An apparatus 10,70 and method is provided that includes a spatial array of unsteady pressure sensors 15-18 placed at predetermined axial locations x₁-x_N disposed axially along a pipe 14 for measuring at least one parameter of a saturated vapor/liquid mixture 12, such as steam, flowing in the pipe 14. The pressure sensors 15-18 provide acoustic pressure signals P₁(t)-P_N(t) to a signal processing unit 30 which determines the speed of sound a_mix propagating through of the saturated vapor/liquid mixture 12 in the pipe 14 using acoustic spatial array signal processing techniques. The primary parameters to be measured include vapor/liquid concentration (i.e., steam wetness or steam quality), vapor/liquid mixture volumetric flow, mass flow, enthalpy, density and liquid droplet size. Frequency based sound speed is determined utilizing a dispersion model to determine the parameters of interest.