An apparatus for measuring at least one parameter of a particle/fluid mixture in a pipe, comprising: a spatial array of at least two pressure sensors, disposed at different axial locations along the pipe, and each measuring an unsteady pressure within the pipe at a corresponding axial location (X 1, X 2, X 3, X n), each of said sensors providing a pressure signal (P 1 (t), P 2 (t), P 2 (t), P N (t)), indicative of the unsteady pressure within the pipe at said axial location (X 1, X 2, X 3, X n) of a corresponding one of said sensors; and a signal processor, responsive to said pressure signals (P 1 (t), P 2 (t), P 2 (t), P N (t)), which is configured to provide a signal indicative of the speed of sound propagating through the mixture in the pipe, said signal being a function of the frequency of the sound propagating through the mixture; and to utilize a dispersion model to determine the at least one parameter of the mixture from the signal indicative of the speed of sound wherein a signal indicating the speed of sound in a first frequency range is used for determining the concentration of the particle/fluid mixture and a signal indicating the speed of sound in a second frequency range is used for determining the particle size within the particle/fluid mixture, wherein the frequencies within the second frequency range are larger than the frequencies within the first frequency range. a spatial array of at least two pressure sensors, disposed at different axial locations along the pipe, and each measuring an unsteady pressure within the pipe at a corresponding axial location (X 1, X 2, X 3, X n), each of said sensors providing a pressure signal (P 1 (t), P 2 (t), P 2 (t), P N (t)), indicative of the unsteady pressure within the pipe at said axial location (X 1, X 2, X 3, X n) of a corresponding one of said sensors; and a signal processor, responsive to said pressure signals (P 1 (t), P 2 (t), P 2 (t), P N (t)), which is configured to provide a signal indicative of the speed of sound propagating through the mixture in the pipe, said signal being a function of the frequency of the sound propagating through the mixture; and to utilize a dispersion model to determine the at least one parameter of the mixture from the signal indicative of the speed of sound wherein a signal indicating the speed of sound in a first frequency range is used for determining the concentration of the particle/fluid mixture and a signal indicating the speed of sound in a second frequency range is used for determining the particle size within the particle/fluid mixture, wherein the frequencies within the second frequency range are larger than the frequencies within the first frequency range. to provide a signal indicative of the speed of sound propagating through the mixture in the pipe, said signal being a function of the frequency of the sound propagating through the mixture; and to utilize a dispersion model to determine the at least one parameter of the mixture from the signal indicative of the speed of sound wherein a signal indicating the speed of sound in a first frequency range is used for determining the concentration of the particle/fluid mixture and a signal indicating the speed of sound in a second frequency range is used for determining the particle size within the particle/fluid mixture, wherein the frequencies within the second frequency range are larger than the frequencies within the first frequency range.