Spectral characteristics of sound transmission in the human respiratory system

The amplitude of sound transmission from the mouth to a site overlaying the extrathoracic trachea and two sites on the right posterior chest wall over the 100-600-Hz frequency range was measured in eight healthy adult subjects. An acoustic driver and a rigid tube were used to introduce sound into the mouths of the subjects at resting lung volume, and the transmission measurements were performed using lightweight accelerometers. Similar spectral characteristics of acceleration were observed in all of the subjects showing peaks in the transmission. These characteristics included (1) two regions of increased transmission over the frequency range of the measurements, (2) a decrease in the magnitude of acceleration of the chest wall as compared to the tracheal site of roughly 20 dB at lower frequencies, and (3) a strong trend of decreasing acceleration of the chest wall with increasing frequency. These spectra agreed favorably with the predictions of a theoretical model of the acoustical properties of the respiratory system. The model suggests the primary structural determinants of a number of the observed characteristics including the importance of the lung parenchyma in sound attenuation.