本研究主要探討母音長度對台灣英語學習者在英語鬆緊母音(/i/-/ι/, /u/-/υ/) 感知能力的影響。先前的研究指出英語母語人士主要使用音頻訊號(formant properties)來分辨英語的母音，而母音長度(duration)的影響則在音頻訊號模糊時出現。然而，非英語母語人士主要依據母音音長來辨別鬆緊母音的差異。
但是，我們發現大部分先前的研究主要探討非母語人士在音韻層面(phonological level)上的感知能力，因為他們把母音的語音環境(phonetic environment)控制住。如此一來，若是非母語人士不熟悉這些語音環境，他們對母音的學習則會受到影響。所以，我們無法確定他們的學習困難是來自音韻層面或是語音層面(phonetic level)的影響。因此，我們認為有必要再重新檢視非母語人士在語音層面的母音感知能力。
此研究中，我們先透過篩選的方式，從二十位英語母語人士和五十位台灣英語學習者中選出能夠辨別單獨出現的英語鬆緊母音(/i/, /ι/, /u/, /υ/)。接著，已被選上的受試者則會參與一個主要的感知實驗，其用意是測試當母音出現在不同語音環境(/h_d/, /h_t/)的時候，母音長度對他們感知的影響。換句話說，台灣英語學習者在音韻和語音層面的母音感知能力皆透過不同的實驗來測量。
為了測出母音長度的影響，我們使用八個包含英語高鬆緊母音的字(/hid/, /hit/, /hιd/, and /hιt/ for the /i/-/ι/ pair; /hud/, /hut/, /hυd/, and /hυt/ for the /u/-/υ/ pair)，然後在每一對鬆緊母音中，逐一將每個字原有長度調整和其餘三個字相同長度，因此每字會各擁有四種長度之態樣。實驗結果顯示，英語母語人士在鬆緊母音的感知能力不會受到母音長度的影響。然而，台籍英語學生在鬆緊母音的感知能力會受到母音音長的影響，亦即當緊母音(/i/, /u/)愈長，其辨別度愈高；反之，當鬆母音(/ι/, /υ/)愈短，其辨識度愈高。另外，當母音長度相似時(/hit/: 94ms vs. /hιd/: 85ms; /hut/: 104ms vs. /hυd/: 91ms)，台灣受試者傾向選擇有緊母音的字(/hit/, /hut/)而非有鬆母音的字(/hιd/, /hυd/)，此結果有可能是來自台灣受試者母語的影響。整體來說，儘管台灣英語學習者可以分辨單獨出現的鬆緊母音，他們在語音層面的感知上面，卻高度依賴母音長度來分辨鬆緊母音的差異。
為了檢視台灣受試者的母語母音系統是否影響他們的英語母音感知，同一批台灣英語學習者參與一個語音同化的實驗。此實驗中，受試者會聽到兩個聲音，一個是英語的音(English /hit/ or /hιd/; English /hut/ or /hυd/)，另一個是中文的音(Mandarin /i/; Mandarin /u/)，他們會利用一個1到5的等級量表來判斷這兩個音的相似程度，1代表非常不像，5代表非常像。結果顯示，英語緊母音(/hit/ and /hut/)較鬆母音(/hιd/ and /hυd/)相似於中文的母音音類(/i/ and /u/)。這個結果可作為台灣英語學習者的英語感知受到母語影響之證明。
本研究透過音韻和語音兩個層面來探討台灣英語學習者的英語感知能力。實驗結果顯示，台灣英語學習者尚未學習好語音層面的母音音類，因為他們的英語感知持續受到母音長度的影響；換句話說，他們建立起來的音類特性和英語母語人士不同，因為他們利用母音長度而非音頻訊號來辨別英語母音。除此之外，實驗也指出台灣英語學習者的英語感知受到母語的影響，我們也發現這個影響即使在新的音類建立起來之後還持續地在作用。

This study investigates how Taiwanese EFL learners’ perception of English vowels /i/-/ι/ and /u/-/υ/ embedded in the syllables /h_t/ and /h_d/ is affected by vowel duration. Previous studies concerning the perception of English vowel categories have shown that native speakers of English primarily use formant properties in judging vowels, and duration plays a role only when formant information is ambiguous. In contrast, L2 learners are reported to rely on duration cues in identifying English vowels. In other words, formant distribution is contrastive in English and duration is non-contrastive, based on native speaker’ performance, but L2 learners take the cue of duration as being contrastive.
Nevertheless, most of those studies tend to examine L2 learners’ perception at the phonological level since the phonetic environments which carry the target vowels are controlled by the researchers. That is, the vowels are still embedded or surrounded by other sounds meaning that L2 learners may have been affected by the unfamiliar phonetic environment in the experimental contexts. Because of this, it is hard to ascertain which level (phonological or phonetic) their perceptual difficulties should be attributed to. Moreover, due to the fact that several factors can affect the duration of English vowels, such as rhythm and coda types (i.e., voicing feature of final consonants), there is a need to reexamine the role of duration in L2 learners’ perception of English vowels at the phonetic level.
A screening test was carried out to examine 50 Mandarin speakers’ ability to identify the tense and lax vowel pairs /i/-/ι/ and /u/-/υ/ in isolation, and the result served as the baseline. Simply, the purpose of the test was to select the subjects who were capable of identifying the isolated vowels. Twenty English native speakers were also recruited as controls.
It was followed by a main identification experiment which aimed to investigate the effect of duration on the selected Mandarin and English speakers’ perception of the vowels (/i, ι, u, υ/) embedded in the syllables /h_t/ and /h_d/. That is, the speakers’ vowel perception was examined at the phonological and the phonetic level in different tests. In order to investigate the duration effect at the phonetic level, the test items (e.g., /hid/, /hit/, /hιd/, and /hιt/ for the /i/-/ι/ pair; /hud/, /hut/, /hυd/, and /hυt/ for the /u/-/υ/ pair) were manipulated so as to give different durations. Specifically, each word in a vowel pair had four duration versions with the vowel quality unchanged: one version with the length of the original recording and another three versions with manipulated length of the other three words. The findings included that (i) English native speakers’ vowel perception was not affected by vowel duration, (ii) Mandarin speakers’ vowel perception was heavily affected by duration; for instance, in the tokens with the shortest and longest duration, the four vowels (i.e., /i, ι, u, υ/) were correctly identified, which echoed previous studies’ findings, and (iii) when the cue of duration became ambiguous (i.e., /hit/: 94ms vs. /hιd/: 85ms; /hut/: 104ms vs. /hυd/: 91ms), Mandarin speakers tended to respond the words with the tense vowels (i.e., /hit/, /hut/) more than the words with the lax vowels (i.e., /hιd/, /hυd/), which was interpreted as the effect of the L1 vowel inventory. Overall, the results suggested that Mandarin speakers who were able to identify the isolated target vowels in the screening test did not show native-like perception since their identification of vowels was heavily affected by duration at the phonetic level.
In order to examine whether Mandarin speakers’ perception of English vowel categories had something to do with the similarity between the target vowels and their L1 vowel category, an assimilation task involving a five-point rating scale was designed. Mandarin listeners were presented with two sounds, one was an English word and the other was a Mandarin vowel (i.e., English /hit/ or /hιd/ vs. Mandarin /i/; English /hut/ or /hυd/ vs. Mandarin /u/), and asked to rate the similarity between the two sounds according to the five-point scale (i.e., 1 indicating least similar and 5 indicating very similar). The result indicated that English tense vowel in the word /hit/ was more similar to Mandarin /i/ than the lax counterpart in the word /hιd/. Similarly, English tense vowel in the word /hut/ was more similar to Mandarin /u/ than the lax counterpart in the word /hυd/. This provided an explanation for Mandarin speakers’ asymmetrical perception that they had a tendency of selecting the tense vowels more than the lax vowels.
The current research investigated Mandarin speakers’ perception of English vowels in terms of the phonological and phonetic level of analysis. It was found that although Mandarin speakers were capable of identifying the vowels at the segmental level, they had not fully acquired the vowel categories since their perception was affected by duration, which did not occur in the perception of native speakers. To put it another way, the phonological representation of vowel categories formed by Mandarin speakers was different from that by native speakers of English due to the fact that Mandarin speakers used the cue of duration rather than the cue of formant to distinguish English vowels. Moreover, the result also suggested that the cross-language phonetic similarity played a role in predicting relative difficulties in L2 learning since Mandarin speakers’ perception of English vowels was affected by their L1 vowel categories. In particular, we found that the influence of L1 was persistent on the Mandarin subjects’ vowel perception after a new category had been established.

ACKNOWLEDGEMENTS.......................................................................i
ABSTRACT............................................................................................iv
TALBE OF CONTENTS.........................................................................vii
LIST OF FIGURES.................................................................................ix
LIST OF TABLES...................................................................................x
CHAPTER ONE INTRODUCTION.........................................................1
1.1 Background and motivation............................................................1
1.2 Organization of present study.........................................................5
CHAPTER TWO LITERATURE REVIEW...............................................6
2.1 English and Mandarin Chinese vowels...........................................6
2.2 The acquisition of English vowels by Mandarin speakers..............10
2.2.1 The Perceptual Assimilation Model.........................................12
2.2.2 The Speech Learning Model...................................................13
2.3 Durational effect on English vowel perception...............................16
2.3.1 Introduction.............................................................................16
2.3.2 Durational effect on English native speakers’ perception.......16
2.3.3 Durational effect on English L2 speakers’ perception.............18
2.4 The present study...........................................................................22
CHAPTER THREE EXPERIMENT I.......................................................25
3.1 Overall introduction.........................................................................25
3.2 Method............................................................................................26
3.2.1 Screening test..........................................................................26
3.2.1.1 Materials......................................................................26
3.2.1.2 Subjects.......................................................................27
3.2.1.3 Procedure....................................................................27
3.2.1.4 Results and Discussion...............................................28
3.2.2 Identification test for phonetic vowel perception......................28
3.2.2.1 Materials......................................................................30
3.2.2.2 Subjects.......................................................................33
3.2.2.3 Procedure....................................................................33
3.2.2.4 Results.........................................................................33
3.2.2.4.1 Identification results of the stimuli from the /i/-/ι/
and /u/-/υ/ pairs...................................................34
3.2.2.4.2 Identification results of /hit/-/hιd/ and /hut/-/hυd/..39
3.3 Discussion.......................................................................................42
CHAPTER FOUR EXPERIMENT II.........................................................46
4.1 Overall introduction..........................................................................46
4.2 Method.............................................................................................48
4.2.1 Materials...................................................................................48
4.2.2 Subjects....................................................................................49
4.2.3 Procedure.................................................................................49
4.3 Results.............................................................................................50
4.3.1 Assimilation of English /hit/-/hd/ to Mandarin /i/.....................50
4.3.2 Assimilation of English /hut/-/hd/ to Mandarin /u/...................51
4.4 Discussion........................................................................................52
CHAPTER FIVE GENERAL DISCUSSION..............................................55
5.1 Summary of present study................................................................55
5.2 General discussion...........................................................................57
5.3 Limitations of present study.............................................................59
5.4 Conclusion.......................................................................................60
REFERENCES........................................................................................62
APPENDIX..............................................................................................66
Appendix A: Research Informed Consent Form...................................66

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