The present invention provides a method of measuring an acoustic impedance of a respiratory system. The method comprises selecting a frequency range for an acoustic wave, directing the acoustic wave into the respiratory system and receiving an acoustic wave from the respiratory system. The method also comprises determining the acoustic impedance for a plurality of volumes or volume ranges of the respiratory system. Each volume, or the volumes within each volume range, is larger than RV or FRC and smaller than TLC whereby the determined acoustic impedances are specific for respective volumes or volume ranges. Further, the method includes determining a volume, or volume range, dependency of the acoustic impedance of the respiratory system and charaterising the respiratory system by analysing the dependency of the acoustic impedance on the volume or volume range.
