Sound spectra of gas dispersion phenomena were obtained using a hydrophone and a spectrum analyzer at three locations (the sparge ring, the impeller and a position in the bulk of the tank) in an agitated tank as a function of gassing rate and impeller rotational speed. For the sparge ring, the spectra made it possible to distinguish between gas sparging controlled by the sparge ring itself and the natural volume pulsation frequency of the bubbles and gas sparging controlled by the impeller flow. Sound pressure level spectra, obtained for an impeller under various flooding and non-flooding conditions, showed harmonies in the sound pressure levels only for non-flooding conditions. The number and amplitude of the harmonics diminished as the impeller approached flooding. The sound pressure level spectra for a position in the bulk of the tank resonated with the impeller rotational speed in some cases in which the impeller was not flooded. In other cases in which the impeller was flooded, the sound pressure level spectra of the bulk of the tank resembled the impeller spectra at frequencies below 500 Hz.
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