As
the attenuation properties of the BL depends on the frequency of
the propagating wave [15], the initial size (R0g) and density (Ng)
of spherical bubbles in water are set to allow the maximum attenuation
coefficient at the driving frequency xm (and all the frequencies
of the input signal, which are close to xm), xm=x0g ¼ 1, and
thereby, the BL behaves like an insulator at xm. xs is chosen to verify
the following condition: xs < x0g.