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Speech timing variability of children and adults
Journal of Phonetics (1986) 13, 477- 480
Speech timing variability of children and adults Gail D. Chermak and Carl R. Schneiderman Department of Speech, Washington State University, Pullman, Washington 99164-2420, U.S.A. Received 25th April 1985, and in revised form 26th August 1985
1. Introduction Timing is a critical parameter affecting motor performance and linguistic output (Kent, 1976). Physiological and acoustic data suggest that the timing of sequential speech gestures changes as a function of neuromuscular maturation (Kent, 1976; Kent & Forner, 1980; Tingley & Allen, 1975; Watkin & Fromm, 1984). Anatomical and neuromuscular maturation of the speech mechanism and consequent motor control improves with age until adult-like performance is achieved by 11 or 12 years of age (Kent, 1976). Tingley & Allen (1975) suggested that young children may not perform as older children or adults in tasks that require a neural-timing control mechanism. Children's speech shows greater temporal variability and longer segment, syllable and phrase duration than adults (DiSimoni, 1974a,b; Eguchi & Hirsh, 1968; Smith, Sugarman & Long, 1983; Tingley & Allen, 1975). Reduction of segment duration and temporal variability with age provide two means of characterizing a child's developmental progress in the attainment of adult speech motor control (Kent, 1976; Kent & Forner, 1980). Kent & Forner (1980) suggested that "an individual's timing precision may vary with factors that influence segment duration. When variability of timing is used to describe developing . .. 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 neuromotor immaturity .. ." (p. 167). If this statistical artifact hypothesis is correct in its simplest form then a relationship between segment duration and variability should be seen independent of age . A neuromotor maturation hypothesis would be supported by finding a relationship between variability and age independent of duration. One means to examine these competing explanations of the source of variability in speech timing is to alter rate of speech, thereby modifying segment durations. Studies of adult's speech timing have employed changes in speaking rate; however, few studies of children's speech have experimentally varied rate of production (Smith eta!., 1983). The results reported here contrast word and phrase duration and variability of children, teenagers and adults speaking at two rates of production. The data were collected during the course of a larger investigation of children's phase-level timing which accounts for the procedural variable of emphatic stress described in the next section. 2. Method
Subjects were five children aged 7 years, five teenagers aged 13 years, and five adults. All 0095-4470/85/040477
+
04 $03 .00/0
© 1985 Academic press Inc. (London) Limited
478
Letter to the editor
subjects were judged to be free of speech and language disorders, had normal hearing sensitivity, and spoke with a General American dialect. Children read at grade level. Each subject produced a total of 120 experimental stimuli consisting of a list of 15 randomized repetitions of four sentences (Bob hit the big dog; Bob caught a bad cold; Bob lost the green kit; Bob fed the black cat). Repetitions of the latter three sentences served as "dummy" items and, therefore, were not analysed. Each sentence was read three times under each of five different emphatic stress conditions, at both conversational and fast rates. For the fast rate production, subjects were instructed to read at a rate twice as fast as their conversational rate. Four of the emphatic stress conditions corresponded to placement of stress on one of the four content words in the sentence. The fifth condition consisted of equally unstressed production (no emphatic stress) of each word in the sentence (neutral). The sentences within the list were coded for the five emphatic stress conditions. Subjects were instructed to read uncoded sentences as they would any other written text. Duration of each of four content words of the experimental sentence, as well as total sentence duration, were calculated. Word boundaries were operationally defined as follows. The interval between the burst associated with the release of initial /b/ and the final glottal pulse of the following vowel was designated as the Bob segment. For the segment designated as hit, onset was defined as the abrupt appearance of aspiration noise, and offset was indicated by the final glottal pulse of the following vowel. The big segment included that time period between the final glottal pulse of the vowel in the noncontent word the and the final glottal pulse of the vowel /I/. Lastly, the dog segment was measured between the release of /d/ and the final glottal pulse of the following vowel. (Weismer & Ingrisano, 1979, p. 519). Total segment included the time period between the burst associated with the release of the initial /b/ (in Bob) and the final glottal pulse of the vowel in dog.
Spectrographs were prepared for analysis using Kay Spectrograph Model 6061 B (wide band filter). 3. Results All data are collapsed across emphatic stress condition as the investigators' primary focus in this report centers on the relationship between rate of speech (duration), and variability. The children spoke more slowly and presented greater variability than adults or teenagers for both rates of production (Table I). Within-group comparisons of word durations across speaking rates revealed an increase in both relative variability and absolute variability (as reflected in coefficients of variation 1 and standard deviations , respectively) as word and sentence duration increased. Relative variability for all word and sentence durations differed significantly as a function of age (F(2, 12) = 9.85; p = 0.003; F(2, 12) = 5.83; p = 0.02; F(2, 12) = 14.82; p = 0.001; F(2, 12) = 5.74; p = 0.02; F(2, 12) = 10.01 ; p = 0.003, for Bob, Hit, Big, Dog, and Total sentence duration, respectively). Fisher's least significant difference tests revealed children's 1 The coefficient of variation (relative variability) is a more accurate measure of variability than the standard deviation when groups present different means (Guilford, 1950). The coefficient of variation is calculated by dividing the standard deviation by the mean. It is customary to multiply this ratio by 100. This value represents what percentage of the mean the standard deviation is.
479
Letter to the editor
TABLE I. Mean word and sentence durations (ms), standard deviations, coefficients of variation and standard errors of the means for 15 subjects Rate Conversational
Fast
Group
Segment
Mean
SD
SEM
cv
Mean
SD
SEM
cv
Adult
Sentence Bob Hit Big Dog
1228 175 168 225 275
230 43 61 56 37
46 9 12 7
19 24 36 25 14
896 135 120 167 217
143 34 41 37 29
29 7 8 7 6
16 25 34 22 13
Child
Sentence Bob Hit Big Dog
2287 297 286 434 337
597 105 108 144 75
119 21 22 29 15
26 35 38 33 22
1698 220 250 304 324
409 82 92 105 98
82 16 18 21 20
24 37 37 35 30
Teenage
Sentence Bob Hit Big Dog
1322 206 186 228 317
!52 60 52 59 51
30 12 10 12 10
12 29 28 26 16
1157 187 166 194 286
194 56 58 50 55
39
17 30 35 26 19
II
II
12 10 II
variability to be significantly greater than the adults or teenagers (p ::::;: 0.05). Betweengroup comparisons of variability when duration was comparable (i.e. word Dog), showed significantly more variability for children than the adults or teenagers (F(2, 12) = 5.74; p = 0.02). Significant correlations of substantial strength (r = 0.90) were seen between variability and duration of words and sentences for children and teenagers, accounting for 80% of the variance. Few correlations reached significance for the adults. 4. Discussion
The results of this investigation indicate that neither the statistical artifact hypothesis nor the neuromotor control hypothesis accounts exclusively for differences in speech timing variability among children, teenagers and adults. Both statistical and maturational factors appear to contribute to children's greater variability. Two outcomes provide support for the statistical artifact hypothesis. (1) Within-group comparisons of word and sentence durations at different speaking rates revealed an increase in variability as duration increased. The neuromotor control hypothesis would predict a constant value of relative variability across rates of production. (2) The strong positive correlations between variability and duration that accounted for approximately 80% of the variance also supports the statistical artifact hypothesis.
Two findings support the neuromotor maturation hypothesis. (1) Between-group comparisons of variability when duration was comparable (i.e. , word Dog) showed significantly more variability for the children than the adults or
480
Letter to the editor
teenagers. The statistical artifact hypothesis predicts comparable variability in the case of comparable duration. (2) Twenty percent of the variance was not accounted for by the correlation between relative variability and duration. Additional factors may be responsible for increased variability seen in children's speech. The authors express appreciation to Mary Veith for her assistance in data collection.
References DiSimoni, F. G. (1974a). Influence of vowel environment on the duration of vowels in the speech of three-, six-, and nine-year-old children, Journal of Acoustical Society of America, 56, 360- 361. DiSimoni, F. G. (l974b) . Influence of consonant environment on the duration of consonants in the speech of three-, six-, and nine-year-old children, Journal of Acoustical Society of America, 56, 362- 363. Eguchi, S. & Hirsh, I. J. (1969). Development of speech sounds in children, Acta Otolaryngo/ogica, Supplement 257. Guilford, J. P. (1950). Fundamental Statistics in Psychology and Education, 2nd Edition. New York: McGraw-Hill. Kent , R. D. (1976). Anatomical and neuromuscular maturation of the speech mechanism: evidence from aco ustic studies, Journal of Speech and Hearing Research, 19, 421- 447. Kent, R. D. & Forner, L. L. (1980). Speech segment durations in sentence recitations by children and ad ults, Journal of Phonetics, 8, 157- 168. Smith, B. L. , Sugam1an, M. P. & Long, S. H. (1983). Experimen tal manipulation of speaking rate for studying temporal variability in children's speech, Journal of Acoustical Society of America, 74, 744-749. Tingley, B. M. & Allen, G. D. (1975). Development of speech timing control in children, Child Development, 46, 186-194. Watkin, F. & Fromm, D. (1984). Labial coordination in children: preliminary considerations, Journal of the Acoustical Society of America, 75, 629- 632. Weismer, G. & Ingrisano, D. (1979). Phrase-level timing patterns in English: Effects of emphatic stress location and speaking rate, Journal of Speech and Hearing Research, 22, 516-532.
Speech timing variability of children and adults Gail D. Chermak and Carl R. Schneiderman Department of Speech, Washington State University, Pullman, Washington 99164-2420, U.S.A. Received 25th April 1985, and in revised form 26th August 1985
1. Introduction Timing is a critical parameter affecting motor performance and linguistic output (Kent, 1976). Physiological and acoustic data suggest that the timing of sequential speech gestures changes as a function of neuromuscular maturation (Kent, 1976; Kent & Forner, 1980; Tingley & Allen, 1975; Watkin & Fromm, 1984). Anatomical and neuromuscular maturation of the speech mechanism and consequent motor control improves with age until adult-like performance is achieved by 11 or 12 years of age (Kent, 1976). Tingley & Allen (1975) suggested that young children may not perform as older children or adults in tasks that require a neural-timing control mechanism. Children's speech shows greater temporal variability and longer segment, syllable and phrase duration than adults (DiSimoni, 1974a,b; Eguchi & Hirsh, 1968; Smith, Sugarman & Long, 1983; Tingley & Allen, 1975). Reduction of segment duration and temporal variability with age provide two means of characterizing a child's developmental progress in the attainment of adult speech motor control (Kent, 1976; Kent & Forner, 1980). Kent & Forner (1980) suggested that "an individual's timing precision may vary with factors that influence segment duration. When variability of timing is used to describe developing . .. 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 neuromotor immaturity .. ." (p. 167). If this statistical artifact hypothesis is correct in its simplest form then a relationship between segment duration and variability should be seen independent of age . A neuromotor maturation hypothesis would be supported by finding a relationship between variability and age independent of duration. One means to examine these competing explanations of the source of variability in speech timing is to alter rate of speech, thereby modifying segment durations. Studies of adult's speech timing have employed changes in speaking rate; however, few studies of children's speech have experimentally varied rate of production (Smith eta!., 1983). The results reported here contrast word and phrase duration and variability of children, teenagers and adults speaking at two rates of production. The data were collected during the course of a larger investigation of children's phase-level timing which accounts for the procedural variable of emphatic stress described in the next section. 2. Method
Subjects were five children aged 7 years, five teenagers aged 13 years, and five adults. All 0095-4470/85/040477
+
04 $03 .00/0
© 1985 Academic press Inc. (London) Limited
478
Letter to the editor
subjects were judged to be free of speech and language disorders, had normal hearing sensitivity, and spoke with a General American dialect. Children read at grade level. Each subject produced a total of 120 experimental stimuli consisting of a list of 15 randomized repetitions of four sentences (Bob hit the big dog; Bob caught a bad cold; Bob lost the green kit; Bob fed the black cat). Repetitions of the latter three sentences served as "dummy" items and, therefore, were not analysed. Each sentence was read three times under each of five different emphatic stress conditions, at both conversational and fast rates. For the fast rate production, subjects were instructed to read at a rate twice as fast as their conversational rate. Four of the emphatic stress conditions corresponded to placement of stress on one of the four content words in the sentence. The fifth condition consisted of equally unstressed production (no emphatic stress) of each word in the sentence (neutral). The sentences within the list were coded for the five emphatic stress conditions. Subjects were instructed to read uncoded sentences as they would any other written text. Duration of each of four content words of the experimental sentence, as well as total sentence duration, were calculated. Word boundaries were operationally defined as follows. The interval between the burst associated with the release of initial /b/ and the final glottal pulse of the following vowel was designated as the Bob segment. For the segment designated as hit, onset was defined as the abrupt appearance of aspiration noise, and offset was indicated by the final glottal pulse of the following vowel. The big segment included that time period between the final glottal pulse of the vowel in the noncontent word the and the final glottal pulse of the vowel /I/. Lastly, the dog segment was measured between the release of /d/ and the final glottal pulse of the following vowel. (Weismer & Ingrisano, 1979, p. 519). Total segment included the time period between the burst associated with the release of the initial /b/ (in Bob) and the final glottal pulse of the vowel in dog.
Spectrographs were prepared for analysis using Kay Spectrograph Model 6061 B (wide band filter). 3. Results All data are collapsed across emphatic stress condition as the investigators' primary focus in this report centers on the relationship between rate of speech (duration), and variability. The children spoke more slowly and presented greater variability than adults or teenagers for both rates of production (Table I). Within-group comparisons of word durations across speaking rates revealed an increase in both relative variability and absolute variability (as reflected in coefficients of variation 1 and standard deviations , respectively) as word and sentence duration increased. Relative variability for all word and sentence durations differed significantly as a function of age (F(2, 12) = 9.85; p = 0.003; F(2, 12) = 5.83; p = 0.02; F(2, 12) = 14.82; p = 0.001; F(2, 12) = 5.74; p = 0.02; F(2, 12) = 10.01 ; p = 0.003, for Bob, Hit, Big, Dog, and Total sentence duration, respectively). Fisher's least significant difference tests revealed children's 1 The coefficient of variation (relative variability) is a more accurate measure of variability than the standard deviation when groups present different means (Guilford, 1950). The coefficient of variation is calculated by dividing the standard deviation by the mean. It is customary to multiply this ratio by 100. This value represents what percentage of the mean the standard deviation is.
479
Letter to the editor
TABLE I. Mean word and sentence durations (ms), standard deviations, coefficients of variation and standard errors of the means for 15 subjects Rate Conversational
Fast
Group
Segment
Mean
SD
SEM
cv
Mean
SD
SEM
cv
Adult
Sentence Bob Hit Big Dog
1228 175 168 225 275
230 43 61 56 37
46 9 12 7
19 24 36 25 14
896 135 120 167 217
143 34 41 37 29
29 7 8 7 6
16 25 34 22 13
Child
Sentence Bob Hit Big Dog
2287 297 286 434 337
597 105 108 144 75
119 21 22 29 15
26 35 38 33 22
1698 220 250 304 324
409 82 92 105 98
82 16 18 21 20
24 37 37 35 30
Teenage
Sentence Bob Hit Big Dog
1322 206 186 228 317
!52 60 52 59 51
30 12 10 12 10
12 29 28 26 16
1157 187 166 194 286
194 56 58 50 55
39
17 30 35 26 19
II
II
12 10 II
variability to be significantly greater than the adults or teenagers (p ::::;: 0.05). Betweengroup comparisons of variability when duration was comparable (i.e. word Dog), showed significantly more variability for children than the adults or teenagers (F(2, 12) = 5.74; p = 0.02). Significant correlations of substantial strength (r = 0.90) were seen between variability and duration of words and sentences for children and teenagers, accounting for 80% of the variance. Few correlations reached significance for the adults. 4. Discussion
The results of this investigation indicate that neither the statistical artifact hypothesis nor the neuromotor control hypothesis accounts exclusively for differences in speech timing variability among children, teenagers and adults. Both statistical and maturational factors appear to contribute to children's greater variability. Two outcomes provide support for the statistical artifact hypothesis. (1) Within-group comparisons of word and sentence durations at different speaking rates revealed an increase in variability as duration increased. The neuromotor control hypothesis would predict a constant value of relative variability across rates of production. (2) The strong positive correlations between variability and duration that accounted for approximately 80% of the variance also supports the statistical artifact hypothesis.
Two findings support the neuromotor maturation hypothesis. (1) Between-group comparisons of variability when duration was comparable (i.e. , word Dog) showed significantly more variability for the children than the adults or
480
Letter to the editor
teenagers. The statistical artifact hypothesis predicts comparable variability in the case of comparable duration. (2) Twenty percent of the variance was not accounted for by the correlation between relative variability and duration. Additional factors may be responsible for increased variability seen in children's speech. The authors express appreciation to Mary Veith for her assistance in data collection.
References DiSimoni, F. G. (1974a). Influence of vowel environment on the duration of vowels in the speech of three-, six-, and nine-year-old children, Journal of Acoustical Society of America, 56, 360- 361. DiSimoni, F. G. (l974b) . Influence of consonant environment on the duration of consonants in the speech of three-, six-, and nine-year-old children, Journal of Acoustical Society of America, 56, 362- 363. Eguchi, S. & Hirsh, I. J. (1969). Development of speech sounds in children, Acta Otolaryngo/ogica, Supplement 257. Guilford, J. P. (1950). Fundamental Statistics in Psychology and Education, 2nd Edition. New York: McGraw-Hill. Kent , R. D. (1976). Anatomical and neuromuscular maturation of the speech mechanism: evidence from aco ustic studies, Journal of Speech and Hearing Research, 19, 421- 447. Kent, R. D. & Forner, L. L. (1980). Speech segment durations in sentence recitations by children and ad ults, Journal of Phonetics, 8, 157- 168. Smith, B. L. , Sugam1an, M. P. & Long, S. H. (1983). Experimen tal manipulation of speaking rate for studying temporal variability in children's speech, Journal of Acoustical Society of America, 74, 744-749. Tingley, B. M. & Allen, G. D. (1975). Development of speech timing control in children, Child Development, 46, 186-194. Watkin, F. & Fromm, D. (1984). Labial coordination in children: preliminary considerations, Journal of the Acoustical Society of America, 75, 629- 632. Weismer, G. & Ingrisano, D. (1979). Phrase-level timing patterns in English: Effects of emphatic stress location and speaking rate, Journal of Speech and Hearing Research, 22, 516-532.