Acoustic properties of vowels /uː/ and /ʊ/of Serbian EFL students with partial

hearing impairment

Nikola MitićUniversity of Niš

Abstract: This paper presents the findings of a study of theacoustic properties of vowels /uː/ and /ʊ/ of Serbian EFLstudents with partial hearing impairment. The participantsincluded 4 Serbian pupils from a special school for deaf andhearing impaired. All of the participants have been learningEnglish as a foreign language at the beginner level (the firstyear of study). Their vowel production was recorded at thelevel of individual word production due to the inefficiency ofthe hearing impaired participants to produce larger linguisticunits. The acoustic measurements included F1 and F2 formantfrequency values and vowel duration. The results show vastdifferences in vowel positioning compared to the measurementsdone on hearing children found in the literature.Key words: EFL students’ vowels, acoustic properties, hearingimpairment, formant frequency, duration

1.Introduction

Acoustic properties of vowel production have not beenstudied enough in the speech of the hearing impaired. Formantfrequencies, especially the first (F1) and the second (F2)formants, are used to provide an acoustic analysis of vowels.The formants are then plotted against each other and points onthe graph for each vowel group into distinctive regions. Therelationship between the acoustic vowel plot and thearticulatory map is fairly close and it tells us that F1 (whichincreases and then decreases as the vowels go from /ɪ/ to /ʊ/)represents tongue height, and that F2 (which decreases from /ɪ/to /ʊ/) represents the constriction of the tongue in the front-back plane (Joe, 1982). Analysis of spectrograms of thispopulation can be problematic due to the tendency of the

fundamental frequency (F0) of hearing-impaired speakers to beoften rather high. This sometimes generates the mismatchbetween the source and the bandwidth of the spectrogram filter,which obscures the important harmonics. Moreover, perturbationsin the source, inappropriate management of intensity andinappropriate nasalization may also make a clean analysisharder to achieve. The purpose of this study is to carry out anacoustic analysis of a set of words produced by partially-hearing Serbian children studying English as a second language,aiming at facilitating the production of a more English-likeperformance.

2.Theoretical framework

A traditional classification of vowels utilized categoriessuch as tongue position (high-low and front-back), tonguetension (tense-lax), and the degree of lip rounding. Manystudies have shown that hearing-impaired speakers are found toproduce back vowels correctly more often than front vowels(Boone, 1966; Geffner, 1980; Mangan, 1961; Nober, 1967; Smith,1975) (as found in Joe, 1982), as well as produce low vowelsmore often correctly than those with mid or high tongueposition (Geffner, 1980; Nober, 1967; Smith, 1975) (as found inJoe, 1982). Boone (1966) (as found in Joe, 1982) argues thatthe hearing-impaired tend to hold their tongue in a low backposition, generating resonance that interferes with theperception of front vowels. On the other hand, Stein (1980) (asfound in Joe, 1982) showed that vowels produced by the hearing-impaired exhibit fronting of back vowels.

Sheridan (1948) (as found in Markides, 1983) conducted astudy over 100 hearing-impaired children, among whom 57, as shefound, had speech defects. By examining their speech, sheconcluded, among other things, that vowel substitution wasrare. She also pointed out that the greater the degree ofdeafness the more distorted the speech of these children was.More recent studies (Markides, 1967) (as found in Markides,1983) dealt with children with partial hearing impairment anddeaf children using the articulation test. The vowel errors thechildren made were assigned to several categories: 1. vowelsubstitution (/ɒ/ for /ɪ/ or /ʌ/ for /ɒ/); 2. vowelneutralization (cases where the neutral vowel /ə/ wassubstituted for another vowel, namely /ə/ for /ɪ/ or /ə/ for

/e/); 3. vowel prolongation (pronouncing /uː/ instead of /ʊ/ or/ɔː/ instead of /ɒ/) etc. The deaf children made 482 vowel anddiphthong errors, an average of 8.32 per child. The partially-hearing children made 35 vowel and diphthong errors, 1.29errors in average per child. The vowel prolongation errors werefound to be among the most frequently made errors by partially-impaired, right under the diphthong errors that were the mostnumerous.

Some studies have shown that formant frequencies of deafchildren’s vowels trend toward that of the natural vowel /ə/(Angelocci et al., 1964; Monsen, 1974, 1978) (as found in Joe,1982). It is worth mentioning that subjects in the Angelocci etal study produced vowels in CVC monosyllabic words, whereas theparticipants of the Monsen studies produced vowels in sentencecontext. The results of these studies suggest that hearing-impaired speakers use a restricted amount of tongue movement toachieve vowel differentiation. Moreover, researchers have alsopointed out that differences in vowels produced by hearing-impaired speakers are achieved primarily by means of variationin fundamental frequency (Angelocci et al., 1964; Martony, 1968)(as found in Joe, 1982). In addition, Monsen (1976) (as foundin Joe, 1982) also stated that the second formant of vowelsremained around 1800 Hz rather than varying as different vowelsare produced. This is due to the fact that many hearing-impaired speakers have residual hearing only in the frequencyrange of F1 and not in the rage of F2. Another reason may bethe relative invisibility of tongue constriction from a front-to-back position that is responsible for the second formants.

3.Methods

The subjects of the study were 3 male pupils and 1 femalepupil with partial hearing impairment. The criterion forchoosing the subjects was the same amount of time spentlearning English as a second language (6 months). All subjectswere approximately the same age (9-11). All the participantswere implanted with a cochlear implant on one of the ears.Moreover, they have all been enrolled in an early interventionsince early age but the implant was not build in until later.Participants combine sign language and the oral mode ofcommunication in their daily activities, but the sign languageis predominant.

The subjects were recorded uttering isolated wordscontaining one of the vowels /uː, ʊ/ in long and shortsyllables. The list of English words contained CVC monosyllabicwords (moon, noon, soon, put, look, and took). The recordings weremade in a soundproof booth of the Special School “Bubanj” inNis. They were analysed using the SFSWin 1.9 program for speechanalysis (Huckvale, 2013). The first and second (F1 and F2)formant frequencies were measured, as well as the vocalicduration. The mean values of each formant were calculated. Dataon formant frequencies in hearing children from the literature(Hillenbard et al., 1995) were used as the control values.Furthermore, the mean values were used as the basis of plottedgraphs of F1 and F2 which visually represent the comparison ofnative production of hearing speakers and the non-nativeproduction of hearing-impaired children.

4.Research results

Let’s start by observing the frequency values (F1 and F2)of the English long vowel /u:/ and the Serbian vowel /u/ inlong syllables shown in Table 1. Frequency values for theEnglish long /u:/ have been taken from the Hillenbrand et al(1995) study, measured in children. Frequencies for the Serbian/u/ in long syllables were taken from Čubrović, Paunović (2009)and then modified in accordance with two facts. Namely, theformant frequencies in children are 25% higher than those inman; and the formant frequencies in women are 18% higher thanthose in man, when we look at Serbian vowels (Kostić, 1971).

FORMANT English /u:/ Serbian /u/F1 494 Hz 402 HzF2 1345 Hz 801 Hz

Table 1. Formant frequencies of the English long vowel/u:/ and the Serbian vowel /u/ in long syllables

FORMANT English / ʊ / Serbian /u/F1 568Hz 423HzF2 1490Hz 906Hz

Table 2. Formant frequencies of the English short vowel / ʊ /and the Serbian vowel /u/ in short syllables

Table 2 represents the first and the second formantfrequencies of the English short vowel / ʊ / and the Serbianvowel /u/ in short syllables. We can note that English /u:/ inchildren is back but has a tendency of fronting, reflected inthe high values of F2. Moreover, it is rather close-mid thanclosed, as can be seen from the high value of F1. Incomparison, the Serbian /u/ in long syllables is almost equallyclosed but occupies a much more back position. When we talkabout short variants, we can observe that high F2 in theEnglish short /u/ leads to more centralized articulation, morecentralized than the English long /u:/, and that higher F1represents semi closed, instead of closed articulation as inmale English speakers. In comparison to Serbian short /u/, wecan see that both F1 and F2 frequencies are lower, thusreflecting less centralized and more closed articulation.

Table 3 shows the mean values of the subjects’ productionof the English long vowel /u:/. The results show that childrenwith hearing impairment have considerably lower F1 values incomparisons to the English F1 value in the realization oflong /u:/. This indicates more closed articulation. Evencompared to the Serbian vowel /u/ in long syllables, F1 isstill lower. Opening up the jaw in pronunciation may result inhigher values of F1. On the other hand, F2 value is lower thanthe one in the realization of the English long vowel /u:/, but372 Hz higher than the value of the second formant of theSerbian vowel /u/ in long syllables. This can be also visibleon the formant chart represented in Figure 1.

FORMANT English /u:/ Serbian /u/ Subjects’/u:/

F1 494Hz 402Hz 383HzF2 1345Hz 801Hz 1171Hz

Table 3. F1 and F2 mean values of the English longsound /u:/, Serbian sound /u/ in long syllables and the English

sound /u:/ produced by the hearing impaired children

Mean values of the subjects’ production of the Englishshort vowel / ʊ / are shown in Table 4. Again, we can observethe similar pattern. Hearing impaired children seem to have F1value lower than the F1 value of the English short / ʊ /, aswell as than the F1 value of the Serbian /u/ in short

syllables. This indicates that their pronunciation in terms ofopenness of the jaw is closer to the Serbian pronunciation ofvowel /u/ in short syllables than to the English short vowel /ʊ /. F2 values of the English short vowel / ʊ / are, incontrast, more close to the realization of the English shortvowel / ʊ /, but still not quite centralized as it should be.This is also represented on the formant chart in Figure 2.

FORMANT English / ʊ / Serbian /u/ Subjects’/ ʊ/

F1 568Hz 423Hz 417HzF2 1490Hz 906Hz 1140HzTable 4. F1 and F2 mean values of the English short

sound / ʊ /, Serbian sound /u/ in short syllables and theEnglish sound / ʊ / produced by the hearing impaired children

This study also measured the duration of these vowels aspronounced by hearing impaired children. The results are shownin Table 5 below. The control values of the duration of theseEnglish vowels are taken from the Hillenbrand et al (1995) studyfor values assigned to child speakers.

DURATION Subjects Englishuː 433ms 278 msʊ 291ms 247 ms

Table 5. Duration of English long and short vowels /u:/and / ʊ / from the literature and as pronounced by the hearing

impaired children

The results show that children with hearing impairmenttend to prolong English long vowel /u: /. It is almost as twiceas long as the one in children with regular hearing. Thisamount of quantity of the vowel /u:/ can have a great influenceon the distortion of the overall quality of the vowel /u: /.Nevertheless, the duration of their English short vowel / ʊ /is not that different from the duration of the short / ʊ / inhearing children.

Figure 1. Plotted F1:F2 graph of the English vowel /u:/produced by hearing children whose native language is English(triangle) and by our subjects (square), as well as of theSerbian vowel /u/ in long syllables

Figure 2. Plotted F1:F2 graph of the English vowel / ʊ /produced by hearing children whose native language isEnglish (triangle) and by our subjects (square), as wellas of the Serbian vowel /u/ in short syllables

5.Conclusion

This paper showed that children with partial hearingimpairment exhibit more closed articulation of the English longvowel /u:/ and that opening up their jaws while speaking mayresult in a more native like performance. The formant chartsshowed us that the articulation of both English vowels /u:/ and/ ʊ / are closer to the Serbian /u/ in long and short syllablesrespectively. As for the duration, the findings are as expectedand in accordance with the previous studies on vowel errors inwhich vowel prolongation is one of the prominent ones. Eventhough the measurements of F1 and F2, as well as of vowelduration, are not enough to describe all the factors that gointo the production of a certain vowel, the findings can beapplied in a direct individual work with children with hearingimpairment in an attempt to reshape the articulatoryconfigurations of their vocal tracts to produce the desiredvowels more accurately.

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