The vowels i and u, when appearing between voiceless consonants or at the end of sentences, may not be easily heard because the vocal chords may not be vibrating when pronouncing these vowels at the above-mentioned positions. This phenomenon is called ‘vowel devoicing’.
In the Japanese language, the consonants of the ka, sa, ta, ha, and pa columns are voiceless consonants. The cases, when vowels are not heard clearly, involve ki, ku ( き・く ), shi, su ( し・す ), chi, tsu ( ち・つ ), hi, fu ( ひ・ふ ), pi orpu ( ぴ・ぷ )when followed by the ka, sa, ta, ha, pa column sounds. For example, the vowels that are between voiceless consonant sounds in suki (すき（好き）like), kiku (きく（聞く） to listen), kusa (くさ（草）grass), tsuki (つき（月）moon), shita (した（下） below) are devoiced, and hence difficult to recognize.（refer to section2.11へ）