Recently, the present authors examined anomalous dispersion of the Lamb modes in isotropic plates as shown in their previous paper, by finding the slope of each mode first at its point of inception and secondly at a point far removed from it. If the slopes at two points differ in sign, it will indicate that a zero group velocity point occurs between them. However, Prada [1] has pointed out that analytical reasoning about the anomalous nature of modes has already been given by Mindlin in [2], where he has calculated the curvature of the modes in the plane, where and , respectively, denote the wave number and frequency of the wave. Shuvalov and Poncelet [3] explained this fact by looking at the sign of the first coefficient in the Taylor series for .
The statement “In all isotropic materials with , only the mode has this “anomalous behavior” and other modes behave normally,” which appears in Section 1 of the paper should be replaced by “In all isotropic materials with , the mode always has this anomalous behavior.”
The paper offers an alternative and somewhat simpler, treatment of the anomalous behavior of Lamb modes.