5

This is not a direct answer to your question but let me explain about difference between voicing/devoicing vowels and prolonging vowels. There are several ways to pronounce です or the likes. des (1 syllable non-moraic 3 morae, /de/ is longer than /s/, sounds chopped foreign) des: (1 syllable 2 morae, /de/ and /s:/ are even, sounds eastern-dialect-ish) desu ...


5

In Japanese phonotactics, high vowels (for Japanese, these are i and u) have a certain property: they become unvoiced when surrounded by unvoiced sounds. Since the "u" in desu is surrounded on the left by "s" (voiceless) and on the right by nothing (nothing is also voiceless), the u is now voiceless. However, this rule is not universally followed; for ...


1

For the older vocabulary of Japanese, we have native "Yamato" terms and borrowed Sino-Japanese terms. For these, gemination as spelled with the small-tsu っ historically only happened with voiceless obstruents. The analogue for geminate voiced obstruents in Japanese was prenasalization, which we do see -- although it is realized as //ɴ// + [following ...


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