From a mathematical point of view, the Landau derivative is a non-dimensional measure of the curvature of isentropes in the pressure-volume thermodynamic plane. From a physical point of view, the definition of Γ Γ tells that the speed of sound increases with pressure in isentropic transformations for values of Γ Γ > 1 , while, by contrast, it decreases with pressure for Γ Γ < 1 .