The main objective of this study was to develop a piezoelectric probe for measuring the impact rate of particles to enable characterization of the shape of particle number density profiles and characteristic particle impact frequencies inside the draft tube of a Wurster coating chamber. Pellets from 800 mu m to 900 mu m were subjected to fluidization in a conventional and swirl Wurster coating chambers. Fast Fourier Transform (FFT) analysis of the raw signal obtained at four heights and three different radial distances from the draft tube centerline was performed for experiments in a conventional and a swirl Wurster coating chamber at different process parameters. Power spectra revealed three characteristic frequency bands for both chambers: 1-5 Hz, 5-7 Hz, and 12-14 Hz. Particle number density profiles demonstrated an evident difference between both chambers. In the conventional Wurster coater, the location of the maximum number of particle impacts was close to the tube center, whereas in the swirl Wurster coater the particle impacts were distributed rather uniformly. The difference in particle number density profiles for both process chambers can help to understand the difference in performance of both Wurster coater designs. (C) 2012 Elsevier B.V. All rights reserved.