Ultrasonic propagation in suspensions—A comparison of a multiple scattering and an effective medium approach

The methods of an effective medium and of multiple scattering for determining the velocity and attenuation of suspensions of spherical particles have been compared in order to identify the source of unphysical results obtained using the former. With a suspension of elastic spheres (polystyrene) in an inviscid fluid (water), the cause of the discrepancy was found to be the approximations used in calculating the scattering coefficients An. When full expressions for the An are used, the results obtained by the two methods agree both for elastic spheres in a fluid and for elastic spheres (glass) in an elastic matrix (epoxy). The computational simplicity of the effective medium method compared to the multiple scattering method is significantly reduced if the approximations cannot be made.