The importance of gas dispersion variables on the flotation process has led to the development of techniques that allow measuring and monitoring them in flotation. In the case of bubble size, the most used methods are based on image analysis and, although these methods are well accepted, they have not been implemented to monitor the flotation process on a regular basis, mainly because they are labor intensive, require the manipulation of a well trained operator, and are difficult to automate. In the current work, an acoustic technique for measuring the bubble Sauter mean diameter, d(32), is introduced: The technique is based on the response of bubbles to an acoustic perturbation consisting of a modulated signal with frequencies F-1 +/- F-2, where F-1 acts as a carrier set at 1 MHz, and F-2 corresponds to the modulating signal that varies from 1 kHz to 10 kHz. A linear empirical relation was found between the average intensity of the acoustic response of the bubbles, I-mean, and d(32), which allows determining the bubble Sauter mean diameter for a wide range of sizes, from 0.75 to 3 mm, covering the range of interest of flotation. The acoustic method represents an alternative to the image analysis technique as a sensor to monitor the flotation process, however, it still depends on image analysis as an independent method for calibration. (C) 2016 Elsevier Ltd. All rights reserved.