Attenuation of Lamb Modes and SH Waves near cut-off Frequencies

Near cut-off frequencies, the shear horizontal (SH) and Lamb modes in a platelet are created by a bulk acoustic wave bouncing up and down between the platelet sides in an almost vertical direction. The small inclination angle corresponds to slow drift of energy along the platelet (low group velocity), but the attenuation of the bulk wave accumulates fast. Therefore, the attenuation of these waves along the platelet near the cut-off frequency is high, being roughly inversely proportional to the group velocity.