:<math>\frac{\partial^{2}P}{\partial z^{2}} + \frac{\partial^{2}P}{\partial x^{2}} = \frac{1}{V^{2}}\frac{\partial^{2}P}{\partial t^{2}}</math> | :<math>\frac{\partial^{2}P}{\partial z^{2}} + \frac{\partial^{2}P}{\partial x^{2}} = \frac{1}{V^{2}}\frac{\partial^{2}P}{\partial t^{2}}</math> |