ヘリウム音声のききとり上の性質

抄録

The use of helium-rich mixture as a breathing gas solves physiological problems in the deep-sea or saturation diving, but it gives rise to a problem of "helium speech ", that is, degradation of speech intelligibility. Needless to say, the effective speech communication must be provided for making sure the life support and efficient action of divers in such a situation. However, the present state of art for overcoming the said problem is unsatisfactory. It is due to the lack of comprehensive studies covering human factors as well as the physical properties in helium speech. Especially, perceptual effects of the helium distortions have been almost unknown. The purpose of this article is to clarify the perceptual nature of helium speech through the articulation tests of Japanese CV-syllables and to study their relation to the physical nature observed by the acoustic analysis of helium speech. Speech materials were recorded in three experiments of the simulated dives of the Japanese Man-in-the-Sea Project (Seatopia). Experimental conditions and factors related to the articulation tests are listed in Tables 1 and 2. Result of analysis of variance on the factors of those tests in listed in Table 4. Among the helium distortions, the most important factor for perception is thought to be a nonlinear upward shift in formant frequency. This upward shift is due sound velocity elevation and the nonlinearity of it is due to increase in density. This relation is approximated by Eq. (1). The representative transposition curves are shown in Fig. 6. The main results are summarized as follows:1. Articulation scores of CV-syllables (Table 3 and Fig. 1) show that speech communication is not performed freely around 5 atm. and becomes almost impossible at a pressure greater than 10 atm. 2. The 5% rise of articulation score on the forth day in the five-day saturation dive (Table 5) is statically significant, which is probably due to the speakers' adaptation to the unusual voice environment. 3. Table 6 shows the confusion matrix of helium vowels. Its structure (Fig. 4(a)) is uni-directional and entirely different from that of the distorted vowels in normal air (Fig. 4(b)-(d)). 4. Confusions among consonants are listed in Table 7. Plosives, fricatives and nasals are difficult to recognize in this order. Confusions from voiced consonants to vowels and /j/, and those from unvoiced to /h/ are remarkable. Consonants which have front place of articulation show more errors than others having the same manner of articulation. 5. Comparison of our results with those in the cases of English (Table 9 and also see Table 1 and Fig. 6) verifies that perception of consonants having front place of articulation is greately affected by the nonlinearity in the formant shift. 6. For designing a helium speech unscrambler, it will be necessary to take account of the nonlinearity of the formant shift as well as the upward linear shift of the formant frequency.