We will first consider mechanical waves in solids. Wave motion in solids is generally a result of both elasticity and geometric stiffness. Elasticity refers to the ability of a deformed material to return to its original shape and size when the deforming load is removed. Geometric stiffness is the result of a applied pre-stress that causes the solid to depart from its equilibrium state. In the case of a one-dimensional stretched string, geometric stiffness dominates and we initially ignore elasticity (or bending stiffness).
