新竹市及新豐紅毛港的閩南語屬於新竹老同安腔，兩地現今央元音［ɨ］、［ǝ］消失的次序不同，元音系統的演變趨勢亦不相同。新豐紅毛港部分音類的［ǝ］比［ɨ］保留得更好，［ɨ］比［ǝ］先消失，符合世界元音發展之趨勢。然而新竹市卻是［ǝ］比［ɨ］先消失，此乃因：（1）在面對外地方音的壓力時，同時分布於文讀及白讀層的［ɨ］，比僅分布在白讀層的［ǝ］更為鞏固；（2）〈居艍_文〉類受到華語「留滯作用」的影響，［ɨ］較易保留；（3）為了避免〈刀高_白〉類的展唇音［ɤ］與老泉腔的央中元音〔ǝ〕混淆，促使新竹市的〔ǝ〕比［ɨ］更快消失。當央元音衰退之際，兩地原本的元音系統，因結構不對稱，引發元音系統的重整，新竹市朝／i、e、a、u、ɤ、ɔ／六元音系統演變；新豐紅毛港則朝／i、e、a、u、ɔ／五元音系統演變。

This study investigates Old Tong'an 老同安-based Southern Min in Hsinchu city and Xinfeng Hongmaogang 新豐紅毛港. Despite both being Quanzhou-based dialect areas, evidence shows that various factors differentiate the decline of the central vowels [ǝ] and [ɨ] in the two regions. In Xinfeng Hongmaogang, [ǝ] in some rhyme groups is preserved much better than [ɨ], which conforms to the trend of world vowel development. However, in Hsinchu city, [ǝ] disappeared earlier than [ɨ]. Three factors favor this [ɨ] against decline. First, in comparison to [ǝ], which is used only in colloquial readings, the vowel [ɨ], which is rooted in both literary and colloquial readings of the dialect pronunciations, has greater resistance against pressure from other prestige dialect variations. Second, influence from Mandarin pronunciation also contributes to the preservation of [ɨ] in the ＂Ju/Ju _(literary) 居艍_文＂ rhyme group. Third, in order to avoid the confusion of [ɤ] in the ＂Dao/Gao _(colloquial) 刀高_白＂ rhyme group with the Old Quan mid vowel [ǝ], [ǝ] disappeared much faster than [ɨ] in Hsinchu city. In addition, due to the decline of the two mid vowels within the asymmetric structure, the six-vowel system in the two regions has been restructured. Language contact drives the systematic transition to /i, e, a, u, ɤ, ɔ/ in Hsinchu city, while the vowel system in Xinfeng Hongmaogang converges to a five-vowel one (/i, e, a, u, ɔ/).