The intelligibility of helium-speech as a function of speech-to-noise ratio. Rep No. 555.

Three experiments compared the intelligibilities of helium-speech and normal speech when both were masked by loud noise. Recordings were made of five talkers reading intelligibility word-lists. Several panels of listeners heard these recordings masked by different levels of background noise. In Experiment I, a fatigue effect seemed at first to be present differentially for the helium-speech, suggesting that short-term auditory fatigue may occur with helium. However, Experiment II, designed to observe the effect on intelligibility of time during listening session that material was heard, refuted the notion of short-term fatigue effects. Experiment III incorporated results of the first two experiments to evaluate the effect of introducing varied levels of noise upon intelligibility of helium-speech and normal speech when order of presentation of gas-mix was minimized. The helium-speech and speech in air were equally intelligible both in the presence of loud noise and in the presence of low level noise. However, heliumspeech was approximately ten percentage points less intelligible than speech in air when the level of noise was close to the level of speech. We conclude from this study that the effects of adaptation and learning on the intelligibility of helium-speech should be investigated further. THE INTELLIGIBILITY OF HELIUM-SPEECH AS A FUNCTION OF SPEECH-TO-NOISE RATIO