Passive acoustic sound, measured by a hydrophone in an air-water bubble column, is used to study the hydrodynamics of the unit. The recorded measurements taken at different Superficial gas velocities are processed using both spectral and chaos-based techniques in Order to characterize the column flow regimes and to predict the transitions points. These processing tools Were supported by digital video imaging of bubbles motion inside the Column. The results of data analysis indicate the applicability of passive sound measurements to identify how regimes in the bubble column. In this regard the analysis of sound spectra gives a useful qualitative comparison of flow regimes. Chaos-based techniques, on the other hand, are more Successful in predicting the transition points between the homogenous and the churn-turbulent flow regimes in the column. The critical gas velocities of the transition are associated with a marked change in some of the calculated chaotic invariants of sound pressures. The calculated superficial gas velocities of the critical points are also found to be consistent with experimental observations. Moreover, a useful visualization Of the dynamics induced in the column by the alternation of small and large bubbles is possible through the inspection of phase-space trajectories reconstructed from time-series measurements. The shape and size of the trajectories are closely linked to the size distribution of bubbles, and they change as the flow moves from one regime to an other. (C) 2008 Elsevier B.V. All rights reserved