苗栗縣是台灣著名的四縣客家話使用區，但縣內後龍鎮的優勢語是閩南語，而四縣客家話的使用人數又多於海陸客家話，海陸客家話處於絕對弱勢的情形下，如何保持其原來的語音，又在接觸的環境下，產生了哪些語音變異，這些都值得深入探討。本文的研究發現，就語音而言，親屬語言之間的影響大於非親屬語言。與新竹海陸客家話比較，後龍海陸客家話在聲母部分沒有[t∫, t∫^h, ∫, □]，韻母部分與唇音結合的[ui]韻轉變成[i]韻，與喉音結合的[ai]韻轉變成[e]，與齒音結合的[au, iau]轉讀[eu, ieu]，名詞後綴異於新竹的[□]而讀[e]。本文就後龍海陸客家話舌葉音的消失深入探討，並將重點放在後龍海陸客家話與新竹海陸客家話齒音的比較，透過家戶語言分布的調查及訪談錄音，整理後龍海陸客家話語音演變的語料，希望進一步了解後龍海陸客家話處在四縣腔客家話為優勢的環境，齒音與韻母的結合呈現出的特色，並說明舌葉音消失的原因。本研究除了描述後龍海陸客家話與新竹海陸客家話的異同之外，並且討論同樣處於弱勢語言的親屬語言在語音上的變異現象，探討接觸環境下的語音演變規律。

Miaoli County in Taiwan is a well-known area where the native language of the majority of residents is Sixian Hakka. But in Houlong Township of the county the major language is Southern Min. There are also Hakka speakers there with Sixian speakers outnumbering Hailu speakers. Thus it is of interest to investigate how Hailu Hakka retains its sound system and what sorts of sound changes have occurred in contact with the coexisting languages. This paper shows that phonetically the influence from more closely related languages is greater than that from remotely affiliated languages. In comparison with the Hailu Hakka in Xinzhu County, we see that the consonants [t∫, t∫^h, ∫, □] in Hailu Hakka of Houlong township has disappeared. In vowels, [ui] has changed to [i] when it is combined with labials, [ai] has changed to [e]when it occurs with laryngeals, and [au, iau] have become [eu, ieu] after dental consonants. Moreover the noun suffix is [e] differing from [□] in Xinzhu. We place our emphasis on the loss of palato-alveolar sibilants in Hailu Hakka of Houlong as we make the comparison. Through surveys of geographic distribution of languages of all households and some language recordings, we have gathered appropriate data for investigation of how palato-alveolar sibilants disappeared. Besides showing the differences between the Hailu Hakka of Xinzhu and that of Houlong, we also discuss phonological changes of minor languages and rules of sound change due to language contact.