本研究探討台灣華語雙元音 /ai/ 與 /au/ 中元音核 /a/ 的聲學特徵，及其在不同前導子音、聲調及音節位置下的差異，透過共振峰 (formant) 數值的變化分析舌體的前後位移是否受到協同發音 (coarticulation) 的影響。研究對象包含 43 位北台灣年輕母語者，分別為 23 位男性高中生與 20 位女性高中生。研究結果顯示，其中 15 位受試者展現協同發音抗拒現象，其元音核在 /ai/ 與 /au/ 中之第二共振峰 (F2) 數值在統計上並無顯著差異，證實元音核 /a/ 在雙元音系統中為穩定的聲學標記。然而，該元音核容易受到前導子音的協同發音影響，並呈現明確的層級關係：具高發音限制 (articulatory constraint) 的軟顎音 /k/ 會引發強烈且普遍的前移效果；齒齦音 /t/ 的協同影響具個體差異；雙唇音 /p/ 則因不需特定舌位而幾乎沒有影響。此外，雙元音的聲調與音節位置對元音核的發音皆無顯著影響。因此，本研究表明，與舌頭相關的發音限制是影響台灣華語雙元音中元音核的主要因素。

This study investigates the acoustic characteristics of the nucleus /a/ in the Taiwan Mandarin diphthongs /ai/ and /au/, as well as their variations across different preceding consonants, lexical tone, and syllabic positions. By observing the differences between formant frequencies, the study analyzes whether the forward-backward movement of the tongue body is influenced by the coarticulation effect. The participants included 43 young native speakers from Northern Taiwan, consisting of 23 male and 20 female senior high school students. The results indicate that 15 of the participants exhibited coarticulation resistance, showing no statistically significant differences in the second formant (F2) values of the nucleus between /ai/ and /au/, which confirms that the nucleus /a/ serves as a stable acoustic target in the diphthong system. However, the nucleus remains susceptible to the coarticulatory effect from preceding consonants, displaying a clear hierarchical relationship. The velar onset /k/, which possesses a high articulatory constraint, induced a significant and consistent tongue-fronting effect. The coarticulatory impact of the alveolar onset /t/ showed individual variations. The bilabial onset /p/ had almost no influence since there was no specific tongue involvement in the articulation. In addition, neither tonal variations nor syllabic positions exerted a significant effect on the articulation of the nucleus. Therefore, this study demonstrates that tongue-related articulatory constraints are the dominant factor affecting the nucleus of diphthongs in Taiwan Mandarin.

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