This paper presents an experimental study of acoustic agglomeration of coal-fired fly ash particles in travelling sound waves. The ranges of variation of the main physical parameters are as follows: acoustic frequency, f = 700-3000 Hz; sound pressure level (SPL), SPL = 130-147 dB; residence time, t = 3-7 s; aerosol number concentration, N-0 = 1.0 x 10(5)-3.7 x 10(5)/cm(3). A 68.4% decrease in total number concentration is gained under an SPL of 147 dB and a frequency of 1400 Hz. Aggregates larger than 10 pm are observed in scanning electron microscopy photographs. The results show that the effect of sound waves is very sensitive to the frequency change, which means that orthokinetic interaction governs the process. There exists an optimum frequency for a given particle size distribution, which decreases slightly as SPL increases. The influences of SPL, residence time and initial total number concentration are also studied (C) 2009 Elsevier B.V. All rights reserved