“…The acoustic hardness of a bubble cluster is less than the acoustic hardness of water, so the initial boundary wave from the cluster will be reflected as from a free boundary [9] (figure 2, a). Over time, due to the two-dimensional wave motion, oscillations change in the region of a highly compressible bubbly liquid [3,4]. Focusing is accompanied by ugly wave bursts (figure 2,b and 2,c).…”