My conclusion is that the volume average formula used by Wang and Nur (1991) apparently gives good agreement with the velocity data for these hydrocarbon mixtures because in all cases the spread in velocities and densities is so small that Wood's formula, Wyllie's time average, and the volume average all give virtually the same results (within probable experimental error). In general, Wood's formula is expected to give reliable estimates even when the fluid constituents have widely differing acoustic speeds and densities (as in the case of liquid/gas mixtures), whereas the volume average formula is expected to have more limited applicability.