Experiment phonetic study on vowel devoicing and Speech rate in Korean and Japanese This paper performed a sound experiment to find out how the devoicing of the vowel sound was influenced by utterance speed in Korean and Japanese. As a result, an occurrence rate of the devoicing was about the same by fast utterance and the slow utterance, but was low by the normal utterance. In addition, a lot of devoicing occurs so that the length of the vowel sound is short. Therefore, what I was concerned with closely was identified as duration of the vowel sound. If utterance speed becomes fast, by this experiment, the devoicing of the vowel sound is affected a little, but an occurrence rate of the vowel sound is low and is judged from the person of utterance to have at random when it is not a decisive factor.