Measurements of the seismic wave velocities as well as the density of the rock material are necessary for calculating dynamic elastic constants. Correct determination of the dynamic elastic constants is the base for correct calculation of dynamic strains and stresses. These calculations use equations that haves been formulated according to some assumptions. This study investigates the effect of the assumptions of 1-D and 3-D wave propagation on the magnitudes of the estimated dynamic elastic constants. Also some relations between densities, seismic velocities, velocity ratio, Poisson's ratio, Young's modulus, and shear modulus will be developed. The study depicted a significant effect of the assumption of 1-D or 3-D wave propagation on the calculated magnitudes of the dynamic elastic constants and came up with some useful relationships between the above mentioned parameters.