Correlating Degree of Thyroid Tilt Independent of f Control as a Mechanism for Phonatory Density with EGG and Acoustic Measures across Loudness Conditions.

Introduction: Traditionally, fundamental frequency increase has been viewed as largely associated with vocal fold length as a consequence of tilting the thyroid cartilage forward and downward, a so-called thyroid tilt, caused by cricothyroid muscle contraction. Recent pilot studies in singers suggest vocal fold elongation independent from f as related to a pedagogical parameter called "phonatory density," suggesting a further discrete mechanism of the thyroid cartilage tilt related to voice quality. This study endoscopically, EGG, acoustically, and auditory perceptually explores different vocal modes in relation to degree of phonatory density independent of changes in f across loudness and voice quality conditions.

Methods: Case-control with 20 professional singers performing sustained-vowel samples (C4 males, B4 females) for 8 different voice quality conditions with different degrees of auditory-perceptual "density" while undergoing endoscopic examination and concurrent EGG and acoustic measurement. Endoscopic vocal tract assessments were blindly rated according to a 33-item systematic assessment tool and a forced consensus paradigm. MANOVA, Spearman's rho, and factor density were calculated at p ≤ 0.05. Auditory-perceptual assessments of 64 samples of the 8 voicing conditions were performed by 33 professional singing teachers. Fleiss' kappa and percentage agreement were used to calculate assessor accuracy and inter-rater reliability.

Results: Forward and downward thyroid tilt was related to the perceptual category of "reduced density (RD)" as the only statistically significant endoscopic assessment variable: "fuller density" conditions exhibited little to no forward visible articulation of the thyroid cartilage, whereas RD conditions exhibited visible to marked forward articulation of the thyroid cartilage across tested conditions suggesting vocal fold elongation for RD conditions while maintaining an unchanged f with high ICC for the assessors (r = 0.70 and r = 0.94 for male/female datasets, respectively). Correlation analyses revealed negative correlations for SPL, shimmer, and CPP measures for RD conditions, while Qx did not vary with statistical significance. Panel assessors accurately assessed the 8 tested conditions with 87% accuracy and good inter-rater reliability agreement (k: 0.772, p < 0.001).

Conclusion: Phonatory density, as an auditory-perceptual denotation of vocal weight, is controlled by the degree of thyroid cartilage tilt. The study documents systematic variations in vocal fold lengths across several conditions of loudness while f is maintained. The findings suggest a further mechanism of the thyroid cartilage related to voice quality beyond the control of f. Further studies are needed to document pitch production mechanisms compensating for the maintenance of f given vocal fold elongation during RD conditions.