The energy velocity of waves guided in plates, such as Lamb waves, has been studied. Vectors of energy within the plate have been calculated and have been integrated to give the energy velocity of the wave modes. It has been shown that when there is no attenuation of the waves, then the conventional group velocity expression agrees perfectly with the energy velocity. However when there is attenuation, which can be due to leakage from an immersed plate or to material damping, then the group velocity expression does not represent the energy velocity. This finding has also been confirmed experimentally using a lossy plastic plate, the measured results agreeing well with the predicted energy velocity.
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