Scaling behavior in crackle sound during lung inflation

During slow inflation of lung lobes, we analyzed a sequence of short explosive transient sound waves called "crackles", each consisting of an initial spike followed by ringing. The crackle time series is irregular, with the distribution of spike size s following a power law, n(s)/spl sim/s/sup -/spl alpha// (/spl alpha/=2.77/spl plusmn/0.05). We develop a model of crackle wave generation and propagation in a tree structure. The agreement between experiment and simulations suggests that (i) the irregularities are a consequence of structural heterogeneity in the lung and (ii) the scaling is a result of successive attenuations acting on the sound spikes as they propagate through a cascade of bifurcations along the airway tree.