Japanese · Pronunciation Guide
na-tsu, two beats — the tsu is one sound, and its vowel usually whispers away.
natsu
Two matched beats with the pitch high on the first. Even when the second beat is almost inaudible, it still takes its full share of time — a native 夏 is not a one-syllable word with a fricative tacked on.
Listen: 夏
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夏は一番好きな季節です。
Natsu wa ichiban suki na kisetsu desu.
Summer is my favorite season.
Listen: 夏は一番好きな季節です。
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夏は一番好きな季節です contains 好き, whose two vowels both devoice into something close to "ski," and 季節 (ki-se-tsu), which ends in another devoicing-prone affricate. So the sentence has three whispered vowels and two affricates in one line.
"NAH-tsoo" stretches the first vowel, stresses it, and rounds and voices the second. Shorten the a, spread the lips, and let the final vowel devoice. Some learners also split the affricate into "t" plus "soo," which adds a beat and removes the sound that makes つ distinctive.