Reaction of Small Enclosures on the Human Voice, Part II: Analyses of Vowels

Abstract

In a previous paper, the specifications necessary for intelligible transmission of speech through enclosures such as gas masks were discussed. In the present paper, harmonic analyses are presented to show the distortion of vowels by enclosures against the mouth in a number of typical cases. A partially successful attempt is made to devise a theory for these distortions on the basis of an equivalent circuit suggested by Crandall for the human voice. Enclosures may be divided into radiating and non-radiating types, according as the major interest is in the sound radiated or the sound pressure developed within the enclosure. In the first category belong gas masks; in the second, noise shields and oxygen masks. Theory predicts that the two formant frequencies characteristic of each vowel will be split into three by a small radiating enclosure, and that the distortion will increase as the aperture and resonant frequency of the enclosure are decreased. The analyses confirm this and indicate that it is largely responsible for the characteristic hollowness of the speech. The distortion is less than theory predicts, however. Theory predicts that the effect of a small non-radiating enclosure will be primarily a rise in the two formant frequencies. According to the analyses, there is such a tendency, but correlation with the theory is poor, and the frequency shifts are often erratic.