The setting level of a hydraulic machine, specially for low head machines, is decided with respect to the possible alteration of the efficiency due to the cavity development. In the case of traveling bubble cavitation, corresponding to the outlet cavitation at the nominal head, the physical phenomenon underlying such an efficiency alteration is not very clear, even though a strong dependence of this drop with the volume of cavities have been noticed. The aim of this paper is to present the results of an experiment intending to explain physically the influence of bubble cavitation on the performance of a hydraulic profile.