Stroboscopy

STROBOSCOPY (from the Greek stroboeo - I turn and skopeo - I look), a method for studying the movements of the vocal cords in living individuals, by means of which their rapid oscillations during phonation can be observed in slow motion. The principle of the method was discovered in 1832 by the physicist Plateau and independently of him in 1834 by Stampfer, who applied it to study the laws of motion of oscillating bodies. In 1876, Muller used it to study freely oscillating membranes. However, it was Oertel who first used stroboscopy to study the movement of the human vocal cords in 1878, and after him this method was used by Rethi, Musehold, and others, and in Russia by Koshlakov (1886), Simanovsky, Malyutin, and others. S. is performed using a special instrument - the stroboscope. The action of the instrument is based on the fact that if the eye receives impressions that are separated in time by no more than 0.2 seconds, the observer will perceive the merging of two adjacent images into one; in addition, the object will appear darker (due to its periodic covering by the edges of the disk holes). If one looks through the rotating holes of a disk at an object oscillating at the same speed as the disk rotation, it will appear stationary, and the eye will perceive the same phase of movement; in this case, the number of interruptions in the illumination of the object will be equal to the number of its oscillations. If the rotation of the disk is somewhat slowed down or accelerated compared to the number of oscillations of the object (disrupting the frequency), then each point of the object will be shown in different phases, and as a result, the eye will generally perceive the sensation of slowed-down movements of the entire object. Thus, the main component of the stroboscope is a disk, along the edge of which are cut circular or slit-shaped holes equally spaced from each other; their number on different models may vary. The rotation of the disk is accomplished by means of a clock mechanism or an electric motor, and the speed of rotation is regulated by a rheostat. The second part of the stroboscope is the light source, which must be sufficiently strong to allow examination of the vocal cords using a laryngoscopic mirror, from which the method is called laryngostroboscopy. The third part of the instrument is a siren, producing a sound of a certain pitch, depending on the speed of disk rotation. The sound of the siren is produced by blowing air through the holes of the rotating disk via a tube; the number of interruptions in the air stream corresponds to the number of oscillations of the sound produced, and if the voice sound is of the same pitch as the siren sound, then the interruptions in the stream and the oscillations of the vocal cords occur with the same frequency. In old stroboscopes, the light source was placed between the disk and the object of observation, so that the vocal cords were viewed through the holes of the disk passing before the eye; in later models, the light source (small electric bulbs) was located in front of the interrupter (disk) and was placed on the forehead of the researcher together with a reflector; this setup provided greater freedom of movement during research (Spiess's stroboscope). In more recent times, use is made of a light source from a voltaic arc, which is enclosed together with the rotating disk in a special box, so that the flashing strong light falls on the ordinary frontal reflector of the observer and, by means of a laryngoscopic mirror, illuminates the oscillating vocal cords (Zumsteeg's, Wethlo's, Schneider's, and Levi-dov's stroboscopes). To produce sound in the siren, air is blown through the passing holes by the observer using a foot pedal. Thanks to the S. method, it was found that, for example, in the chest voice the cords oscillate with their entire mass, while in the falsetto only their inner edges participate in the oscillation. It was noted that the oscillations of the vocal cords occur mainly inward and outward, while vibrations upward and downward have secondary importance. The phase of closure of the cords is longer than the phase of their opening. Furthermore, it was established that both vocal cords during phonation often oscillate unevenly alternately, or one of the cords remains stationary. In pathological cases, by means of the stroboscopic method, even weakly expressed forms of disorders in the motor apparatus of the vocal cords can be recognized early. However, the stroboscope is not yet widely used in practice among either laryngologists or vocal pedagogues, since from these data it is still difficult to draw definite practical conclusions.