In the case of a stress pulse propagating longitudinally through a bar, the bar acts much like an infinite number of springs in series and can be taken as an extension of the equation derived for Hooke's law. A uniform bar, i.e. of constant cross-section, made from a linear elastic material has a stiffness K given by K = E A L , where A is the cross-sectional area, and E is the Young's modulus of the material. The wave equation becomes ∂ ∂ 2 u ( x , t ) ∂ ∂ t 2 = E A L M ∂ ∂ 2 u ( x , t ) ∂ ∂ x 2 .