A model oj voiced-sound generation is derived in which the detailed acoustic behavior of the human vocal cords and the vocal tract is computed.The vocal cords are approximated by a self-osdllaiing source composed of two stiffness-coupled masses. The vocal tract is represented as a bilateral transmission line. One-dimensional Bernoulli flow through the vocal cords and plane-wave propagation in the tract are used to establish acoustic factors dominant in the generation of voiced speech. A difference-equation description of the continuous system is derived, and the cord-tract system is programmed for interactive study on a DDP-516 computer. Sampled waveforms are calculated for: acoustic volume velocity through the cord opening (glottis); glottal area; and mouth-output sound pressure. Functional relations between fundamental voice frequency, svbglotial (lung) pressure, cord tension, glottal area, and duty ratio of cord vibration are also determined.Results show that the two-mass model duplicóles principal features of cord behavior in the human. The variation of fundamental frequency with subglottal pressure is found to be 2 to 3 Hz/cm H/), and is essentially independent of voivel confixjuration in the programmed tract. Acoustic interaction between tract eigenfrequencies and ghttal volume flow is strong. Phase difference in motion of the cord edges is in the range of 0 to 60 degrees, and control of cord tension leads to behavior analogous to chest/falsetto conditions in the human. Phonation-neutral, or rest area of cord opening, is shown to be a critical factor in establishing self-oscillation. Finally, the complete synthesis system suggests an efficient, physiological description of the speech signal, namely, in terms of subglottal pressure, cord tension, rest area of cord opening, and vocal-tract shape.
The effects of asymmetrical tension on the vibratory pattern of the vocal cords were studied in two kinds of experiments: 1) high speed motion picture photography of artificial voice production in excised canine and human larynges, and 2) computer synthesis of voice and vocal cord vibration via a theoretical model incorporating the physiological parameters required for phonation. In both approaches the asymmetrically tensed vocal cords consistently vibrated in three distinct modes which depend partly on the rest positions of the vocal cords; Type I. For rest positions at or near closure, the two cords vibrate at the same frequency with glottal closure every period, and with tense cord preceding the lax one in phase and with the line of contact moving toward the tenser cord during the closed phase. The voice produced is not hoarse; Type II. For wider rest positions glottal closure occurs irregularly, the vibrations become complex and less periodic, and the voice becomes hoarse; Type III. The glottis never closes and the vibrations become more periodic with reduced amplitude. Supplementary stroboscopic observations suggest a precedure for diagnosing tension asymmetry and the implications for surgical treatment for disorders of vocal pitch are discussed.
A dynamic model of the vocal cords, namely, a bilaterally symmetric two-mass model, is extended to a bilaterally asymmetric two-mass model to simulate pathological conditions of the vocal cords. Asymmetric behavior of the computer model is investigated for various conditions of imbalance in bilateral tension. The computer model is found to behave in three basic vibratory modes that are similar to those observed in physiological experiments on larynges under tension imbalance. The three distinctive modes are (1) a vibratory pattern with differences in phase and amplitude of cord vibration; (2) a nearly periodic motion without glottal closure; and (3) an unsteady, dicrotic or tricrotic motion. The three modes are found to be a strong function of the subglottal pressure and the glottal rest area, as well as the imbalance conditions of the cord parameters. The asymmetric vocal-cord model is incorporated into a dynamic vocal-tract synthesizer to simulate speech with a hoarse voice. Subject Classification: [43]70.20, [43]70.50.
The input acoustic impedance looking into the trachea was measured on several laryngectomized subjects. The results show remarkable inconsistency with van den Berg’s data. The subglottal resonances are found at approximately 640, 1400, and 2100 Hz. The acoustic input impedance at the first peak ranges between 25 and 50 cgs acoustic ohms. The results are analyzed by means of a digital simulation based on anatomical studies, and the tract wall impedance necessary to produce the measured subglottal impedance is calculated. Subject Classification: [43]70.20, [43]70.40, [43]35.20.
We describe a computer model of the human vocal cords and vocal tract that is amenable to dynamic control by parameters directly identified in the human physiology. The control format consequently provides an efficient, parsimonious description of speech information. The control parameters represent subglottal lung pressure, vocal‐cord tension and rest opening, vocal‐tract shape, and nasal coupling. Using these inputs, we synthesize vowel‐consonant‐vowel syllables to demonstrate the dynamic behavior of the cord/tract model. We show that inherent properties of the model duplicate phenomena observed in human speech; in particular, cord/tract acoustic interaction, cord vibration, and tract‐wall radiation during occlusion, and voicing onset‐offset behavior. Finally, we describe an approach to deriving the physiological controls automatically from printed text, and we present sentence‐length synthesis obtained from a preliminary system.
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