An apparatus 10,70 and method is provided that includes a spatial array of unsteady pressure sensors 15-18 placed at predetermined axial locations X.sub.1-X.sub.N disposed axially along a pipe 14 for measuring at least one parameter of a solid particle/fluid mixture 12 flowing in the pipe 14. The pressure sensors 15-18 provide acoustic pressure signals P.sub.1(t)-P.sub.N(t) to a signal processing unit 30 which determines the speed of sound a.sub.mix of the particle/fluid mixture 12 in the pipe 14 using acoustic spatial array signal processing techniques. The primary parameters to be measured include fluid/particle concentration, fluid/particle mixture volumetric flow, and particle size. Frequency based sound speed is determined utilizing a dispersion model to determine the parameters of interest.