THE EFFECT OF FREQUENCY PASSBAND UPON THE INTELLIGIBILITY OF HELIUM-SPEECH IN NOISE By

The alteration in the pitch of speech produced by breathing helium mixtures suggests the possibility of increased intelligibility of such speech by selective frequency filtering. This study evaluated speech, spoken after breathing an 80 % — 20 % mixture which was combined with broadband noise and then passed through a selection of bandpass filters. Results indicate that while intelligibility did not suffer appreciably when frequencies of 300 cps and below were eliminated, the loss of frequencies below 600 cps caused a marked deterioration. Even with the severe filtering restrictions of a 600-1200 cps bandpass and a 1-db Speech/Noise ratio, 38% intelligibility was achieved. Apparently the intelligibility of speech in a helium mixture, as in air, is quite distortionresistant. The estimated frequency at which highand low-pass filtering would have equal effects on intelligibility was lower than 1000 cycles/seconds in contrasts to approximately 1600 c/s for speech in air. No condition of filtering increased intelligibility over the no-filter condition.