A modal perspective of lung response.

The qualitative modal structure of lung response is investigated. Asymmetry of the branching airway network of the lung leads to clumping of system eigenvalues into distinct and well-separated clusters at low frequency. The breadth of the eigenvalue clusters increases with the degree of asymmetry and frequency. At high frequency the breadth of the clusters exceeds the cluster spacing so that distinct eigenvalue clusters are no longer observed. Symmetric airway networks lead to degenerate mode clusters with large eigenvalue redundancy. The average density of eigenvalues of the undamped system is estimated to be higher than 0.5 per Hz. All modes appear to be in modal overlap.