Lung sound spectra at standardized air flow in normal infants, children, and adults

To investigate the effect of age and body size on normal lung sounds, we studied 10 newborn infants within 3 d after birth, nine children between 6 to 8 yr, and 10 adults between 25 and 37 yr of age. Lung sounds were recorded with a contact transducer over the posterior right lower lobe, and air flow was measured at the mouth. Computer analysis provided average power spectra of lung sounds at flows of 15 ml/s/kg. In children and adults measurements were also made at flows of 30 ml/s/kg. Lung sounds were referenced to background noise, measured at zero air flow. The spectra in infants contained less power below 300 Hz compared with children and adults, resulting in significantly higher quartile and spectral edge frequencies. Resonances of the thoracic cavity may explain some of the differences among the study groups. Sound attenuation above 300 Hz was similar at all ages. At increased air flows, lung sounds in children and adults were above background noise at frequencies as high as 2,000 Hz. High-frequency expiratory lung sounds of low intensity were present in all children and in eight of 10 adults at increased flows. Normal lung sounds of low intensity are present above traditionally accepted frequency limits and warrant further investigation.