ACOUSTIC-WAVE SPEED AND ATTENUATION IN SUSPENSIONS

Acoustic wave propagation in a monodisperse suspension of varying solids concentration was modeled exactly for wavelengths much larger than the particle size, including unsteady viscous effects, for the situation where particle interactions are predominantly inviscid. Inviscid particle interactions are addressed in terms of the added mass coefficient, which is sensitive to the solids concentration, the direction of insonification and the anisotropy of the particle arrangement. The sound speed and attenuation were calculated and compared to experimental results for a wide range of ka, where k is the wavenumber (=2-pi/lambda) and a is the particle radius. The attenuation, which is a strong function of ka, is seen to have non-monotonic behavior with respect to the solids fraction at low frequencies and it becomes monotonic at high frequencies. In general, the effect of ka on sound speed is seen to be small in comparison. The comparison with experiments shows that at values of ka near 1, effects associated with multiple scattering begin to affect acoustic propagation sufficiently to cause marked deviation between the present theory and measurements.