論 文Interaction of the Factors Aﬀecting the Accuracy of L2
Pronunciation — based on the acoustic analysis ofEnglish voiced plosives in word-ﬁnal position

Toru Isono

要

旨

第二言語学習者の発音は，学習者の母国語音がそのまま使用されているような母国語なまりの発音から，その第二言語の母語話者に近い発音まで，実に多種多様である。また，同じ学習者の発音だけをみた
場合でも，状況によってその正確さに大きな差が観察され，この点においても学習者言語は多様であるといえる。中間言語研究において，
この「多様性」の研究は主要課題の1つとされ，主に，「学習者が自分の第二言語発音に注ぐ注意の度合い」と「発音の正確さ」の関係から議論されてきた。本論文では，「注意の度合い」に，「第二言語音自体の難しさ」，「学習者の学習レベル」という2つの要因を加え，それらがどのように関係しあいながら第二言語学習者の発音の正確さに影響を与えているのかを，日本人英語学習者から得た英語破裂音の音響分析
データを踏まえながら，実証的に考察していく。
キーワード：L2 pronunciation（第二言語発音），plosives（破裂音），interlanguage（中間言語），the amount of attention（発音への注意の度合い），intrinsic
diﬃculty（言語音自体に備わる自然言語的観点からみた発音の難しさ）

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1. Introduction
In the acquisition process of Second Language (L2) pronunciation, as Nemer(1971) reported, L2 learners tend to have the stages to produce many examples of
elements which do not belong to either First Language (L1) or L2 sound systems
before they master the accurate L2 pronunciation. We also often hear the examples of
L2 learners who can produce native-like L2 sounds in some cases but cannot in othercases. These facts support the idea that “learners do not progress from a state of nonacquisition to a state of acquisition, but rather pass through a series of stages” (Ellis,
1994, 96). In the developmental process, L2 learners are supposed to construct anddevelop their own regular and systematic linguistic systems, and it causes the productionof the above pronunciation particular to L2 learners (See details in Isono, 2005). The
linguistic systems which lie midway between L1 and L2 are named interlanguage, andthe aim of interlanguage study is to clarify the characteristics of the developmental
process of the linguistic systems.
The concept of interlanguage has been revised and developed as the result of
being aﬀected by various trends of Second Language Acquisition (SLA) research.
Yamaoka (1997) summarises the characteristics of the current accounts of interlanguage
into the six features presented, as referred to in the studies of Hatch (1983) and Richards(1985).
1. Systematicity
4. Universality

2. Permeability
5. Variability

3. Transitionality
6. Fossilisation(Hatch, 1983; Richards, 1985, cited in Yamaoka, 1997: 75-76)1

Among the above six characteristics, this paper focuses on variability in interlanguage,
and examines what factors aﬀect the accuracy of L2 pronunciation, based on the
acoustic analysis of English plosives produced by Japanese learners.

2. Variability
When SLA researchers face the issue as to how they explain the variability ofinterlanguage theoretically, their views diﬀerentiate depending on whether they―2―

Interaction of the Factors Aﬀecting the Accuracy of L2 Pronunciation
approach it from the standpoint of the homogeneous competence model or theheterogeneous competence model. The diﬀerence between the two approaches is whether
the source of the variability of interlanguage refers to non-linguistic factors aﬀecting alearner’s linguistic performance or a learner’s linguistic competence itself.
In the homogenous competence model, linguistic competence is separated fromlinguistic performance. To sum up, all speakers’ eﬀorts at performance, and the factthat they do not always manifest the ideal system in the way they speak are dismissed
as irrelevant to an understanding of the ideal system (Chomsky, 1965, cited in Ellis,
1985). Linguistic competence as the ideal system is regarded as a homogeneous one,so variability of interlanguage is considered as the phenomenon in the process of
performance, and all variability is classiﬁed as non-systematic variability. Since thehomogeneous model distinguishes linguistic competence from linguistic performanceand mainly focuses on the former only, the variability of interlanguage has seldom
become an object of study for SLA researchers supporting this model. This notion iscondensed in Gregg’s (1990: 379) comment, “since variability is a part of performance,
it is perfectly possible that one could formulate an interesting theory of IL variation aspart of a theory of IL performance. But to try to force variability into a theory of theacquisition of competence by claiming that competence is itself variable is selfdefeating”. In this sense, as Ellis (1985) noted, the homogeneous competence model
discounts stylistic variability or rather treats it only as one of the aspects ofperformance.
In contrast to the homogenous model, in the heterogeneous model, the variability
is explained at the level of linguistic competence. In short, in the model, a learner’s
linguistic competence is regarded as being heterogeneously formed by various kinds oflinguistic knowledge, and this fact is considered as a reason of the occurrence of thevariability. Some scholars (e.g., Bialystok, 1982; Bialystok & Sharwood Smith, 1985;
Hulstijin, 1990) present various theoretical frameworks within this model to explain
the variability in interlanguage.
As far as the experimental research in L2 pronunciation which was conductedwithin the heterogeneous model is concerned, it has become common to explain thevariability in interlanguage phonology in relation to the amount of attention that L2
learners are thought to have given to language production. In short, it is predictedthat the more attention is given, the greater the accuracy observed.
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A study which became the basis of SLA research was Labov’s (1970) experiment
which investigated the variability in a natural language. He investigated the speech
patterns of New Yorkers in the following ﬁve tasks: (1) casual speech; (2) careful
speech; (3) reading aloud; (4) the reading of word lists aloud; and (5) minimal pairs.
There was an assumption that little attention was paid in task 1, but greater attention
was given in task 5. He found that the more attention was given, the more instancesof [ ], which is a prestige sound in New York English, were observed, but the lessattention the subjects paid, the more instances of non-prestige variants, such as [t],were found.
For SLA research, Dickerson (1975) investigated the production of the English[z] in the speech of 10 Japanese learners three times in a nine month period using thefollowing three tasks: (1) free speech; (2) reading dialogues aloud; and (3) reading
word lists aloud. She found that the correct [z] variant was observed most frequently
in task 3, followed by task 2, and least often in task 1. Similarly, other researchers
also reported on the ﬁnding of higher frequencies of native-like pronunciation intasks involving reading aloud or imitating a model sound, and of less native-likepronunciation in spontaneous speech (e.g., Tarone, 1982, 1983; Beebe & Zuengler,1983).
The main aim of the current research which will be outlined in later sections is to
examine the relationship between the amount of attention and the accuracy (=variability) of L2 pronunciation, and to propose some other factors which should betaken into account, in order to clarify the interaction of the factors aﬀecting theaccuracy of L2 pronunciation.

3. The Outline of the Research
3-1. Subjects
Isono (2004) investigated Japanese learners’ preferred strategies (whether it was‘epenthesis’ or ‘devoicing’ or ‘deletion’) for English plosives in word-ﬁnal position, andclariﬁed that the preference might vary according to learners’ learning proﬁciency.
The analysis was based on the data which were produced by 30 Japanese subjects
consisting of 3 groups (the US, the PS, and the AS groups) and 8 native speakers of―4―

Interaction of the Factors Aﬀecting the Accuracy of L2 Pronunciation
English who had been teaching English in Essex, East Anglia, England (the NS group).
Both Isono (2004) and the current research were conducted in a series of the study on
English plosives in word-ﬁnal position, so the same subjects who participated in Isono(2004) were also enrolled in the current research.
The US groups (Male = 4, Female = 6) consisted of 3rd and 4th year university
students in Japan, whose major was English and American literature. In this group,
no one had studied abroad, except 1 subject who stayed in England for a month. The
PS group (Male = 4, Female = 6) was made up of postgraduate students in a Japanese
university, who majored in linguistics and literature2. 2 of the subjects had experiencestudying abroad. One studied in America for a year and the other in Australia for ayear. The AS group (Male = 3, Female = 7) was comprised of postgraduate students
who had been studying in University of Essex, England. All of them were in the
Language and Linguistics Department, and had stayed in England for more than 3
years3. According to the result of the reading-aloud test, it was clariﬁed that theJapanese groups were at diﬀerent levels in their pronunciation proﬁciency, whichmeans, the most advanced Japanese subjects in order were the AS group, then the PS
group, and the US group (See details in Isono, 2004).

3-2. Method and Procedure
The subjects were enrolled in the following two tasks. In the ﬁrst task, the subjects
were asked to produce sentences including the following six target words: cap – cab;pat – pad; and back – bag, which manifested the [CæC] syllable, and contained thetarget English plosives which had voiceless — voiced contrasts in word-ﬁnal positions.
According to Yavas (1993, cited in Yavas, 1994), ﬁnal voiced plosives are more accurately
produced when the preceding vowel is a low vowel rather than a high vowel, so thesetarget words were relatively suitable for producing accurate voicing of a voiced plosive
in word-ﬁnal position. In this task, when the test sentences were constructed, thefollowing three environments were conditioned to elicit the speech data in equal
conditions as done in the second task, described below, as far as possible. Firstly, theﬁnal phoneme of a word which preceded the target word was always /ɪ / (including
diphthongs /aɪ / and /eɪ /). Secondly, the ﬁrst phoneme of a word which followed thetarget word was always /ə /. Finally, the test sentences were constructed by using words
which were normally learnt before the 2nd-year in high school in Japan. In addition,
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No. 17the test sentences were constructed so that they made sense whichever one of theminimal pairs was used, in order to prevent the subjects from sensing the target words
and to minimise the eﬀect of intonation on the target words. Two diﬀerent sentenceswere made for each pair, so we had four possible sentences for each pair (See Appendix).
In each test sentence, the upper sentence was included in Test A, and the lower sentencein Test B. Half of the subjects in each group were asked to read Test A, and the otherhalf were instructed to read Test B. In both tests, the sentences were mixed with someother sentences for another experiment. The subjects were asked to repeat the test
sentences three times in a row.
In the second task, the subjects were instructed to produce the target words inthe carrier sentence ‘I say
again.’. The target words were shown to the subjects
before starting the task. Each target word was randomly presented at regular intervals
to each of the subjects three times.

3-3. Acoustic Measurements
The speech production data were digitally recorded, and 1368 diﬀerent
productions (38 subjects × 2 tasks × 6 words × 3 repetitions) were elicited. According
to Isono (2004), most of the Japanese subjects had a great diﬃculty with voicing
English voiced plosives in word-ﬁnal position. On the other hand, they succeeded indevoicing English voiceless plosives in word-ﬁnal position. Therefore, for the aim of
the current research, we focus on the 684 productions of voiced plosives in word-ﬁnal
position (38 subjects × 2 tasks × 3 words × 3 repetitions).
By using a digital waveform editor and a digital sound spectrogram, each of the
productions was characterised according to the following two acoustic features: 1.
Stop closure duration time (SD); 2. Voiced closure duration time in the stop closure
duration time (VD). ‘SD’ was regarded as the interval from the end of a vowel to therelease of a ﬁnal plosive. ‘VD’ was deﬁned as the interval from the end of a vowel tothe point where voicing energy was no longer detected. Based on the two features,‘degree of voicing’ (‘VD’ divided by ‘SD’) was calculated.

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Interaction of the Factors Aﬀecting the Accuracy of L2 Pronunciation
Figure 1. An example of ‘pad’ ����� in English

Figure 1. An example of ‘pad’ [pæd] in English

|VD |
|SD |
The mean score of the three repetitions for each of the tasks was regarded as eachsubject’s representation for a target word in each task.

4. Research Questions and Hypothesis
Past studies (Ohala & Riordan, 1979; Yavas, 1993, cited in Yavas, 1994) show that avelar plosive [ɡ] is the most intrinsically diﬃcult plosive to voice in word-ﬁnal position.
In addition, the alveolar plosive [d] is assumed to be more diﬃcult to voice than thebilabial plosive [b] in word-ﬁnal position. The reason of this hierarchy in the intrinsic
diﬃculty of voicing is attributed to the diﬀerent eﬀects of aerodynamics, which arecaused by the diﬀerent places of articulation, as Yavas (1994: 273) explains as follows:“the larger the supraglottal area is, the better it can accommodate glottal ﬂow for sometime before oral pressure exceeds subglottal pressure and stops the vocal cord
vibration”.
Based on the above discussion, as far as the degree of voicing is concerned, it isexpected that the bilabial plosive [b] would be produced more accurately than thealveolar plosive [d], and, in turn, the alveolar plosive would be more accurately
produced than the velar plosive [ɡ]. However, a more interesting question is how
these diﬀerences in the intrinsic diﬃculty aﬀect the accuracy of the productions
exhibited by L2 learners who are at various levels of learning. In other words, theresearch question is that what diﬀerent kinds of patterns of improvement emerge
according to the diﬀerences in the intrinsic diﬃculty, when comparing advanced
learners’ productions with less advanced learners’ productions.
The most likely assumption is that a signiﬁcant improvement would be observed

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No. 17as the intrinsic diﬃculty eases. To put it in an extreme way, great and noticeable
improvements would be obtained for [b] and [d], but no signiﬁcant improvement
would be found for [ɡ] because of the intrinsic diﬃculty.
However, taking the large similarity in voiced plosives between English andJapanese into consideration, we can propose another assumption which predicts thata learner’s learning level would be a signiﬁcant factor only for the accuracy of [ɡ], but
not for [b] and [d], since the latter voiced plosives would be accurately produced
irrespective of a learner’s learning level. In this case, a signiﬁcant improvement would
be found only for [ɡ], but not for [b] and [d], although the latter plosives would
constantly have high degrees of accuracy.
As far as the data gathered on Japanese learners are concerned, these two
assumptions are illustrated as Figure 2.
Figure 2. Descriptions of two hypotheses for the accuracies of voiced plosives
Figure 2. Descriptions of two hypotheses for the accuracies of voiced plosives
Assumption 1[b]Accuracy[d]�

[ɡ]More advanced learners
Assumption 2[b]�
Accuracy

[d]�
[ɡ]�

More advanced learners―8―Interaction of the Factors Aﬀecting the Accuracy of L2 Pronunciation
This paper hypothesizes that whether we ﬁnd the ﬁrst assumption or the second
assumption depends on how much a learner’s attention is drawn to the voicing
contrasts. In casual speech style, we would ﬁnd the ﬁrst assumption since it is the stylein which little attention is paid to form, and the essence of the linguistic rule would bedirectly reﬂected in the productions. On the other hand, in formal speech style, inwhich a large amount of attention is given to form, we would expect the second
assumption. This is due to the fact that revision, which is caused by attention to form,in the accuracy of [b] and [d] is activated regardless of a learner’s learning level, whilerevision for that of [ɡ] is activated only by advanced learners, because of the intrinsic
diﬃculty of the motor control.
To put it more concretely, the hypothesis predicts that the results in the ﬁrst task
in which the target words were hidden will look like the graphs in the assumption 1 inFigure 2, meaning that a signiﬁcant improvement in the degree of voicing is observedin [b] and [d], but not in [ɡ], in the Japanese subjects’ data. On the other hand, theresults in the second task in which the target words were not hidden are assumed tobe similar to the graphs in the assumption 2, which suggest that a signiﬁcant
improvement is found only in [ɡ], but not in [b] and [d].

5. Results
In order to investigate the eﬀect of the interaction among the amount of attention,
the intrinsic diﬃculties of English plosives in word-ﬁnal position, and the subjects’learning levels on the diﬀerences in the accuracy, the productions in the ﬁrst and thesecond tasks were analysed. As explained earlier, the target words in both tasks wereplaced according to the same condition in which the last phoneme of a word precedinga target word was /ɪ / and the ﬁrst phoneme of a word following a target word was/ə /.
Figure 3 shows the results of the ﬁrst task. The degree of voicing of [b] in wordﬁnal position is represented by a thin solid line, that of [d] by a thin dotted line, andthat of [ɡ] by a thick solid line. This also applies to Figure 4.

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Figure 3. Results for Task 1

Figure 3. Results for Task 1
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Firstly, it should be noted that the degree of voicing by the NS group was
signiﬁcantly higher, compared with the results of past studies (e.g., Yavas, 1993, cited in
Yavas, 1994). These results might suggest that this experiment suﬃciently attained theaim that desirable conditions were provided for voicing in voiced plosives in wordﬁnal position.
Looking at the diﬀerences in the degree of voicing among the voiced plosives, we
ﬁnd that, as predicted, the degree of voicing for the velar plosive [ɡ], which is said tobe the most diﬃcult plosive to voice, was constantly lower than for the other two
voiced plosives in all groups.
We direct our attention to the issue of how the degrees of voicing of these voiced
plosives were improved according to the learning levels of the subjects. As Figure 3
indicates, for the Japanese groups, a much sharper ﬂuctuation is found in [d] than in[b] and [ɡ]. A series of one-way ANOVAs revealed that a signiﬁcant group diﬀerence
was found for [d] at the .01 level, and for [ɡ] at the .05 level, but it was not obtainedfor [b] at the .05 level. According to Bonferroni’s Post Hoc Tests, the [d] values of the
PS and AS group were signiﬁcantly greater than the value of the US group, although
they were also signiﬁcantly smaller than the value of the NS group. On the other
hand, no signiﬁcant group diﬀerence for both the [b] and [ɡ] values was found among
the Japanese groups at the .05 level. The diﬀerence in the results between [b] and [ɡ]values was that there were no signiﬁcant diﬀerences between the Japanese groups’ [b]― 10 ―

Interaction of the Factors Aﬀecting the Accuracy of L2 Pronunciation
values and the NS group’s [b] value; however, all the Japanese groups’ [ɡ] values weresigniﬁcantly smaller than the value of the NS group, at the .05 level.
The above statistical results conﬁrm the hypotheses for the acquisition processesof the degree of voicing of [d] and [ɡ], but not for the hypothesis regarding [b], in thecase of the ﬁrst task. The developmental process of the voicing of [b] was assumed to
be a process such that the sharpest ﬂuctuation would be found from the US group to
the AS group, because it was intrinsically the easiest phoneme to voice among thevoiced plosives. However, as it was revealed by the above statistical results, there wasno signiﬁcant diﬀerence in the degree of voicing of [b] between the Japanese groups
and the NS group. This result clariﬁes that, in the degree of voicing, even the least
experienced Japanese group could attain a performance which was not distinguishedfrom the native speakers, and this could be interpreted to mean that the voicing of [b]is comparatively easier than that of the other two voiced plosives. The reason why the
degree of voicing in [b] was lower than that in [d] in most of the groups will be
discussed by referring to the results in the second task.
Figure 4 shows the results in the second task in which the target words were nothidden.
Figure 4. Results for the second task

Figure 4. Results for the second task�������

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Concerning the intrinsic diﬃculties of voicing [b] and [d], we ﬁnd a relatively
clearer picture in the second task than in the ﬁrst task that the voicing of [b] might be― 11 ―

愛知大学

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No. 17

easier than that of [d]. This result also suggests that the reason why low degrees ofvoicing in [b] were obtained in the ﬁrst task can be partly attributed to the possibility
that the target words containing [b] in word-ﬁnal position were placed in theenvironments where it was diﬃcult to voice them, despite of the eﬀort of ﬁxing thelast phoneme of words preceding the target words and the ﬁrst phoneme of words
following them. For example, in the test sentence ‘I saw a grey cab at the right-handcorner.’, some subjects might have made a pause between ‘cab’ and ‘at’, and generally,
the possibility of devoicing a word-ﬁnal voiced plosive becomes high when it isfollowed by a pause compared with the case when it is followed by a low vowel (Yavas,
1994). However, even though we acknowledge this possibility, it is still not clear why
these conditions aﬀected only the productions of the PS, AS and NS groups, but not
those of the US group.
Regarding the developmental processes in the voicing from a less advanced to amore advanced group, we ﬁnd that all the graphs showed a rise toward the right-handside, which means that the voiced plosives were more accurately voiced as the subjects
became more advanced learners. However, going from the US group through to the
PS group, a much sharper ﬂuctuation is found in [ɡ] compared with [b] and [d].
One-way ANOVA revealed that a signiﬁcant group diﬀerence was not found for [b]and [d]: F (3, 34) = 2.53, p > .05; F (3, 34) = 1.83, p > .05, respectively. On the other
hand, a signiﬁcant diﬀerence was obtained for [ɡ]: F (3, 34) = 5.86, p < .01, andaccording to Bonferroni’s Post Hoc Tests, the value of the US group was signiﬁcantly
smaller than the other groups at the .05 level. These results for the second task conﬁrm
the hypothesis that the subjects’ learning levels are a signiﬁcant factor in the
improvement of [ɡ], but not for [b] and [d], in the formal condition, in which theamount of attention given to the items is relatively large.

6. Conclusion
To sum up the above results, ﬁrstly, the degrees of intrinsic diﬃculty in voicing
English voiced plosives in word-ﬁnal position were conﬁrmed, which is that the velar
plosive [ɡ] is more likely to be devoiced than the alveolar plosive [d], and it is also
more likely to be devoiced than the bilabial plosive [b]. Regarding the question of
how these diﬀerences in intrinsic diﬃculties contribute towards characterising the― 12 ―

Interaction of the Factors Aﬀecting the Accuracy of L2 Pronunciation
productions of Japanese learners, the current experiment suggests that the eﬀect varies
according to the amount of attention being paid to the ﬁnal voiced plosives, as well as
learners’ learning levels. Concretely, in the results, when little attention was given, theaccuracy in the voicing of the alveolar plosive [d] was the only feature distinguishingadvanced learners from less advanced learners. This was due to the fact that, forJapanese learners, the bilabial plosive [b] was easy, and the velar plosive [ɡ] was diﬃcult
to voice, regardless of their learning levels. On the other hand, when a large amountof attention was given, the accuracy of the velar plosive [ɡ] became the distinguishingfeature. This was due to the fact that less advanced learners could not accurately
produce it regardless of how much attention was given to it, but more advanced
learners could according to the amount of attention. On the other hand, concerning[b] and [d] in word-ﬁnal position, which are easier plosives to voice than [ɡ], they were
accurately voiced regardless of learners’ learning levels provided suﬃcient attention
was given to the features.
As previously discussed, it is said that the variability of interlanguage is conditionedby the amount of attention paid to an item. In short, the amount of attention given
to an item had been assumed to be directly reﬂected in the accuracy of production.
However, the current study implies that this is not always true, and that the eﬀect of
the amount of attention devoted to the improvement in accuracy is conditioned bysomething resembling a ‘ﬁlter’, which is the intrinsic diﬃculty of an L2 item. Whenthe ﬁlter is thin, which means the target item is intrinsically easy, the amount ofattention tends to eﬀectively aﬀect the accuracy of production. On the other hand,when the ﬁlter is thick, which means the target item is intrinsically diﬃcult, the eﬀectof the amount of attention is minimised since the eﬀect tends to be obstructed by the
thick ﬁlter. In both cases, the ﬁlter is supposed to be made thinner according to alearner’s proﬁciency level, to some degree. This paper suggests that, in order to clarify
the mechanism of the variability of interlanguage, not only the amount of attention
but also at least a learner’s proﬁciency level and the intrinsic diﬃculty of the L2 item
should be taken into account, since the latter are the features controlling the eﬀect oflearners’ attention on the accuracy of their L2 productions.

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No. 17Notes
1. The translation from Japanese to English was made by the author of this paper.
2. No one had studied a subject connected with any kind of pronunciation as their majors.
3. In the same way as the PS group, only the subjects whose majors were not relevant to any kindof pronunciation study were selected.

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Appendix
Test sentences for [p] – [b] in word-ﬁnal position★ Target words – cap & cab
Test A I saw a grey cap at the right-hand corner.
Test B ◦I saw a grey cab at the right-hand corner.
Test A He moved his dirty cab aside.
Test B ◦He moved his dirty cap aside.

Test sentences for [t] – [d] in word-ﬁnal position★ Target words – pat & pad
Test A I pat a chair.
Test B ◦I pad a chair.
Test A I found my pad at the door of my house.
Test B ◦I found my Pat at the door of my house.

Test sentences for [k] – [g] in word-ﬁnal position★ Target words – back & bag
Test A Her pretty back appealed to everyone in the class.
Test B ◦Her pretty bag appealed to everyone in the class.
Test A He said he saw my bag at the station.
Test B ◦He said he saw my back at the station.

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