GAS-DENSITY DOES NOT AFFECT PULMONARY ACOUSTIC TRANSMISSION IN NORMALMEN

Fremitus, the transmission of sound and vibration from the mouth to th e chest wall, has long been used clinically to examine the pulmonary s ystem. Recently, modern technology has become available to measure the acoustic transfer function (TF) and transit times (TT) of the pulmona ry system. Because sound speed is inversely proportional to the square root of gas density in free gas, but not in porous media, we measured the effect of air and Heliox (80% He-20% O-2) breathing on pulmonary sound transmission in six healthy subjects to investigate the mechanis m of sound transmission. Wide-band noise (75-2,000 Hz) was ''injected' ' into the mouth and picked up over the trachea and chest wall. The av eraged power spectra, TF, phase, and coherence were calculated using a fast Fourier transform-based algorithm. The phase data were used to c alculate TT as a function of frequency. TF was found to consist of a l ow-pass filter property with essentially flat transmitted energy to 30 0 Hz and exponential decline to 600 Hz at the anterior right upper lob e (CR) and flat transmission to 100 Hz with exponential decline to 150 Hz at the right posterior base (BR). TF was not affected by breathing Heliox. The average TT values, calculated from the slopes of the aver aged phase, were 1.5 +/- 0.5 ms for trachea to CR and 5.2 +/- 0.5 ms f or trachea to BR transmission during air breathing. During Heliox brea thing, the values of TT were 1.5 +/- 0.5 ms and 4.9 +/- 0.5 ms from th e trachea to CR and from the trachea to BR locations, respectively. Th ese results suggest that sound transmission in the respiratory system is dominated by wave propagation through the parenchymal porous struct ure.