Equations that describe wave propagation in orthorhombic media are derived under the acoustic assumption. These include a dispersion relation, which is a key equation for imaging, and a wave equation, which is important for modeling P-waves. Though the Earth subsurface is always elastic in anisotropic media, these acoustic approximations yield accurate kinematic and dynamic descriptions of P-wave propagation in elastic media. In addition, the acoustic equations are simpler than the elastic ones, and thus, they are more efficient and easier to use.