Comparison of Lung Sound and Transmitted Sound Amplitude in Normal Men

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Abstract
Within the last 5 yr, several studies have suggested that if the lung sound intensity at the chest wall is compared with the intensity of white noise transmitted from the mouth to the same site, the acoustic transmitting properties of the thorax can be assessed and separated from the properties of the lung sound generator. In this study, we used a computer-aided rapid amplitude measuring technique to study this question over a much greater area on the chest wall in 7 subjects. Study locations were at 2cm intervals, apex to base, over both hemithoraxes, anteriorly and posteriorly. Lung sound intensity (LS) was measured by an air-flow-corrected technique. Colored noise (50 to 500 Hz) introduced at the mouth (TRANS) was measured on the chest wall and adjusted for glottic aperture by comparison with the signal from a reference microphone at a fixed location on the chest. The LS amplitude patterns that we observed were similar to those previously determined in this laboratory and were bilaterally equal for the group. The TRANS sound patterns were always of greater intensity near the apex and, over the right hemithorax, were approximately twice the amplitude of those over the left. Similar LS and TRANS maps performed at locations completely encircling the upper thorax in one upright subject revealed wide intensity variations in each that appeared unrelated to each other or to the presumed distribution of ventilation. The transmission of sounds introduced at the mouth appears to be influenced by factors other than simple propagation down airways in certain lung regions. Therefore, the use of such transmitted sounds may not be an appropriate way to correct for transmission characteristics affecting normal lung sounds in these regions.