The term ‘environmental shielding’ has been used to refer to a class of processes in which the phonetic realisation of a nasal stop depends on its vocalic context. In Chiriguano, for example, nasal consonants are realised as such before nasal vowels (/mã/ → [mã]), but acquire an oral release before oral vowels (/ma/ → [mba]). Herbert (1986) claims that shielding protects a contrast between oral and nasal vowels: if Chiriguano /ma/ were realised as [ma], [a] would likely carry some degree of nasal coarticulation, and be less distinct from nasal /ã/. This article provides new arguments for Herbert's position, drawn from a large typological study of South American languages. I argue that environmental shielding is contrast preservation, and that any successful analysis of shielding must make explicit reference to contrast. These results contribute to a growing body of evidence that constraints on contrast are an essential component of phonological theory.

中文翻译：

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环境屏蔽就是对比保存

术语“环境屏蔽”已被用来指代鼻塞音的语音实现取决于其发声上下文的一类过程。例如，在 Chiriguano 中，鼻辅音在鼻元音 (/mã/ → [mã]) 之前实现，但在口元音 (/ma/ → [mba]) 之前获得口头释放。Herbert (1986) 声称屏蔽保护了口元音和鼻元音之间的对比：如果 Chiriguano /ma/ 被理解为 [ma]，[a] 可能带有某种程度的鼻音发音，并且与鼻 /ã/ 的区别较小。本文为 Herbert 的立场提供了新的论据，该论据来自对南美语言的大型类型学研究。我认为环境屏蔽就是对比保存，任何成功的屏蔽分析都必须明确提及对比。