A unified relation among the hydrodynamic velocity V _ϕ, the density ρ, the mean atomic weight m, and Poisson's ratio σ is proposed for oxides, silicates, and germanates as V _ϕ = V _ϕ ⁺ − 0.058(m − m ₀) − 2.62(ρ − ρ₀), where V _ϕ ⁺ = −1.84 + 7.51 σ + 1.98ρ and ρ₀ is the density of the crystallographically isostructural material of m = m ₀ = 20.2. The proposed relation can be considered as a linear form of the power law relation proposed by Shankland and Chung (1974). However, the incorporation of σ into the present relation would give a better correlation among the various parameters. It would also accommodate the velocity data of α quartz as well as those of materials undergoing various phase transformations. The value of the mean atomic weight calculated from this relation for various earth models implies that iron enrichment in the lower mantle with respect to the upper mantle is not necessary.