Abstract: In one exemplary embodiment, the efficiency of an acoustic-energy-type cleaner for vessels, such as heat exchangers, is substantially increased by propagating through the fluid in the vessel, a plurality of opposing acoustic wave trains, and continuously varying at least the frequency or phase relationship of said opposing wave trains for causing constructive interference between said opposing acoustic waves to create a series of augmented acoustic waves that are spatially displaced in relation to each other and successive in time interval for sweeping over the surfaces of the body to be cleaned.