the words the preceding vowel, which always had lexical stress, was a phonologically short vowel /a :J c I/; in the rest it was a long vowel I a o e/.

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TABLE I. List of disyllabic words used in the e licitation task with four- and six-year-o ld children . All words belo ng to a young chi ld's vocabulary

Short vowe l preceding intervocalic plosivc appcl stappcn pop pe n kippc n ba tten kattc n pette n patten

l op~I J [ s top~ n J

[ b"t~n J

' apple ' 'steps' 'do lls' 'chicke ns· ' bo nes'

l kot ~ nJ

~ ca t s'

[ pct ~ nJ

'caps 'po ts'

lP"P~n l [ klp~n]

[p "t~nJ

krabbcn ladder ma dder ridd er midde n redden bed de n

[ k rob~nl

[ mtd~n ]

' to scratch' ' ladde r' ' mud ' ' knight' ' middle '

[ rcd~nJ

' to rescue

[ b cd ~ n]

'beds'

[lod ~ rJ

[ m"d~r ] [nd~rJ

sokkcn takkc n vlckkc n wa kkc r

[ s ;)k ~ n ]

·socks'

i tok ~ n l

'branches'

[ v l ~kdnj

'spots'

[ wok ~ r]

·awa ke'

[k o : k ~ n]

' to cook' ' to make' ' blanket' 'sheet' 'cup

Long vowel preceding int ervocalic plosive bo t en gra te r pot e n

l bo:t ~ n]

water

kopen lcpcl schapen stapcl slape n

[g ro:t ~ rl

' boa ts' ' bi gger'

[po:t~n ]

'paws '

[ wa : t ~ rJ

'wa ter

lk o :p~nl

·to bu y ·spoon 's heep ' pile ' ' to slee p'

[l e: p ~ ll

l sxa:p~ n l l s t a :p ~ IJ

isla:pc n]

rode zadel vade r kabel

[ ro:d ~ J

i za :d~ IJ [ va: d ~ rJ

[ ka:b~ I J

' red' 'saddle' ' fath e r' 'cable'

ko kc n ma ken de ken Ia ken bekc r

l ma:k~n l

l d c :k~nl

[la :kdnj [b e :k~rl

Our purpose was to obtain the same set of words from each child and to control for the voicing contrast in intervocalic VCV sequences . All words had to belong to a young child's vocabulary and they all had to be elicitable with pictures in orde r to avoid imitative speech forms. Because the vocabulary of the younger childre n was too restricted to obtain exact minimal pairs matched for the intervocalic consonants /p b/ vs . It d/, initi al consonants could not always be identical in the word pairs , e.g., petten [pet;}n] 'caps' vs. bedden [bed;}n] ' beds'. Selection of an appropriate set of words was not without problems and it was impossible to find as many words with inte rvocalic voiced consonants . We used two-syll abic words with unstressed j;} j as nucleus of the second syllable. 2.3. Task

Each word was represented by a picture on a separate card . Sometimes the picture was chosen such that some close relationship would give rise to an appropriate elicitation. For instance, the picture of a bicycle with a red saddle (zadel) gave rise to the elicitation sente nce , " Deze fiets he eft een rood .. . " (this bike has a red ... ) , but also to the re lated elicitation sentence, " Wanneer de fiets stuk is dan moet iemand hem .. . " (maken ) (whe n the bike is broken someone has to make it) . With all picture cards, elicitation sentences were used in such a way that the child had to complete them by the word itself. The subjects only pronounced the word that was needed for further analysis and in this way we prevented production of longer sentences , which could influence the segmental characteristics of the test item . Moreover, the procedure would account for a spontaneous but controlled speech production of the children without straightforward imitation. With the four-year-old children the mother was asked to do the elicitation task . She was carefully instructed about the procedure of non-imitation , while the author attended

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the tape recorder. With the six-year-old children the author was the "elicitor" and a second experimel)ter was present to attend the instrumental recordings.

2. 4. Recordings Recordings of the four-year-olds were made at their homes with a Tandberg tape recorder and a Sennheiser MD21HN microphone. The six-year-old children were recorded in a laboratory setting using a Revo A 77 and a Sennheiser MD421N microphone. In addition , exact timing of glottal activity in the six-year-olds was registrated with an electro-laryngograph (Fourcin & Abberton , 1971) and was used to verify whether or not voicing was present in the intervocalic plosives . (In a pilot experiment it was shown that we had severe difficulties in attaching the electrolaryngograph around the small necks of four-year-old children and they were very afraid to speak and nearly whispered. Therefore, we decided not to use the electro-laryngograph in the younger age group. With the six-year-old children we did not meet any of these problems .) Before recording all pictures were shown to the child so that he or she would become familiar with the words . All the children were recorded twice using the same elicitation procedure and the same set of words. Between the first and the second recording a brief pause was inserted . Both recordings were used for analysis . In total 410 and 432 phonologically correct realizations by the four- and the six-year-old children, respectively, were available for analysis. In the two age groups, more than 90% of both voiced and voiceless consonants were pronounced "correctly" by the children . That is, during segmentation with the speech editing system SESAM (Broeder, 1989) both visual display of the waveform and our auditory observation indicated that the voiced plosive was not substituted for an intended voiced one or vice versa. Only the consonants that were produced correctly were included in the statistical analyses . 2.5. Measurements

The microphone signals, as well as the electro-laryngograph signals for the six-year-olds, were stored digitally on a Digital microVAX II computer, using a 20kHz sampling frequency with a 12-bit quantization and a 9.6 kHz low-pass filter. Several measures were derived for further analysis: (1) (2) (3) (4) (5) (6)

presence/absence of voice activity in the intervocalic consonant duration of voicing during the intervocalic consonant duration of closure duration of burst duration of vowel preceding intervocalic consonant duration of the word.

The speech editing system allowed us to use both visual and auditory information during segmentation. Judgments of voicing and the durational measurements of vowels and consonants were made using the speech waveform display in the utterances of the four-year-olds and the synchronic speech waveform display and electro-laryngograph traces in those of the six-year-olds. However, to be consistent in the measurement procedure for the two age groups, we based our segmentations

Voiced -voiceless distinction in Dutch children

319

mainly upon the speech waveforms and the electro-laryngograph traces were used mainly as verification. The beginning of the vowel was marked at the first regular vocalic period. When the initial consonant was a sonorant, changes in regularity and/or amplitude of the waveform were taken as the beginning of the vowel. The end of the vowel was marked at the last regular vocalic period. When the vowel preceded a voiced consonant, the changes in regularities and/or amplitude of the waveform indicated the end of the vowel. The end of the vowel was marked as the beginning of the intervocalic consonant. Closure duration and burst duration were labeled separately. If no burst was present in the voiced plosives, the transition in periodicity and amplitude of voicing was marked as the end of the closure period (see Fig. 1). Data were pooled across subjects in both age groups. 3. Results 3. 1. Voicing in medial consonants

In both age groups, the voiceless plosives /p t/ were characterized consistently by a silent closure period followed by a voiceless noise burst. The voiced plosives /b d/, however, showed some variation and dissimilarities between the four- and six-yearold children. Three different types of realizations in voiced plosives could be identified. First, complete voice continuation with no identifiable noise burst interval after closure release (CV) occurred in 58.4% of the productions by the four-yearolds and 27.6% by the six-year-olds. Second, voice continuation followed by a clearly visible burst noise (VB) occurred in 29.2% of the four-year-olds' and 25 .0% of the six-year-olds' productions. Finally, voice period followed by a silent period and then a burst noise (VsB) occurred in 12.4% of the four-year-olds' and 47.4% of the six-year-olds' productions . Figure 1 illustrates the first and the third of these types. The distribution of the three types CV, VB and VsB was found to be significantly different between the two age groups (x 2 = 36.32, p < 0.001), suggesting that in far more of the younger children's productions the oral obstruction in the voiced stops is not so long or tense as to make voicing cease momentarily around the burst (cf. the production of flaps in English) . The older children clearly display Jess voice continuation, and voice is interrupted by the articulatory movements. We could call it a difference in "degree of voicing" between the two age groups. 3.2. Closure durations in medial voiced vs. voiceless plosives Table II gives mean closure durations and burst durations for the two age groups. In both groups the differences in closure duration between the voiced plosives /b d/ and the voiceless plosives /p t/ were analysed. In both age groups, a two-way MANOVA upon closure duration with factors Voice (voiced vs. voiceless) and Vowel (short vs. long) indicated a significant difference for the main effect Voice. Closure duration of voiced consonants /b d/ was significantly different from closure duration of voiceless consonants /p t/ [F(1, 15) = 36.01, p < 0.001 for the four-yearolds; F(1, 15)=326.97, p<0.001 for the six-year-olds]. In both age groups no significant differences were found for the main effect Vowel or for the interaction

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320

(a)

Closure

End

Begin (b)

el. Jar.

I I

I

v

I

Is: B l l

I

End Closure Burst

Begin 50ms

Figure 1. Production of vowel /a/ and voiced consonant /d/ extracted from the word vader 'father' pronounced as [fadgr] by (a) a four-ye a r-old chi ld a nd (b) a six-year-old child. Also the electro-laryngegraph trace (el.lar.) for the six-year-old child is given. Segmentation criteria a re indicated for the " Begin" of the vowel , the " End" of the vowel, the " Closure" and the " Burst" . In the electro-laryngeal trace the type of realization YsB , i.e. , voice (V) followed by silence (s) and burst (B) , is al so indicated.

II. Mean closure durations (and standard deviations) in ms for the four-year-old children (top) and the six-year-old children (bottom). Measures are given for the subjects pooled

TABL E

Burst duration

No. of utterances

Four-year-old children 121.85 (60.78) /p/ 70.55 (26.72) /b/ 111.12 (63.06) It/ 55.13 (33.25) /d/ /k/ 73 .83 (26.63)

16.59 (12 .20) 13 .88 (8.04) 33.64 (20.93) 10 .94 (14.97) 21.83 (10.28)

97 22 94 87 105

Six-year-old children 108.90 (20.94) /p/ 71.73 (12 .53) /b/ 97.92 (18.25) It/ 56.58 (15.92) /d/ 101.02 (17 .92) /k/

14.79 (8 .03) 12.46 (8.00) 35 .38 (13 .38) 17.66 (13.11) 29.23 (9 .87)

104 20 95 73 101

Closure duration

I

Voiced-voiceless distinction in Dutch children

321

Voice x Vowel. A three-way MANOVA with factors Age by Voice x Vowel showed no significant difference across ages for the interaction Age x Voice or for the interaction Age x Voice x Vowel. The voicing contrast in terms of closure duration is comparable in speech of four-year-old and six-year-old children. A closer look at the segmental data in Table II shows that mean closure durations and standard deviations of the voiced consonants /b d/ are almost equal in the utterances of the two age groups, while the voiceless consonants /p t/ are longer and display a considerably larger standard deviation in the four-year-olds relative to the six-year-olds. In addition, it can be said that the six-year-old children behave as a relatively homogeneous group and already seem to approach an overall adult-like consistency ; that is the intersubject variability approximates 20 ms (see Kent & Forner, 1980) . The four-year-olds have a slower and more variable performance; their word durations averaged 10% longer than those of the older children (mean word duration of 550 ms for the six-year-olds and 614 ms for the four-year-olds) and their intersubject variability ranges from 25 ms to 65 ms. Some of the four-year-old children seem to control segmental speech durations better than others and these results are in general agreement with those of DiSimoni (1974a,b), Smith (1978) and Hawkins (1984) concerning English-speaking children of the same age. The most curious result in Table II concerns the consonant /k/, which has no voiced cognate in Dutch. A two-way MANOVA with factors Age x Voice indicated a significant difference for the main effect Age with closure duration in the four-year-olds being considerably shorter (F(1, 10) = 20.05 , p < 0.01]. No differences were found for the interaction Age x Vowel; in the two age groups the consonant /k/ behaves similarly whether preceded by a short or a long vowel and in both groups the variability of the closure duration is low .

3. 3. Short and long vowels Table III gives mean vowel durations pooled over the voiced and voiceless consonant contexts. In each age group, a two-way MANOV A with factors Vowel x Voice indicated a significant difference for the main effect Vowel (irrespective of the following consonant) [F(1 , 15) = 191.95, p < 0.001 for the four-year-olds; F(1, 15) = 374.04, p < 0.001 for the six-year-olds]. In a three-way MANOV A, no differences were found for the interaction Age by Vowel. Absolute durations are clearly longer in the younger age group, but the relationship between the durations of short vowels /a;:,£ 1/ and long vowels /a o e/ appears to be the same in the two groups. In order to compare durational differences of short vs. long vowels across groups, ratios are given with mean duration of short vowels divided by mean duration of long vowels . Durational differences of short vs. long vowels is given by the ratio 0.61 for the four-year-olds and 0.59 for the six-year-olds which develops towards the ratio of 0.54 found in adult productions of isolated words (Koopmans-van Beinum, 1973). 3. 4. Vowel duration in relation to the following consonant

Figure 2 shows mean vowel durations preceding voiced vs. voiceless stops for each subject separately. The four-year-olds did not differentiate durations of vowels before voiced /b d/ vs. voiceless /p t/ , as indicated by a two-way MANOV A with

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322

III. Mean vowel durations (and standard deviations) in ms for the four-year-olds , six-year-olds and adults per vowel and grouped in short and long vowels ; ratios are calculated for short vowel duration divided by long vowel duration. Adult data are taken and ratio is calculated from Koopmans-van Beinum (1973) TABLE

Four-year-olds

Six-year-olds

Adults

152.00 (52.37) 147.03 (37.38) 146.30 (46.87) 140.43 (41.97)

128.11 (27.53) 124.96 (46.87) 119 .94 (28.18) 119.28 (38.13)

117 (20) 111(21) 124 (22) 104 ( 16)

/a/ /e/ /o/

238 .94 (53.59) 234.06 (63 .24) 218 .27 (64. 72)

217.26 (34.07) 183 .71 (33.89) 208.34 (33. 78)

211 (42) 215 (44) 206 (29)

Short Long Ratio

146.84 (48.83) 239.65 (55 .37) 0.61

123.49 (30.68) 207.88 (36.60) 0.59

114.00 210 .67 0.54

Short vowels

/£1 /a/ hi

/II Long vowels

factors Vowel x Voice [see Fig. 2(a) for four-year-olds and Fig. 2(b) for six-yearaids]. The six-year-olds , in contrast, displayed a significant difference [F(l, 15) = 35.03 , p < 0.001)], with vowel duration preceding voiceless plosives being clearly shorter. Again , no difference was found for the interaction Voice x Vowel; short and long vowels seem to behave the same way . This contrast in vowel duration patterns was significant, as shown by the interaction Age x Voice in the three-way MANOVA [F(1, 10) = 37.28, p < 0.001)]. Thus, as a group the four-year-old children do not vary vowel duration as a function of presence or absence of voice in the following consonant. In addition, separate !-tests were performed on vowel durations for each subject. In the four-year-old group, subject 1 showed a lengthening effect of short vowels before voiceless consonants at p < 0.05, and subject 4 showed such a lengthening effect for (a) Four-year-olds

!§ "

300 240

.g"

e 1so

::J "0

~

(b) Six-year-olds

E:;~~~-, f?;

120

::s-1

0

>

60 0+-----r----------r--~

+voice

-voice

+voice

-voice

Figure 2. Differences in mean vowel duration before voiced vs. voiceless consonants for each of the six subjects in the age groups. 0---- 0 =long vowels; .....__.=short vowels.

Voiced-voiceless distinction in Dutch children

323

long vowels. Many more of the six-year-olds produced a shortening of the vowel duration preceding a voiceless consonants. The separate t-tests showed a significant difference at p < 0.05 for short vowels for all subjects except subject 2, and a significant effect for long vowels for subjects 2, 3 and 5. In order to assess whether older children learn to lengthen the vowel before voiced or to shorten before voiceless consonants, analyses of covariance were carried out with word duration being the covariate, i.e., the measure of speaking rate. Vowels before /k/ were not included. In the analyses, vowels followed by a voiced consonant were kept separate from vowels followed by a voiceless consonant while short and long vowels were pooled. Homogeneity of regression within groups was found to be present . No significant difference was found between the two age groups for vowel duration preceding voiced consonants that could not be attributed to the generally faster speaking rate of the older children. Vowel duration preceding voiceless consonants, however, showed a significant difference beyond the effect of speaking rate [F(l, 387) = 7.64, p < 0.01]. It seems that vowel duration before voiced consonants (like the consonant closure itself) does not change with age, whereas vowel duration before voiceless consonants clearly tend to be shorter for the older children than would be expected on the basis of speaking rate alone. As we have seen above, the relation between vowel duration and subsequent closure duration turned out to be different between the two age groups. This relation can be expressed in terms of a V /C ratio, i.e., vowel duration divided by closure duration. Figure 3 plots the mean V /C ratios. Each point in the scattergram represents the mean value of V /C ratios for one word and averaged over subjects per repetition. In this figure, values for the V /C ratios situated on the diagonal mean a perfect correlation between V /C ratios of four- and six-year-olds. Values of the V /C ratios in the words with voiceless consonants show disproportionately long vowels in the younger age group compared to the older age group (filled circles in Fig. 3). In the short vowels, and to a lesser extent in the long vowels, most values are situated 3·5

6 (a) Short vowels

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5

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0 0

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.... • •

0·5 0·5

1·5

2·5

3·5

2

3

4

Ratio V /C four-year-olds Figure 3. Scattergrams of vowel/consonant ratios for words with final voiced (V /C( +voice], 0) and final voiceless consonants (V /C( -voice], e) in utterances of four-year-old children plotted against those of six-year-old children. Words with vowels preceding the consonant /k/ are not included. The values for (a) short vowels and (b) long vowels in VCV sequence are kept apart. Data points indicate V /C ratios for separate words.

5

6

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almost horizontally to the right of the diagonal. In utterances of the older children , vowel duration relative to voiceless closure duration is shorter than we would expect given the values for the younger children. Finally , the correlation between duration of vowel and closure was calculated over all tokens pooled for each age group. The utterances of the younger age group show a positive correlation [Pearson's r = 0.27, p < 0.001] whereas the older children exhibit a significant negative correlation [Pearson's r = -0.32, p < 0.001] and thus already tend to manifest a temporal compensation . 4. Discussion and Conclusions We have examined temporal aspects of intervocalic voiced and voiceless stops from a developmental point of view. Conclusions can be drawn concerning consonant and vowel duration per se as well as concerning the temporal co-ordination between vowel and consonant. In the Introduction , four questions were formulated regarding (i) closure duration of voiced vs. voiceless plosives , (ii) duration of short and long vowels, and (iii) vowel duration as a function of the subsequent consonant and the correlation between vowel and closure duration . E ach point will be discussed in turn below. (i) For those places of articulation for which Dutch has a voicing contrast , the relative closure durations of voiced vs. voiceless stops show no differences that could be attributed to differences in developmental stage ; the closure durations of /p t/ as opposed to those of /b d/ are comparable in the two age groups (although the variability in the voiceless stops is higher for the younger children) . Concerning voiced consonants, another measure of voicing has been taken. With respect to the internal structure of voiced consonants , the four-year-olds show a preference for continuant voice, while the six-year-olds tend to produce the voiced consonants with voice dying away during the consonantal obstruction . The conclusion can be drawn that, although no differences exist in voiced closure duration per se, four- and six-year-old children do manifest a certain difference in "degree" of voicing. Therefore, it would be interesting to know whether the perception mechanism of children , in the age of four or younger , also relies more heavily upon this " degree of voicing" than upon a durational cue . One interesting result is that closure duration of the consonant /k/ differs across ages. This is in contrast to durations of /p/ and /t/ which do not differ between fourand six-year-old children; closure duration of the consonant /k/ is clearly shorter in the speech of the younger children. Two alternative interpretations can be made of this difference : The first involves physiology and motor control. Within the obstruents involving tongue body movements , the consonant / k/ requires the least spatial accuracy. The posterior closure can be made almost anywhere and demands less refinement , less time and less activity than anterior closure (Kent & Moll , 1972). Also, the obstruction for /k/ demands more activity of extrinsic tongue musculature than of intrinsic tongue musculature by comparison to It/ . It seems that in young children extrinsic musculature is more developed and easier to use than intrinsic musculature (Kent , 1979). Therefore , accuracy and physiology are factors that possibly determine the short closure duration and low standard deviation of the consonant /k/ in the youngest age group . The older children are no longer influenced by these factors and closure durations of /p t k/ are more alike .

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The second interpretation of the data involves the lack of contrast. Four-year-old children seem to produce similar closure durations in /p t/ , contrastive to /b d/, but closure duration of /k/, having no voiced cognate, deviates strongly . It seems that the youngest children would not necessarily have generalized a rule about voiceless stops that groups /k/ together with /p/ and /t/ . These two interpretations ("physiology" and " lack of contrast") might be distinguished by comparison to other languages. If it is physiological factors which cause the closure duration of /k/ to be shorter than closure durations of /p/ and /t/, English and Dutch children should manifest the same tendency in production of the velar stop. On the other hand, if it is the lack of contrast which causes the closure duration of /k/ to be shorter than /p/ and /t/, English children, who do have the /k/-/g/ voicing contrast, should manifest a different pattern from Dutch children and they would certainly group /k/ with /p/ and /t/. As far as we know , no studies have been done that pay attention to the production of word-medial /k/ vs. /g/ by English children. (ii) The durational correlates of the short-long opposition in Dutch vowels seem to be established correctly in the two groups of children . Taking into account a difference in speaking-rate, the durational properties of these vowels indicate that young children (aged four and six) already have durational rules that attribute the appropriate values to these speech segments. (iii) We have also seen that vowel duration is shorter in a voiceless than in a voiced subsequent environment only in the speech of the six-year-old children. Thus , the temporal stability of the VC unit can be said to be absent in the four-year-olds. Our findings confirm statements by Kent (1976) and Hawkins (1984) about a more segment-oriented production mechanism in young children ; vowe l and consonant duration are first acquired independently of each other and temporal interaction between vowel and closure duration only grows with age. Our results show that between the ages of four and six the sequence VC[ -voice 1 (i.e ., vowel followed by voiceless consonant) develops towards an adult-like pattern of relative vowel and stop closure duration , whereas the sequence VC[ + voice], in which the two segments share the feature [+voice], does not change with age . While vowel durations in the sequence VC[ - voice1 and VC[+voice1 are identical in speech of four-year-old children , only the sequence VC[ -voice 1 develops towards an accurately timed unit with shortening of the vowel in speech of six-year-olds . We can conclude that children between the ages of four and six learn to shorten the vowel before voiceless stops rather than to lengthen them before voiced stops. In English , the voicing contrast in word-final VC and in medial VCV position is characterized by a lengthening of the vowel before voiced consonants with the effect biggest in word-final position. This may be related to syllable affinity. In medial position , the intervocalic stops are syllabified differently according to the context : lone medial stops after stressed vowels in VCV sequence are often considered ambisyllabic, i.e ., both final to the preceding stressed syllable and initial to the following unstressed syllable. The voicing contrast in medial VCV position, however, does not result in a durational adjustment of the unstressed preceding vowel (Davis & Summers, 1989) and the medial stop is considered initial to the following stressed syllable. Thus, the durational effect might be considered an intra-syllabic coarticulatory effect. In the present data , the relationship between vowel duration and the voicing feature of the consonant tends to develop in two stages , reflected in the two different age groups , that can be related to effects of syllable structure. First, the

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four-year-old children do not show the adjustment of vowel duration as a function of the voicing characteristics in the following consonant. It could be that in VCV utterances the syllable boundary blocks the temporal adjustment. Ingram (1978) shows that the favoured syllable structure in children's first words is CV. He also shows that children, up to almost three years of age, give preference to disyllabic words with CVCV and CVCVC structure . Thus, it would not be astonishing that the four-year-old children still prefer the so-called CV-CV structure with intervening syllable boundary preventing vocalic adjustment. Later, in utterances of the six-year-old children, the vowel and following consonant do interact. These data could be interpreted as showing that the preference for the CV structure has been abandoned by the older children. The results support the view that young children (three to four years old) acquire sound sequencing ability at an early age, captured in what we called the " articulatory" model, and that anticipatory coarticulation is a fine tuning of temporal information acquired only gradually during childhood (Kent, 1983; Sereno, Baum, Marean & Lieberman, 1987; Katz, Kripke & Tallal, 1991). Although the latter studies come to this conclusion on the basis of spectral studies in CV syllables, the basic idea is the same as in our temporal study, i.e., young children seem to produce speech utterances segment by segment, and the ability to produce temporally well coordinated speech gestures develops with age. So far we have presented methods and results concerning developmental aspects of the voiced-voiceless distinction in speech of four- and six-year-old children. To obtain a more complete overview of this topic, our further research concentrates upon two other groups of subjects, 12-year-old speakers and adults, using exactly the same methodology. This research, now being conducted, observes the changes and similarities that occur during the developmental years and leads to a better understanding of children's speech production mechanism. The results reported here are part of a Ph.D. study, financially supported by the University of Amsterdam. We would like to thank Louis Pols , Florien Koopmans-van Beinum, Els den Os and two anonymous reviewers for comments on earlier drafts of this paper.

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