A new concept of effective agglomeration length, which measures the maximum particle separation distance for effective collisions, is proposed for study of acoustic agglomeration of polydisperse aerosols with respect to the separate and combined effects of orthokinetic collision and acoustic wake in a horizontal acoustic wave when particle gravity is included. Particle gravity is found to be significant for the acoustic wake effect while the particle collision efficiency is important for the orthokinetic collision. Results indicate that orthokinetic collision dominates at low frequencies for intermediate size ratios while the acoustic wake effect is more significant at higher frequencies for all particles. The optimum frequency for orthokinetic collision is confirmed but shifts downward with the increase of sound power. For the acoustic wake effect, the agglomeration increases monotonically with sound frequency. Results also show that the orthokinetic collision is not effective for agglomeration of sub-micron particles because of low particle collision efficiency. (C) 2005 Elsevier Ltd. All rights reserved.