Effects of breathing pathways on tracheal sound spectral features

The spectra of sounds recorded over the trachea of adults typically reveal peaks near 700 and 1500 Hz. We assessed the anatomical determinants of these peaks and the conditions contributing to their presence. We studied five adult subjects with normal lung function, measuring sounds at the suprasternal notch and on the right cheek. The subjects breathed at target airflows of 15 and at 30 ml sec(-1) kg(-1) both through the mouth with nose clips and then through the mouth and nose using a cushioned face mask. The mouth breathing maneuvers were performed with three lengths (3.6, 21.1 and 38.6 cm) of 2.6 cm diameter tubing between the mouth and the pneumotachograph. The nose breathing maneuver was performed with the longest tube (between the mask and pneumotachograph). The signals occurring at the target flows +/- 20% were used to create averaged, spectral estimates. We found that all subjects had two predominant spectral peaks; a approximately 700 Hz peak loudest over the cheek and a approximately 1500 Hz peak loudest over the trachea. The frequency of both peaks negatively correlated with body height (and presumably, airway length). There was no systematic effect of breathing phase, flow rate or length of the tube connecting the mouth to the pneumotachograph on the spectral peaks. Breathing into the mask and breathing through the nose did markedly alter the spectra. We conclude that the higher tracheal sound peak reflects resonance within the major airways and is relatively independent of extrathoracic influences during mouth breathing through a tube.