A model which describes the movement of a powder bed subjected to vibration and aeration through a porous support plate is proposed and solved. The calculated bed movement is shown graphically both as a function of time and - taking discrete Fourier transforms - of frequency. To validate the model experimentally, a new experimental technique was developed. This involved measuring the movement of a porous disc inserted in the bed and following the bed movement. The methodology and the difficulties involved are discussed. The experimental results are presented as Fourier transforms of the bed movement. Both model and experiment show that at the higher vibration intensities, where the bed looses contact with the support plate, bed movement with a period of more than one vibration cycle causes the appearance of a peak in the Fourier spectrum at half the plate excitation frequency. The intensity at which this first occurs is acceptably well predicted by the model. On the whole, taking into account the assumptions made in the model and unavoidable experimental errors, predictions and experimental results show good agreement.