Effect of Air Viscosity on Sound Propagation in Human Bronchial Tree

Abstract

A physical model of sound propagation in a transitional zone of human respiratory tract has been developed within the long-wave approximation. A basic notice was given to analyzing the influence of viscous interaction between the bronchial air, oscillating owing to acoustic excitation, and bronchial wall. To avoid a considering of the effects of wall compliance and sound radiation into pulmonary parenchyma, within this study the respiratory airways were modeled by the tubes with rigid walls. It is shown that the allowance for air viscosity results in not only attenuation, but also profound dispersion of sound waves propagating in the elements of the bronchial tree. These phenomena rapidly increase with the decrease of the bronchus caliber. Also, the viscosity essentially alters frequency dependencies of the input impedance of terminal bronchioles loaded on the system of respiratory airways. Moreover, the impedance of terminal bronchiole depends on boundary conditions at the entrance of the respiratory tract - the glottis (open or closed vocal chords).