The research proposes a method of aggregation of metallic copper particles in colloidal aqueous solutions under the action of underwater diaphragm discharge initiated in the sound frequency range. This paper considers the mechanisms of the underwater diaphragm discharge effect on a colloidal solution of metallic copper stabilised by surface-active agents (surfactants). According to the research, these mechanisms can be quite complex. They include energetic, hydrodynamic, chemical, and electrochemical effects on both copper particles and surfactant molecules. Moreover, as a result of short-term plasma exposure to the solution it is possible to increase the size of copper particles on average by 2-2.5 times. The colloidal system degrades under prolonged exposure of the solution to plasma, and in extreme cases one can even observe the formation of copper oxides.