Speech Production as Revealed by Vocal Cord Photography and Sound Wave Analysis
John C. Steinberg · The Journal of the Acoustical Society of America · 1940
For more than a century, wide differences of opinion have been held as to the function of the vocal cords in speech production. In the present paper, this problem is approached by utilizing high speed motion picture photography to determine the vibrations of the cords during phonation and a method of Fourier analysis to determine the characteristics of the sound waves. By taking pictures at a rate of 4000 per second and projecting them at the normal viewing rate of 16 per second, the motion is slowed down by a factor of 250 to 1. Thus, if the cords execute 250 vibrations in one second, they appear to make one vibration per second and the details of their movements may be seen. Pictures suitable for monocular viewing, and also stereoscopic pictures showing the movements of the cords in full relief, have been obtained. For the lower pitches, the cords appear to be completely relaxed, and there appears to be a phase difference between the movements of the lower and upper surfaces. They do not move as units, but more as folds or lips, the motion of the upper surfaces lagging behind that of the underneath surfaces. The tension increases as the pitch is raised, the cords elongate and tend to move more nearly as units with somewhat smaller amplitudes than for low pitches. The movements appear to begin toward the front end of the cords and to extend progressively toward the back, so that the motion has a horizontal as well as vertical phase difference. The analysis of the vocal cord vibrations indicates a vibration form composed of a fundamental having a frequency equal to the vibration rate of the cords and overtones, which diminish in amplitude with increasing order or number of the overtone. Such vibration forms do not possess the distinguishing characteristics of the different speech sounds. The latter characteristics are produced by the passage of the sound waves through the throat, nose and mouth cavities. The paper will be illustrated by motion pictures showing the movements of the cords in ultra-slow motion, and by variable area recordings of the sound waves and their corresponding analyses. Stereoscopic motion pictures will be available for limited viewing so that the cord movements may be seen in relief.