Why fluid loading has an opposite effect on the velocity of dilatational waves in thin plates and rods

From almost every respect, the lowest-order dilatational waves in thin plates and rods are very similar. Therefore, it is rather intriguing that fluid loading increases the phase velocity of such waves in a plate, but decreases it in a rod. Although this difference can be easily demonstrated by inspection of the well-known exact dispersion equations, the underlying physics remains somewhat obscure. Simple physical models are introduced for thin immersed rods and plates that provide a better insight into the fundamental difference causing the opposite behavior of these otherwise analogous modes of wave propagation. It is shown that the inverse effect of fluid loading is caused by the fact that the radiation impedance exerted by the fluid on plates is purely real while it is dominantly imaginary in the case of thin rods. © 1997 Acoustical Society of America. @S0001-4966~97!01212-5#