Effect of changes in lung volume on acoustic transmission through the human respiratory system

The variation of acoustic attenuation with lung density was determined in e xperimental studies on seven healthy human volunteers, using a change of lu ng volume as a means of varying lung density. White noise between 50 and 68 0 Hz was introduced into the mouth and the transmitted signals were recorde d with four microphones on the posterior chest wall (left/right, top/base) at 24, 40, 60 and 80% of total lung capacity. The change in lung volume had a frequency-dependent effect on acoustic attenuation in all subjects. A fr equency between 177 and 243 Hz was identified, where altering the lung volu me between 24 and 80% of total lung capacity induced a change in attenuatio n of only 1.0 (+/-0.5) to 2.7 (+/-1.8) dB, while at a frequency of 364-436 Hz marked variations in attenuation 8.9 (+/-2.0) to 21.5 (+/-4.8) dB occurr ed with similar lung volume changes.