A model of acoustic transmission in the respiratory system

A theoretical model of sound transmission from within the respiratory tract to the chest wall due to the motion of the walls of the large airways is developed. The vocal tract, trachea, and first five bronchial generations are represented over the frequency range from 100 to 600 Hz by an equivalent acoustic circuit. This circuit makes it possible to estimate the magnitude of airway wall motion in response to an acoustic perturbation at the month. The radiation of sound through the surrounding lung parenchyma is represented as a cylindrical wave in a homogeneous mixture of air bubbles in water. The effect of thermal losses associated with the polytropic compressions and expansions of these bubbles by the acoustic wave is included, and the chest wall is represented as a massive boundary to the wave propagation. The model estimates the magnitude of acceleration over the extrathoracic trachea and at three locations on the posterior chest wall in the same vertical plane. The predicted spectral characteristics of transmission are consistent with previous experimental observations.