Airflow Error Measurement Due to Pneumotachograph Mask Rim Leaks

Airflow during speech production is recorded using a pneumotachograph system wherein typically a mask is placed upon the face. Accurate measures of airflow require mask calibration and a complete seal of the mask rim to the face. Literature frequently cites mask rim leaks as causing flow measurement inaccuracies, but quantitative studies of the inaccuracies are needed. The purpose of this study was to determine the degree of inaccuracy of flow measurement using a Glottal Enterprises aerodynamic system for a variety of mask rim leak conditions. Air was pushed and pulled through the Glottal Enterprises mask system over a wide range of airflow with leaks simulated by small metal tubes of various cross-sectional areas placed between the mask rim and a face-like calibration mold. Two leak locations, single versus multiple leaks, and two different leak tube geometries were used. Results suggest that (1) as leak area increases, the amount of leak flow increases; (2) the amount of flow leak is relatively independent of location; (3) given equivalent leak areas, multiple leak locations provide less leak flow; and (4) quasi-elliptical tubes were more resistive to airflow than rectangular tubes. A general empirical equation was obtained that relates the leak flow between the mask rim and the face, the size of the leak region, and the amount of the upstream airflow toward the mask: Leak(cm³/s) = 0.33 × Area(cm²) × Flow(cm³/s) for the range of ±2000 cm³/s. This equation may provide researchers and clinicians with a tool for generalizing airflow leak effects.