In pharmaceutical manufacturing, control of particle size is crucial for achieving reproducible products of acceptable quality. Robust technologies to measure particle size online during operations such as blending, granulation and tabletting are required, but to date have not been implemented. A previous study showed acoustic emissions generated during high-shear granulation are correlated to changes in particle size; however, the origin of the relationship is not well understood. In this work, the correlation between audible acoustic emissions (AAEs) and particle size was investigated by isolating particle-particle and particle-equipment interactions MCC and sugar spheres of known size were rotated in 1.2 and 12 L stainless steel beakers and a PMA-10 granulator. AAEs were collected using a condenser microphone positioned above the moving bed. The results showed the acoustic 10 Hz total power spectral densities (TPSDs) between 20 and 20,000 Hz were predictive of particle size. The correlations were positive for the majority of the 10 Hz frequency groups and were shown to correspond to an increase in surface velocity with increasing particle size. Some of the low frequency groups were negatively correlated, in agreement with a reduction in AAEs generated by interparticle friction. The investigation also demonstrated a predictive relationship between the 10 Hz TPSDs and size for wet granules in the 12 L beaker that agrees with the results for the dry spheres. The findings support correlations developed between AAEs and particle size using process data and encourage further development of AAEs for use in online, non-intrusive monitoring. (C) 2011 Elsevier B.V. All rights reserved.