Studies of children’s acquisition of speech sound can be a major guideline in a diagnosis of speech sound disorders.
The purpose of this study is to investigate age-specific features of 2:1 to 4;1 year-olds’ production of monophthong and diphthong through both auditory perceptual analysis and acoustic analysis.
Participants of this study were 102 children of 2;1 to 4;1 year-olds residing in Daejeon city, who were later classified into three groups for this study: 2;1 to 2;8 years-old, 2;9 to 3;4 years-old, and 3;5 to 4;1 years-old. Criteria used for the classification were as follows: The children 1) were reported not to have emotional, behavioral, and intellectual impairment, 2) achieved a consent to participation into this study from their parents, and 3) belonged to a normal scope which showed, combined language age was different less than 1 year old, compared with their chronological age based on the results of the Preschool Receptive- Expressive Language Scale: PRES.
Test task included {vowel+’da’} consisting of 7 monophthongs and 10 diphthongs and meaningful words beginning with vowel. Percentage of vowels correct was used for perceptual analysis and Praat(5.2.12) was used for acoustic analysis, analyzing 41 variables related with monophthong and 2 variables related with diphthong.
The results of this study are as follows: First, perceptual analysis provided that children from an age group of 2;1 to 2;8 years-old showed significant difference in the accuracy level both monophthong and diphthong, compared with those from one of 2;9 to 3;4 years-old and 3;5 to 4;1 years-old. In specific, they showed more than 90% of accuracy in producing monophthong, compared with both 2;9 to 3;4 years-old and 3;5 to 4;1 years-old. In addition, observation on production of diphthong provided that accuracy level was 47%~48% from 2;1 to 2;8 years-old; 77%~79% from 2;9 to 3;4 years-old; and 75%~82% from 3;5 to 4;1 years-old. Thus, the findings of this study lead to development order of monophthong as /i, ?/→/?, u, ?/→/?, o/. Difference in accuracy level of vowel between {vowel+’da’} and meaningful words was not found. Also, correlation between the number of wrong productions from each child and disagreed number (more than one) on target sound in transcribing from three transcribers were significantly different.
Analysis of the rate of correct articulation of monophthong based on phonemes by age group showed that more than 90% of correct articulation in /i, ?/ from 2;1 to 2;8 years-old; /i, ?, ?, ?, u/ from 2;9 to 3;4 years-old and 3;5 to 4;1 years-old was observed in both {monophthong+’da’} and meaningful words. On the other hand, monophthong /o/ showed relatively low, compared with other phonemes. Thus, two-way ANOVA was conducted where independent variables were age group and type of words to explore main effect and interaction. According to the results of multiple comparisons revised by Bonferroni, only in correctly articulated /o/ F2 of {monophthong+’da’}, significant difference was observed from 2;1 to 2;8 years-old and 3;5 to 4;1 years-old. The significantly different values of F2 /o/ showed that compared with 2;1 to 2;8 years-old, /o/ of 3;5 to 4;1 years-old tended to go backward in the acoustic vowel space, which can be assumed that /o/ F2 (backwardness) develops later in the course of development than F1 (highness).
The rate of correct articulation of diphthong based on phonemes showed that 2;1 to 2;8 years-old had a low rate of correct articulation in all phonemes. 2;9 to 3;4 years-old were observed to have more than 80% of correct articulation rate of /j?, j?, jo, w?, w?/; and 3;5 to 4;1 years-old were of /j?, j?, w?, w?/ in both {diphthong+’da’} and meaningful words.
Second, the results of acoustic analysis provided that formant (F1 and F2) of monophthong tended to decrease in general as age went up. In addition, in a formant /?/ F2 from a correctly articulated {monophthong+’da’}, 3;5 to 4;1 years-old were observed to have backing of the tongue, compared with 2;1 to 2;8 years-old. Also, it was found that 3;5 to 4;1 years-old showed vowel rising in F1 of /i/, compared with other age groups. In terms of the Euclidean distance difference of {monophthong+’da’}, the Euclidean distance of /?-?/ showed the distance from 2;1 to 2;8 years-old was longer than that of 2;9 to 3;4 years-old, which was about the same as the one from 3;5 to 4;1 years-old. In addition, the Euclidean distance of /?-?/ showed that the distance from 3;5 to 4;1 years-old was about the same as that of 2;9 to 3;4 years-old, which was longer than that from 2;1 to 2;8 years-old; the Euclidean distance of /?-?/ from meaningful words monophthong showed that the distance from 2;1 to 2;8 years-old was longer than that from 2;9 to 3;4 years-old, which was longer than that of 3;5 to 4;1 years-old.
In terms of F2 differentiation slope and regression slope, which were diphthong related variables, age group of 3;5 to 4;1 years-old showed a big slope change in general and the statistically significant difference between two variables was observed in diphthongs from meaningful words such as /j?, j?/. In specific, F2 differentiation slope and regression slope of /j?, j?/ were observed as biggest in the age group of 3;5 to 4;1 years-old, which is assumed to produce diphthong more clearly compared with younger age group, by expanding movement scope in articulating.