“…Figure 2а shows the wave surfaces Ψ(t 3 ,r,θ) at a certain time instant t 3 (shown in figure 3с) at two-frequency small-amplitude oscillations. These surfaces are also attributed to a spherical wave, but the spherical layer in this case can be divided into three sublayers: a sublayer adjacent to the inner sphere where Ψ(t 3 ,r,θ) > 41, a sublayer adjacent to the outer sphere where Ψ(t 3 ,r,θ) < 9,5, and a narrow transient layer (shown in figure 2а in white) with a large phase gradient 9.5 < Ψ(t 3 ,r,θ) < 41, which increases in time (figure 2с shows the dependence of the phase of oscillations on the radius in the equatorial plane for three time instants, where the transient layer is shown in gray).…”