Objectives: In previous work, we found that airflow at the superior edge of the vocal folds, in the excised canine larynx, can be laminar even when the tracheal airflow is predominantly turbulent. Turbulent flow directly above the folds may lead to an irregular or "rough" voice. Thus, it is important to determine the mechanism of turbulence reduction. From fluid mechanics, it is known that a smoothly converging duct will reduce turbulence. In this study, we tested the hypothesis that the majority of the turbulence reduction is due to the smooth converging shape of the subglottis.
Methods: In 3 excised canine larynges, hot-wire anemometry was used to measure the turbulence intensity (TI) below the cricoid cartilage and 2 to 3 mm above the superior edge of the vocal folds. Laminar flow was seen when the TI was approximately less than 2%. For our measurements, flow into the subglottis had an average TI of more than 20% (high turbulence) in the shear layer and a TI of more than 15% in the center of the jet. The larynges were tested under steady conditions (folds not phonating) with the vocal processes approximated.
Results: For the center of the jet, there is moderate turbulence below the cricoid cartilage and laminar flow 2 to 3 mm above the folds. For the shear layer, there is very high turbulence below the cricoid cartilage and low turbulence 2 to 3 mm above the folds.
Conclusions: The smooth converging shape of the subglottis can produce a significant reduction in turbulence. These findings may have important voice implications for operations that may change the subglottal shape (such as vocal fold medialization or airway reconstruction).