Effect of level difference between left and right vocal folds on phonation: Physical experiment and theoretical study

The vocal folds, which are constituted by muscles covered with a mucous membrane, generate a primary sound called the voice source, as airflow passes them. In some voice disorders, asymmetry between left and right vocal folds was observed. We focus on level difference, which is defined as the distance between the upper surfaces of the bilateral vocal folds in the inferior-superior direction and is caused by symptom of such disorders. Physical models of the vocal folds were utilized to study the effect of the level difference on the phonation threshold pressure. For three types of different self-oscillating synthetic models, our experiments reveal that the phonation threshold pressure increases significantly as the level difference is extended. Furthermore, based upon a small amplitude approximation of the vocal fold oscillations, a theoretical formula was derived for the phonation threshold pressure. Our theory was in good agreement with the experiments, especially when the phase difference between the left and right vocal folds is not too large. From these results, we conclude that the level difference affects voice production, therefor the effect of the vocal fold geometry needs to be taken into account for the observation of voice disorders.