Parametric phase-delay estimation of sound transmitted through intact human lung.

Sonic noise between 300 and 1600 Hz is introduced into the mouths of 11 healthy adult male subjects at resting lung volume and is detected over the anterior extrathoracic trachea and at three sites on the right posterior chest wall. To overcome the difficulties associated with non-parametric phase unwrapping due to thoracic anti-resonances, the phase delay tau(f) of propagation between the trachea and the chest wall is estimated using a linear parametric ARX-type statistical model with the non-parametric magnitude spectra as a guide. The resulting tau(f) estimates are unambiguous and reliable, and show a clear trend of decreasing tau(f) with increasing frequency, indicating that sound at higher frequencies reaches the chest wall faster than that at lower frequencies. This finding indicates that respiratory sound transmission is highly dispersive, most probably owing to frequency-dependent airway and parenchymal wavespeeds.