Sweep-tone measurements of vocal-tract characteristics.

The vocal tract was excited transcutaneously at a point just above the glottis hy an e • ternal sweep-tone signal, in order to measure its transfer characteristics acoustically as continuous frequency functions. An analysis-by-synthesis procedure derived reliable data of vowels, in particular of the formant bandwidths, for three male and three female normal subjects. It has been shown for the closed glottis condition that the first formant bandwidths are higher for close vowels (typically 70 Hz for male subjects} than fnr semi-open vowels (typically 35 Hz for male subjects). Stationary consonantal articulations including stops, nasals, and nasalized vowels also have been studied, as well as the effect of opening the glottis on the vocal-tract transfer characteristics. The stop articulations give rise to a first-formant frequency slightly below 200 Hz. This fact and the high dissipation of the first formant is explained by assuming nonrigidness of the surrounding wall. Characteristics of nasalized vowels and nasal murmurs are also discussed hased on the data ol,tained in this experiment. II • ITRODUCTIOl • I Direct measurements of the vocal-tract transmission characteristics by use of sinusoidal waves as the excita-tion have been reported by van den Berg, who applied a sweeping pure-tone signal at the larynx of a hemi-laryngectomized subject. t Fant adopted a similar method to a normal subject by exciting his vocal tract through the skin near the larynx. His sound source was a small moving-iron-type transducer, and by applying the vibrating diaphragm directly on the outer surface of the throat somewhat above the larynx, he recorded frequency response curves of the vocal tract for different vowels and some nasalized vowels. By visual inspections of the continuous response curves near formant peaks, he estimated the bandwidths of the formants under a closed-glottis condition • and derived a graph of the bandwidth values plotted against frequency over a frequency range up to above 4000 Hz. • These sweep-tone techniques as a method for exploration of the acoustic characteristics of the vocal tract are characterized by two major advantages. The continuous frequency response curves that can be obtained by these methods can reveal all details of the transfer characteristics, whereas the harmonic structure of natural voiced speech samples obscures these spectral details. More important is that these measurements exclude the unknown factor of the source spectrum, which we usually, in the case of analyses of natural utterances, cannot separate from the transfer function. The …