Do native speakers think of prolonged vowels as one long vowel, or two vowel sounds following each other?


In a native speaker's mental model, are long vowel sounds - for example the おう
in 「教室」 the ああ in 「唐揚げ」, the えい in 「先生」 or just 「いい」 (I chose these examples
quite deliberately, since they represent fairly different "categories" of long
vowels; please consider them all) - a long vowel sound or two vowel sounds
following each other? In other words, when they say a long vowel, are they
deliberately saying one long vowel sound or two of them directly following
each other?

Does it perhaps depend on which vowel it is? Does it depend on the word?
Perhaps neither, or something else entirely? This is something I've been
wondering for a while now, and I feel knowing the answer would help my latent
understanding of the language and pronunciation.

I realize there's probably no definite answer, at least not without conducting
some scientific research, but having the input of some native speakers or
someone who's talked to native speakers about this would be invaluable.




I'm a native speaker. When you tell a native Japanese speaker to say these
words veeeeery slowly, they would say:

  * きょ、う、し、つ。 （or きょ、お、し、つ。）
  * せ、ん、せ、い。 （or せ、ん、せ、え。）
  * か、ら、あ、げ。
  * こ、ん、ぴゅ、う、た、あ。 （コンピューター）

And if you ask "How many 'sounds' are there in those words?", they would count
using their fingers, and say 4, 4, 4 and 6, respectively. So this means so-
called "long vowels" are fundamentally two separate sounds in native speakers'
minds.

Of course it doesn't mean we _deliberately_ say two vowels in succession in
everyday conversations. We do it simply unconsciously.

Reference: [Japanese _On_ (or
morae)](https://en.wikipedia.org/wiki/On_\(Japanese_prosody\))

Japanese morae system and English syllables are so much different that it took
me years to understand why English 'strike' is one "sound" (syllable) to
English speakers, while it was clearly five sounds (す、と、ら、い、く) to me.

