Abstract: The present work is intended to investigate the changes of speech of adult young men breathing a mixture of helium and oxygen under normal atmospheric pressure. The experimental results are as follows:
1. When the subjects are breathing a mixture of helium and oxygen the voice is higher in pitch and nasal in quality when compared with that of breathing air. The fundamental frequency of speech is raised, and the intelligibility is decreased.
2. The first and second formants of vowel and consonant with the exception of nasal ones shift to high frequency. The first formant of nasal tone shifts to low frequency, but the second forment, to high frequency.
3. The characteristics of aspiration of consonants do not change during breathing a mixture of helium and oxygen at normal atmospheric pressure.
4. The change of speech during breathing a mixture of helium and oxygen is affected not only by physical factors, but also by variations in the tension of the vocal cords induced reflexly by changes in gas density, gas flow, and respiratory resistance.