Speech segment durations in sentence recitations by children and adults

Journal of Phonetics ( 1980) 8, 157- 168

Speech segment durations in sentence recitations by children and adults R. D. Kent*and L. L. Forner University of Wisconsin , Madison, Wisconsin, U.S.A . Received 2nd June 1978

Abstract:

Spectrograms were used to study speech segme nt d uratio ns in recita tions of three si mple senten ces by 10 adu lts a nd by 10 children in each of three age grou ps; 4, 6 and 1 2 years. The 4-year-olds typically had longer segment duratio ns and greater variability of segment durat io ns than adults and the old er children. T he degree to whic h segments are lengthened by young child ren appears to depend o n va rious segme nt al, suprasegm ental , and linguistic fac tors which have not yet been explored . In addition, it is possible that greater variances in segme nt duration are co ncomitant wit h lengthening o f segmen ts; that is, t here see ms to be some truth to the assertion that slow speakers are more var ia ble t han fast speakers in timing control.

Introduction In many impo rtant respects , the development of motor control for speech is one instance of the more genera l problem of the development of skilled actio n. In defining t his general problem, Bruner ( 1973) viewed it as the co nstruction of serially ordered acts, the performance of which is modified to achieve diminishing variability , increased anticipation, and improved econom y. These attributes seem highly appropriate to describe the development of motor contro l fo r speech. An adult's speech movement patterns are characterized by precise timing (Kozhevnikov & Chistovich, 1965 ; Nooteboom, 1972 ; Kent & Moll, 1975), considerable anticipation , which results in extensive coarticulation , or overlapping of articulations for phonetic segments (Kozhevnikov & Chistovich 1965; Daniloff & Hammarberg , 1973; Kent & Minifie , 1977) , and economy (consider , for example , the "principle of least effort " that is frequently cited in the study of phonetics) _Just as Lashley ( 1951) used spee ch to exemplify and explicate the general problem of serial ordering in behavior , so speech can be taken as an empirical wedge for analyzing the general problem of the acquisition of sk illed action. Of co urse , speech as a study of skilled action is made es pecially challenging because of its employment as the primary expression of human language . Thus , t he problem of speech development is not readily distinct from that of language development, except through comparisons o f language learned through speech vers us non speech modalities (e.g . American sign language). It seems inescapable that an understanding of a chi ld 's mutual acquisition of speech and language requires systematic and thorough investigation of developmental processes in speech motor control. 0095 - 4470/80/020157+12 $0200/0

© 1980 Academic Press Inc. ( Lo ndo n) Ltd .

*Correspondence to: R. D. Kent, Boys Town Institute, 555 N. 30th Street , Omaha, Nebraska 68131 , U.S.A.

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R. D. Kent and L. L. Forner

A fairly consistE,nt result of studies on children's speech production is that children younger than about six years have · longer speech segment durations than adults and older children (Subtelny , Worth & Sakuda, 1966; Naeser , 1970; Hawkins, 1973; DiSimoni, 1974a, b, c; Gilbert, 1977; Smith, 1978) . For example, Smith (1978) reported that durations of nonsense utterances were 15% longer for four-year-olds than for adults and 31% longer for two-year-olds than for adults. Because few data have been reported on the durations of segments in children's connected , meaningful speech, it is not clear at this time if lengthening of segments is a uniform property of children's speech. This issue is of interest for at least two reasons . First, reduction of segment duration with age may be a consequence of neuromuscular maturation; therefore , durational measurements may be one way of characterizing a child's developmental progress in attaining adultlike speech motor control. A second reason is that developmental patterns in the control of duration are a necessary substrate for research on the acquisition of phonological processes. Smith (1978 : p. 39) comments on this issue: It is also important to assess this factor in order to obtain a perspective for interpreting various phonological issues. If children do require longer to produce segments, there are at least two alternatives that could occur in the development of phonological processes. They might learn to modify segments in an absolute sense that is equal in duration to adult performance, or they could make relative adjustments commensurate with their own rate of production. Knowing which alternative occurs will help in understanding various phonological observations. (Emphasis in the original) Another developmental pattern emerging from studies of children's speech is an age-dependent decline in variability of performance , (Eguchi & Hirsh , 1969; DiSimoni, 1974a, b, c; Tingley & Allen, 1975) If variability is taken as an index of maturation of motor control, then it appears that a child's speech production continues to improve in precision until at least 11 - 12 years of age (see review of pertinent data in Kent , 1976). This gradual decline in performance variability as a function of age accords with part of Bruner's (1973) definition of skilled act development. The developmental study of timing in speech is but one facet of a rather large and rapidly growing literature on children's control of timing . To take one rather intensively studied area as an example, it is well established that reaction time (RT) decreases with children's age in both simple RT experiments (Bellis, 1973; Philip, 1934; Goodenough , 1935; Jones, 1937; Surwillo, 1971; Eckert & Eichorn, 1977) and choice RT experiments (Gilbert, 1894; Surwillo, 1971; Surwillo, 1977) . In addition, it has been shown that both average variability and relative intra-individual variability for simple RT decline with increases in age during childhood (Eckert & Eichorn , 1977). Thus, developmental variation in timing control is by no means unique to speech but rather is common probably to all psychomotor activities. The data in the present paper address the possibility of developmental effects on segment durations of connected, meaningful speech. Children from three age groups, as well as young adults, participated in a simple task of sentence recitation, which was part of a larger study of speech development. Of primary interest were two basic questions regarding timing control : (l) Do younger children have consistently longer segment durations than older children and adults? (2) Are younger children more variable than older children and adults in controlling segment durations? Method

Participants Ten subjects in each of four age groups , four-year-olds, six-year-olds, 12-year-olds, and young (college-age) adults, participated in the study. The two middle age groups were equally divided into males and females but the four-year-old and adult groups were

Sentence recitations

159

constituted of six females and four males. All subjects were judged to be free of speech disorders and the children had successfully completed a task of speech sound imitation (Kent, in press) that required basic listening and articulation skills.

Task and measurements A list of 12 randomly ordered sentences (three different sentences repeated four times each) were spoken by the investigator and repeated by the subjects. Wide-band (300 or 500 Hz) spectrograms were obtained from the recorded acoustic speech signal and a variety of time intervals were measured from the spectrograms. The following measurements were made to describe the temporal structure of the sentence productions. 1. "We saw you hit the cat" (a) duration of frication for [s] in saw; (b) interval from the end of [s] frication to the point at which the F 2 frequency stabilized for [j] in you; (c) voice onset time for [k] in cat; (d) duration of vowel [::e] in cat; (e) duration of the word cat [sum of measures 1(c) and 1(d)]; (f) phase duration , measured as the interval from the initiation of frication for [s] in saw to the instant of closure for [t] in cat. 2 . "The bo x is blue and red" (a) voice onset time for [b] in box; (b) duration of vowel [a] in box ; (c) duration of closure for [k] in box; (d) duration offrication for [s] in box; (e) duratiDn of the word box , or the interval from the release of [b] to the end of frication for [s] ; (f) voice onset time for [b] in blue; (g) durt~tion of the F 2 formant transition for [r] in red; (h) dw-ation of the word red , up to 'closure for [d] ; (i) phase duration , measured as the interval from the rele ase of [b] to the steady-state segment of {r] in red; 3 . " I took a spoon and a knife" (a) voice onset time for [t] in took; (b) duration of vowel [U] in took; (c) duration of closure for [k] in took; (d) duration of the word took (sum of measures (l(a) , (l(b) , and (1(c) ) ; (e) duration of vowel [;;J] in a; (f) duration offrication for [s] in spoon; (g) duration of stop closure for [p] in spoon; (h) voice o nset time for [p J in spoon ; (i) phrase duration , measured as the interval from the release of [t] in took to the beginning of voicing for the [u] in spoon. The total sentence duration was not determined for all three sentences because this measurement could not be made reliably in each case . For instance, the sentence "I took a spoon and a knife " ends with a weak fricative [f] which sometimes barely exceeded the noise floor of the recording environment. Consequently, the longest overall measures of phrase duration were the intervals defined in l(f), 2(i) and 3(i) . The voice onset time measures 2(a) and 2(f) were omitted whenever voicing was carried over from the preceding phonetic segment; however , most subjects ceased voicing momentarily.

160

R. D. Kent and L. L. Forner

All measurements were made to the nearest 0·25 mm and converted to ms values. For each measurement defined above , calculations were made of the subject mean. standa rd deviation , and coefficien t of variation as well as the gro up mean and standard error of the mean. The mean appeared to be generally suitable as a measure of central tenden cy, although the frequency distributions for several measures were skewed to the right. Me as urement reliability , assessed through replicate analysis for arbitrarily selected measures . was within 7 ms (about 1 mm linear measure) on over 90% of the values. Results

Phrase duration Summary statistics fo r the measures of phrase length ( measures 1(f), 2(i), and 3(i) ) are compiled for each age group in Table I. The statist ics include the mean duration (o r mean of the individual subject means), group standard deviation, and the coefficient of variation, for the group data (group standard deviation divided by the group mean) . Three major conclusions can be drawn from the results . First, for each measure of phrase duration , the mean value declines with the age of the subject group . The mean duration for adults varied from 0 ·73 to 0·77 of the mean duration for the four-yea r-old children. Second, the group standard deviation, or intersubject variability , was larger for four-year-olds than for the other age groups. Third , the coefficient of variation , or relative intersubject variability , also was la rger for the four-year-olds than for the other age groups. Consistent developmental differences in intersubject va riabilit y are not apparent for the three olde r age groups , although the adult subjects tended to be less variable as a group than the younger subjects. Table 1 Statistics on the phrase duration measures l(f), 2(i), and J(i) for the four age groups of subjects. The data are in ms, except for the coefficient of variation which is dimensionless

Age group

Measure

Gro up mean

l(f)

s.d. Coe ffi cient of variation Group mean

2(i)

s.d. Coefficient of variation Group mean

3(i)

s.d. Coefficient o f variation

4 years

6 yea rs

1241

1062

965

904

2 10

96

71

81

0·169

0·090

0·074

0·090

1118

1016

89 0

846

197

70

27

77

0·17 6

0 ·069

0·030

0·091

482

421

415

372

73

39

22

24

0 ·151

0 ·093

0·053

0·064

12 years

Adult

Sentence recitations

161

An illustration of the data for measurement 3(i) is presented in Fig . I. Intrasubject standard deviations are shown in the upper graph , with each data point representing the result for one subject. The lower graph shows the mean duration for each age group, bracketed by ± one standard error. This graph clearly illustrates the reduction in phrase duration as a function of subject age and the large intersubject variability for the four-yearold group . The upper part of the figure is evidence of a rather weak developmental trend in individual variance , with the younger subjects being somewhat more variable than the older subjects. However , the performance of the subjects within a group is not uniform , in that some four-year-olds have standard deviations within the adult range (about 30 ms or less) whereas other four-year-olds have standard deviations well outside the range of values for adults. Hence , some of these young children are capable of a much more reliable control over speech production than the others .

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lntras ubject standard deviation (upper graph) and gro up mean duration bracketed by the intersubject standard deviation (lower graph) for phrase measurement 3( i) in recitations of "I took a spoon and knife" by four age groups of subjects.

Segm ent durations, selected data Figure 2 displays for two measures of voice onset time the intrasubject standard deviations (u pper graph) and the group means bracketed by ± one standard error (lower graph). The results for [k] show that as subject age increases , reductions occur in the mean duration , intersubject variabiiity , and int rasubject variability . That is , the younger subjects have longer voice onset times , greater heterogeneity within the group , and a poorer individual accuracy.

162

R. D. Kent and L. L. Forner

However, even some four-year-olds fall within the adult range of values for intrasubject standard deviations . The results for [t] do not show a well-defmed developmental effect for any of the three statistics, except possibly for the intrasubject standard deviations, for which the more mature subjects, as a group, have smaller values. Thus, it cannot be inferred that the four-year-olds have consistently longer and more variable voice onset times , although they may have longer and more variable values than adults in certain circumstances. That is , it appears necessary to consider place of articulation, phonetic context, stress pattern , syntactic pattern , and other phonetic or linguistic factors as they relate to developmental differences in acoustic segment durations. Data for other selected segment durations are displayed in Fig. 3. Results are shown for the duration of the vowel [a] and the [k] closure in box and the voice onset time for [b] in blue. For all three segments, the mean duration and intersubject variability decline with age , and there is a weaker trend for the intrasubject variability to decline as well. But, as already noted for the data in Figs 2 and 3, the intrasubject standard deviations for some four-year-olds are within the range of values for the adults. Thus, four-year-olds are not necessarily less accurate in their speech production control than adults but they tend to be less accurate for a variety of acoustic segment measurements .

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lntrasubj ect standard deviation (upper g raph) and gro up mean bracketed by th e inter subj ec t standard deviation (lower gra ph) for three segment durations : vowel ]a I in box, ]k I closure in box, a nd voice onset time ] VOT] for ]b] in blue. Data arc shown for three age gro ups .

Lengthening effects in fo ur~vear-u lds' speech Beca use the four-year-olds were the only age group of children to be almost uniformly longe r than adu lts in the duration of all measurements, their results are considered by way of graphical summary in Fig . 4 . In this .graph , the mean values for temporal measurements of the four-year-old s' speech are plott ed versus the mean values for temporal measurements of th e adults ' speech. Nearly all of the measurements made in the study are represented in thi s figure with the exception of the phrase durations. If th e four -year -olds had temporal measurements equal to those of the adults, then all the data points wou ld fall along the di ago nal x = y. In fact , the data points are displaced toward vertical from the diagonal, which means that the four-year-olds' mean values are greater than those for adults in the great majority of cases. It should be noted that the lengthening of the four-year-olds' measurements with res pect to the adults' applies to small time values as well as to longe r ones . Thus, the difference between the speech of young children and the speech of adults is not res tricted to a nrnge of values such as those expected for vowel segment durations (100 - 200 rns fo r stressed vowels) or sy llab le s. Finally , because the various temporal measure s are not ind ependent, on ly a ve ry cautious interpretation should be made of the visually determined lin e of fit. Dcpcndcucy of l'ariability on segment duration Th e possibility exists th at both intersubject and intrasubject variabi lity depend upon the mean dura tion of a segment. For exa mple, note in Figs 1- 3 that large valu es of the variability measures accompany large value s of the mean segment duration. Conversely.

164

R . D. Kent and L. L. Forner

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when the four-year-olds have a segment duration similar to that for older children and adults (as in the data for [t] in Fig . 1) the variability measures for the four-year-o ld s also are like those for the older subjects . To test this possibility further , scattergrams were prepared to show the relationship between an individual subject's standard deviation and mean segment duration. Results are shown in Fig. 5 for two measures, the voice onset time for [k] in cat and the phrase duration , measurement I (f) , for sentence "We saw you hit the cat." The least squares Jines of regression are associated with r2 coefficients of determination of 0-42 for the voice onset time and 0-45 for the phrase (both are significant at 0·05 level of confidence) . Thus, it appears that at least part of the variance in the duration measures is related to speaking rate , given that speaking rate determines segment durations. The younger children had slower speaking rates (hence longer segment durations) and therefore a greater variability, both as a group as well as individually . Given the indicated relationship between the measures of variance and the measure of central tendency, it is appropriate to compare the subjects with respect to a relative measure of variation, the coefficient of variation (the standard deviation divided by the mean). Values of this coefficient , C, are shown for three temporal measurements in Fig 6. Each point for each age group represents a value of C calculated for one subject. The data pertain to the voice onset time for [k] in cat, the duration of frication for [s] in box , and the duration of frication of [s] in spoon. A developmental trend in the direction of smaller Cs with advances in age is apparent. The adults as a group have the smallest values , although they are nearly matched by the twelve-year-olds, and the four- and six-year-olds have the largest values . For any one of the three measured segments, no more than three of the fouryear-old children fall within the adult range of C values. Thus , the calculations of C distinguish fairly well at least the youngest versus the oldest subjects tested. It also is worthy of note that the values for the two older groups of subjects have a smaller group scatter than the values for the two younger grou ps. Tha t is, the older subj ect s are more uniform in their individual accuracy .

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166

R. D. Kent and L. L. Forner

Discussion The data presented in this report are in agreement with earlier results , reviewed in the introduction, showing that young children, at least younger than about six years , tend to have longer speech segments than adults. In addition , these longer mean segment durations often were accompanied by a greater variability in repeated productions of the segments. However, because the present study examined only a small number of sentences and a small set of measurement s, it is not warranted to draw sweeping generalizations about segment lengthening and variability at this point. Rather, these data are viewed as raising certain questions and cautions that should be considered in future research on timing in children's speech production. First, studies of lengthening of speech segments in children's speech, compared to adults' speech , should take careful note of the influences of both segmental and suprasegmental factors. It is conceivable that properties of the carrier phrase, style of pronunciation, and stress pattern may strongly affect lengthening of segments in children's speech. Second, datl! on timing variability in repeated productions should be considered with respect to the mean segment duration to determine if a dependency exists. It also appears important to investigate how the variability of segment durations interacts with various segmental and suprasegmental characteristics. The present data indicate that both lengthening of mean segment duration and increased variability of segment duration are more likely to occur under some segmental-suprasegmental combinations than others. For example, the increased mean and variability of voice onset time seen for children's productions of /k/ in cat, but not /t/ in tuok, might be related to the fact that cat is a stressed, utterance final word containing a vowel of inherently long duration. If further research bears out the finding that lengthening of segments in children's speech depends upon various segmental and suprasegmental (not to mention linguistic) factors, then studies of child phonology will have to consider these interactions as they relate to the child's acquisiton of phonological processes . The problem becomes even more serious if predictions from phonological theory are tested against physical measures such as segment duration. Comprehensive data on the development of timing control in children's speech also are needed for the quantitative study of speech disorders. Many disorders , particularly those of neurologic origin, involve disturbances of timing control. For both diagnostic and rehabilitative purposes , it is useful to know how these abnormalities of timing may be similar to, or different from, the ways in which children's timing control deviates from that of normal adults. As a case in point , consider the individual with cerebellar disease. Kornhuber (1977: p. 32) comments on the cerebellum's role in speech production as follows. Cerebellar lesions result in a slow, hesitant or dysarthric speech which agrees with other ce rebellar motor symptoms, such as dysmetria , adiodochokinesia , and lack of coordination of fast movements . The cerebe llum see ms to be a device for the generation and adjustment of fast move ment s that are too quick to be regulated continuously by outer (e.g. visua l) st imuli and that therefore must be preprugrammed and adjustl:'d by motor learning. In add ition, the cerebellum seems to contain a mechanism for holding those positions reached by the fast movements and for temporal coordination of quick successive se ries of movement. .. (Emphasis added) Recent speculation on the role of the cerebellum in motor control often emphasi7.es the need for the cerebellum to gain experience in motor accomplishment and to "learn" from that experience to predict and modify as required the motor consequences of efferent outflow. By this reasoning, the cerebellum must be an active participant in the motor learning of speech production. It is thus of interest to kn ow if t he timing control of a person with cerebellar disease resembles in any way that of a young child who is learning a motor skill.

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167

There is at least a superficial resemblence insofar as both young children and individuals with dysarthria of cerebellar origin tend to have speech segments that are longer and more variable in duration than those for normal adults (Kent, Netsell , & Abbs, 1979). But there are some important differences as well. For example, it appears from the data of individuals with dysarthria of cerebellar origin that a frequent abnormality in timing contro l is failure to reduce certain segment durations in keeping with an intended stress pattern. In a sentence like "We saw you hit the cat" normal adult speakers produce the vowel segments in the and cat with mean durations of about 40 and 170 ms , respectively. When the ratio of the two vowel segment durations u~; to /re/] is calculated for individual subjects, the mean value of this ratio for normal adults is approximately 0 ·24 . The corresponding value for the four-year -old s in the present study was 0 · 18 , whereas the ratios for three dysarthric subjects were 0·35 , 0 ·53, and 0·81. The children had a smaller ratio than normal adults (because of a relative lengthening of /re/ in the children's data) whereas the dysarthric subjects had appreciably larger ratios than normal adults (primarily because of abnormally long 1~1 durations in the dysarthric data) . Hence , although four-year-olds and cerebellar dysarthrics share a tendency to prolong speech segments, the timing control for stressdetermined duration does not seem to be fundamentally similar for these two groups. In this way , systematic studies of temporal regulation in developing and disordered speech should be helpful in testing hypotheses about the structure of motor programs in speech production and the ways in which these programs are acquired and maintained. Variability of timing control is important in characterizing both developing and disordered motor control , whether for speech or motor behavior generally. But it has been reported in the speech literature that variability of timing may vary with segment duration. Klatt ( 1974) suggested in a study of the duration of frication for /s/ that the precision of timing in speech produ ction may vary with speaking rate , such that the variance of temporal measurements decreases as speaking rate increases (and as segment duration is reduced). Similarly, Lehiste ( 1972) reported data for base durations in words such as stick, sticky, stickiness, in which durational variance clearly declines as the base duration (stick- for the words listed here) decreases. It is entirely possible, then, that an individual's timing precision may vary with factors that influence segment duration. When variability of timing is used to describe developing or disordered speech, it is important to recognize the possibility that increased variability may be related simply to a slower speaking rate (hence longer segments) and not necessarily to neuromuscular immaturity or neurologic damage . Some reports showing heightened variability for young children or persons with speech disorders may really be evidence for the general rule that slow speakers are more variable in timing control than fast speakers. This work was supported by NIH research grants NS 12281 and NS 13274. The authors are grateful for the co-operation of the following persons who arranged for the participation of the children as subj ects in th e study: Rita Hibray, Director of the Salvation Army Day Care Center in Madison , Wisconsin; James Clark, Principal of the Oregon , Wisconsin Elementary School ; and Edward Guziewski, Principal of the Oregon , Wisconsin Middle School. References Bellis , C. J. ( 1933). Reactio n time and chronological age . Pro ceedings of th e Society for Experimental

Biology and Medicine 30,80 1- 803. Bruner, J. S. (197 3). Organization of early sk illed action. Child Development 44, 1- 11. Daniloff, R. G. & Hammarberg, R. F.. ( 1973). On defining coarticulation. Journal of Phonetics l,

239 - 248. DiSimoni , F. G. (1974a) . Influence of vowel environment on the duration of consonants in the speech of three-, six-, and nine-year o ld children . Journal of tlu: Acoustical Society of A me rica 55, 360 - 361. DiSimoni, 1:. G. (1974b). Influence of co nsonant environment on duration of vowels in the speech of three-, six-, and nin e-year old chi ldren . Journal of the Acoustical Society of A me rica 55, 362 - 363 .

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