Guided waves energy velocity in absorbing and non-absorbing plates

This paper presents a study of the velocity of the propagation of energy in guided waves in plates. The motivation of the work comes from the practica l observation that the conventional approach to predicting the velocities o f pulses or wave packets, that is, the simple group velocity calculation, b reaks down when the guided waves are attenuative. The conventional approach is therefore not valid for guided waves in absorbing materials or for leak y waves. The paper presents a theoretical derivation of an expression to pr edict the energy velocity of guided waves in an isotropic plate, based on t he integration of the Poynting energy vectors. When applied to modes with n o attenuation, it is shown analytically from this expression that the energ y velocity is always identical to the group velocity. On the other hand, wh en applied to attenuative modes, numerical integration of the expression to yield the true energy velocity shows that this can differ quite significan tly from the group velocity. Experimental validation of the expression is a chieved by measuring the velocity of wave packets in an absorbing plate, un der such conditions when the energy velocity differs substantially from the group velocity. Excellent agreement is found between the predictions and t he measurements. The paper also shows the Poynting vectors in the various m odel studies, and some interesting phenomena relating to their directions. (C) 2001 Acoustical Society of America.