Measurement and Interpretation of Irregular Vocal Fold Vibrations

Background: Irregular vibrations of the vocal folds are regarded as a main reason for the common symptom of hoarseness. With real time resolution, they can be observed only using highspeed recording techniques (8804; 10000 images/s). Method: During the last years, we developed a recording device and an image processing software, dedicated for extraction of vocal fold motion curves from the image series. Irregular vibrations can be understood by using Hirano's "body-cover-model": one vocal fold is assumed to act as a system of two coupled oscillators (the vocal muscle and the epithelium, coupled by the connective tissue of Reinke's space). Results: By applying this biomechanical two mass model and inversely solving Bernoulli's equation, in selected cases the driving parameters of the vibration were computed from the highspeed image series. These parameters, like the subglottal pressure und the tension of the vocal muscle on both sides, can in general not be measured directly. Discussion: From the model it can be supposed, that laryngeal asymmetry (either in mass or tension) is the deeper reason causing irregularity. The consequence of asymmetry in medio-lateral direction are different fundamental frequencies on each side, Medio-lateral asymmetry corresponds to organic voice disorders. Asymmetry in dorso-ventral direction leads to longitudinal mode vibrations and seems to be the correlate of functional voce disorders.