The consistency of a pulp slurry in a pipe 12 is determined using a pair of effective sound speeds a.sub.1eff and a.sub.2eff of the fluid/pipe system. The pair of effective system sound speed measurements are taken at two sensing regions X.sub.1, X.sub.2 along the pipe wherein each of the sensing regions comprises a different system cross sectional area compliance. The pair of effective system sound speeds a.sub.1eff and a.sub.2eff are provided to signal processing logic 160, which determines at least the composition of the fluid 12 flowing in the pipe 112. The effective system sound speeds a.sub.1eff and a.sub.2eff may be provided by a pair of sound speed meters positioned at sensing regions X.sub.1, X.sub.2 wherein the sound speed meters utilize a spatial array of acoustic pressure sensors placed at predetermined axial locations along the pipe 112. One technique uses acoustic spatial array signal processing techniques with the direction of propagation of the acoustic signals along the longitudinal axis of the pipe 112.