The effects of sonic energy waves on the settling velocity of small particles in water were studied. A design of experiment (DOE) with five variables (frequency, amplitude, particle diameter, particle density and fluid viscosity) at two or three levels was conducted to obtain the particle settling velocity as the response. The DOE data were analyzed both experimentally and by a statistical multiple regression software. It was concluded that when sound frequency and amplitude in the range of 0 to 500 Hz and 2 to 3 Vrms (root mean square) respectively were applied to plastic particles of three different diameters (2,381 μm, 3,175 μm, and 4,763 μm) and two different densities ... their effects on the particle settling velocity in hydroxypropyl cellulose (HPC) solutions of three different viscosities ... were insignificant. The regression analysis gave an equation that is in good agreement with the experimental data.