[Measurement and interpretation of irregular vocal cord fold vibrations]

Irregular vibrations of the vocal folds are regarded as a main reason for the common symptom of hoarseness. With real time resolution, they can only be observed using high speed recording techniques (up to 10,000 images/s).

During the last years, we have developed a recording device and image processing software dedicated to the 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).

By applying this biomechanical two-mass model and inversely solving Bernoulli's equation, the driving parameters of the vibration were computed from the high speed image series in selected cases. These parameters, like the subglottal pressure und the bilateral tension of the vocal muscle cannot, in general, be measured directly.

From the model, it can be supposed that laryngeal asymmetry (either in mass or tension) is the primary reason causing irregularity. The consequence of asymmetry in the medio-lateral direction are different fundamental frequencies on each side, while asymmetry in the anterior-posterior direction leads to ap-mode vibrations.