For most common gases, Γ Γ > 0 , whereas abnormal substances such as the BZT fluids exhibit Γ Γ < 0 . In an isentropic process, the sound speed increases with pressure when Γ Γ > 1 ; this is the case for ideal gases. Specifically for polytropic gases (ideal gas with constant specific heats), the Landau derivative is a constant and given by