A wave equation has been formulated for the propagation of sound in suspensions and emulsions, taking into consideration the different viscosities and thermodynamic properties of the suspension components. The suspension has been assumed to be homogeneous in the sound field for the particle size to be much smaller than the incident wavelength and to be a continuous fluid which enters and leaves the control volume element at volume‐averaged values of velocity and momentum. This equivalent continuous fluid has been assumed to possess a density equal to the volume‐averaged density and a compressibility equal to the effective compressibility of the suspension or emulsion (calculated from the scattering theory). The formulated wave equation immediately yields the acoustical properties of suspensions or emulsions, such as sound velocity and viscous and thermal attenuation coefficients.