Self-Oscillating Vocal Fold Model Mechanics: Effects of Bowing and Shallow Geometries
Open AccessThe effects of bowing and shallow geometries, such as those common in aged persons, on self-oscillating vocal fold (VF) model mechanics were evaluated experimentally in a vocal tract simulator to replicate physiological conditions. Supported by previous success in fabricating and testing two-layer silicone vocal fold models in a female geometry, three versions of a male two-layer vocal fold model were fabricated based on the M5 geometry for this study. The first VF model adhered to the accepted M5 geometry and featured a muscle layer stiffer than the lamina propria layer. The second VF model, or ‘bowing’ model, maintained the same material properties but featured a reduction in the volume according to work presented by Murray et al. to mimic an initial glottal gap which is common in aged persons. The third VF model, or ‘shallow’ model, was developed to replicate the thinning of the superficial layer of the lamina propria and featured a physically thinner lamina propria layer (depth reduced by 53.92%) while maintaining the material properties for the two layers as each of the other male VF models evaluated in this study. Measurements were taken from pressure transducers and a high-speed camera to correlate vocal fold motion and pressure signals along the vocal tract to the output of a Rothenberg Mask, a tool that is typically employed by clinicians rather than experimentalists. Data included phonation onset pressure, fundamental and formant frequency of oscillation, AC/DC ratio, open quotient, and maximum flow declination rate (MFDR). The Rothenberg Mask was proven to be a useful and viable experimental tool for in vitro studies as validated against other experimental methods in this study. Further, the in vitro experimental data for the male two-layer, male bowing two-layer, and male shallow two-layer were within acceptable ranges of human data values, confirming that silicone cast in two layers is an effective method for modeling human vocal folds. The lack of a clear mucosal wave and sharp lateral peaks in the bowing and shallow vocal fold models calls for further investigation of these two-layer bowing and shallow models and suggestions for future work are explored in this paper.
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