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

Abstract

As an alternative factor to produce asymmetry between left and right vocal folds, the present study focuses on level difference, which is defined as the distance between the upper surfaces of the bilateral vocal folds in the inferior-superior direction. Physical models of the vocal folds were utilized to study the effect of the level difference on the phonation threshold pressure. A vocal tract model was also attached to the vocal fold model. For two types of different models, experiments revealed that the phonation threshold pressure tended to increase as the level difference was extended. Based upon a small amplitude approximation of the vocal fold oscillations, a theoretical formula was derived for the phonation threshold pressure. This theory agrees with the experiments, especially when the phase difference between the left and right vocal folds is not extensive. Furthermore, an asymmetric two-mass model was simulated with a level difference to validate the experiments as well as the theory. The primary conclusion is that the level difference has a potential effect on voice production especially for patients with an extended level of vertical difference in the vocal folds, which might be taken into account for the diagnosis of voice disorders.

Influence of level difference due to vocal folds angular asymmetry on auto-oscillating replicas

“…In order to compensate the reduced movement of the paralyzed VF, the position of the normal VF in the inferior-superior direction is changed. Consequently, a vertical level difference between the left and right VF emerges, where the paralyzed VF is often inclined in the sagittal plane (right VF in in literature [3][4][5] . Quantitative characterization of this spatial asymmetry from imaging on human subjects is extremely tedious [1][2][3] .…”

“…Recently 4,5 , the impact of level difference between parallelly located VFs, hereafter referred to as parallel level difference, was investigated. The preceding study, however, did not take into account inclination of the paralyzed VF, which is physiologically more close to reality.…”

“…Therefore, the MRI shape is more complex than the M5 shape and more faithful to the shape of the human VF. Note that, compared to the MRI VF replicas reported in literature 4,15,17 , an epithelium layer is added in this work. The EPI replica is molded with the M5 cast as compared in 17 .…”

Experimental study on level difference between left and right vocal folds

“…Such advanced measurements may provide more precise information about the left-right vertical asymmetry and enable us to study the relation between the level difference and the difficulty of phonation. Lastly, we note that further investigations [16] are needed to deepen the present study. For instance, the effect of the supraglottal vocal tract, which have not been considered in our experiment, should be carefully examined, since the supraglottal resonance may facilitate the vocal folds vibrations.…”

“…The theory, which will be reported in detail elsewhere [16], is based upon the small amplitude approximation of the vocal fold oscillations [15]. By neglecting the acoustic loads from both sub-and supraglottal systems and assuming symmetric dynamics between the left and right vocal folds, the oscillation threshold pressure is given by…”