In this paper there is derived a set of approximate equations governing the extensional motion of a sandwich plate, together with the associated initial and boundary conditions. Both the sandwich core and the facings are assumed homogeneous and isotropic, and the facings are assumed thin and identical to each other. The coupling effect of the thickness deformation of the core is considered in the derivation.Frequency‐wavelength curves are then obtained from these equations for an infinite plate in plane strain. The corresponding curves for the same plate according to the three‐dimensional exact theory of elasticity are also obtained. These two sets of curves are then used as a basis for discussion of the accuracy of the approximate equations.The curves indicate that for commonly used core materials and for wavelengths about three times the plate thickness and longer, the approximate equations are reasonably good up to frequencies somewhat higher than the fundamental thickness‐stretch frequency of the plate.