Mechanism for Vibrational Difficulty of Unilaterally Scarred Vocal Folds

Vocal folds can be scarred by trauma, inflammation, or thermal burns after treatments such as surgery or radiation therapy. When vocal folds are scarred, it is known that their vibration amplitude decreases and their phonation threshold pressure (PTP) increases, but the mechanism for the vibrational difficulty of scarred vocal folds is unclear. Previously, by using modal analysis and one-dimensional fluid analysis, we proposed a symmetric vibration model for normal vocal folds. Here, we propose an asymmetric vocal fold model for analyzing the vibration of vocal folds when one side is scarred and the scarring covers that entire vocal fold. To validate the proposed model, we compare the numerical results with experimental results, and find that they correspond. We also aim to determine why one-sided vocal fold scarring increases significantly the PTP and decreases the vibration amplitude even though the PTP does not increase significantly when one side is a straight rigid wall. We find that the work done by the air on the normal vocal fold decreases at an upstream point in the glottis because of its always-convergent shape and decreases at a downstream point because of protrusion.