As is distinct from general gas-liquid two-phase flow, a large number of bubbles with different diameters belong to ship wakes. Feasibility of Laplace equation, used to calculate wake sound speed (WSS), is confirmed based on differential postulation. Defect for calculating the adiabatic sound speed of gases in references is showed, and a concept of WSS is proposed clearly. A minimum WSS of 24.5 m/s is got when bubble ratio reads 0.5 according to the calculation when bubble dimension is less than that of resonance. Also a weak dependence of WSS on pressures is predicted. WSS from calculation corresponds with the experimental data of references well in high frequency domain, when the actual scale of bubbles is greater than the resonant scale.