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Several methods for noninvasively evaluating the instantaneous flow rate from tracheal sounds are compared. All of the eight methods are independent of a mathematical relationship and can be used whatever the breathing level. In spite of different signal processing, all the methods lead to equivalent results, and these results confirm that tracheal sounds allow flow evaluation. However, for a given subject there is a method which gives a flow evaluation with a mean precision of 15%. This can be considered as acceptable for clinical monitoring. In addition, continuation of this work may contribute to understanding lung-sound generation mechanisms. It may also be used in pulmonary pathology characterization. Hierarchical clustering analysis appears to be especially well suited for this objective, because it permits the organization of spectra in homogeneous classes without using variables not belonging to sound.