Speech intelligibility for space vehicles, using nitrogen or helium as the inert gas

A laboratory investigation was carried out to help evaluate verbal-communication intelligibility in a man-rated altitude simulator when either helium or nitrogen was added to the oxygen atmosphere. Some eight operators and 37 male subjects were tested with a total of 16 500 random word events at pressures of 5 psia (pounds pressure per square inch, absolute), using 70:30 mixture of O2:He or O2:N2 mixtures and also 100% oxygen at 3.5 psia. An increased lack of intelligibility has been known to occur as gas densities have been reduced. Differences in test scores following the substitution of helium in place of nitrogen at the pressures and mixtures employed resulted in no increased loss of intelligibility other than that associated with the reduced gas density, although some modification of speech can be detected by listeners.