Crackles were recorded from six patients, four with tuberculosis and two with chronic bronchitis. By waveform analysis we observed initial “high-pitched segments” with characteristics of shock waves and subsequent “low-pitched segments” with characteristics of damped sinusoids. We speculate that the initial segment represents a shock wave caused by an abrupt opening of the airway (Forgacs) and that the latter segment is a damped sinusoid caused by this shock wave exciting a resonator in the lung. The resonant frequency and the quality factor of this resonator were about 200Hz and 3-6 respectively, and both are supposedly determined by the geometry and the acoustic characteristics of the lung. To test our hypothesis, we experimentally excited a resonator with a shock wave and were successful in obtaining waveforms similar to those of crackles.
