When i and u appear between voiceless consonants or when they appear at the end of a sentence, the vocal chords do not vibrate while pronouncing these vowels; consequently, there are times when it is difficult to hear them. We call this phenomenon, ‘vowel devoicing’.
The consonants of the ka, sa, ta, ha, pa column sounds in Japanese are voiceless. It is difficult to hear the vowels of ki, ku, shi, su, chi, tsu, hi, fu, pa, pu when ka, sa, ta, ha, and pa column sounds appear after them. For example, suki (すき（好き） like), kiku (きく（聞く）to listen), kutsu (くつ（靴） shoes), tsuki (つき（月） moon) and kikai (きかい（機会）chance) are some words that undergo devoicing in the vowels that appear between voiceless consonants.
Moreover, when these sounds appear at the end of a sentence, vowel devoicing occurs easily. The su in ‘～desu’ ( ～です ) and masu ( ～ます )forms of verbs is usually devoiced.
Practice is needed as it becomes difficult to catch the whole word when vowel devoicing occurs. (For pronunciation practice,（refer to section 3.3.）.