How light element addition can lower core liquid wave speeds

Sound speeds in liquid mixtures are governed by an interplay between the properties of their end-members. The mixture's bulk modulus and density are each affected by end-member volumes and how those end-members mix. Particularly non-intuitive effects can arise when end-member mixing volumes are non-ideal, even mildly so. Non-ideality could also arise in metallic liquids of the Earth's outer core and exhibit some counterintuitive effects, whose origin is explained here.