Bubble site affects mass transfer and mixing hydrodynamics significantly in a gas-liquid agitated vessel. It is important that accurate and reliable techniques are developed to measure bubble size to improve design and scale-up of such vessels. Existing bubble-size measurement techniques are time-consuming (e.g., photography) or complex and expensive (e.g., ultrasound). The ideal technique, capable of on-line measurement in any type of liquid, has yet to be developed. It is possible to size gas bubbles from sound measurement within a gas-sparged agitated tank. A significant amount of the noise, measured close to the impeller, is made up of transient, damped, sinusoidal pressure pulses, which are heard only in the presence of gas bubbles. Analysis of the individual sound pulses in terms of magnitude, frequency and damping, combined with photographic validation indicates that the sound is caused by bubbles oscillating at frequencies dependent on bubble size. The bubbles are excited into oscillation when formed at the impeller. Sound from other sources (e.g., turbulence) is differentiated from bubble sound. Theoretical modeling and experimental results are used to estimate the sound contribution of a single bubble to a sound spectrum and calculate the bubble-size distribution at the impeller directly from experimentally measured sound-pressure spectra.