In this paper, we considered the scattering of a plane sound wave in a medium which contains many spherical particles, for example, a concentrated suspension in a liquid. The sound interaction field and the equivalent scattering cross section of a partical have been calculated. When dimensions of the particles are much less than the sound wave-length, the following conclusions can be drawn, a) On account of the interaction, the scattering cross section must have a multiplying factor Q. Geometrically, it is due to the masking effect on each other, and Q= |1-(γ0A0(1)+γ＇1A1(1)|2 is called masking factor, b) The dependence of scattering coefficient on partical concentration is no longer a linear one. c) When the dimensions of them are much less than the sound wavelength but larger than viscous wavelength, the scattering coefficient of them will obey Rayleigh's law. But when their radii are near or less than viscous wavelength, the scattering co-efficients will show a dependence on frequency that is higher than the fourth power.