the resistance of the lung is measured by the present technique both for unidirectional air flow and for alternating direction air flow at various points as a person rapidly exhales a maximum breath of air , and these measurements , together with other lung characteristic measurements provide comprehensive data for lung diagnosis and analysis . the equipment which is used is a closed system , and includes an input spirometer from which the subject draws air , and an output plastic air bag into which exhaled air is exhausted . a loudspeaker operated at 10 to 12 cycles per second supplies alternating air pressure signals to the mouth through a substantially conical or tapered transition to provide a good impedance match . signals are recorded from an esophageal balloon system which measures lung pressure from inhalation and exhalation air rate sensors , and from a transducer which measures the pressure at the mouth . suitable compensation and coupling networks are employed to compensate for the frequency response of the transducer systems , such as the esophageal balloon sensor . the input gas from the spirometer is free of nitrogen , and a mass spectrometer is employed to detect nitrogen exhaled by the subject to determine residual levels of air in the lungs .