TABLE OF CONTENTS When the particles contained in an elastic medium are removed from their position of static equilibrium assumed here to be stable the stresses related to the local change of configuration have the tendency to take them back to the position of static equilibrium, but the inertia forces are acting to the opposite, having the tendency to make the particles overshoot it. As a result, they start to oscillate. Due to the principle of action and reaction, the particles lying in the immediate vicinity are also excited and start to oscillate too. In this way, the motion is found to propagate throughout the whole solid. In the absence of inertia, the propagation would have the instantaneous character of the elastic forces. The inertia introduces however a delay in the propagation, in such a way that the speed is finite. Such progressive oscillations are termed travelling waves. It is important to point out first that in this chain reaction what propagates is not matter but mechanical energy. A discrete version of material waves was already discussed in [AXI 04], Chapters 7 and 8. As schematically illustrated in Figure 1.10, two kinds of waves can be distinguished. They are termed transverse waves if the particles oscillate in a direction perpendicular to that of wave propagation and longitudinal waves if the particles oscillate in the direction of wave propagation. On the...
