Sound propagation in suspensions of colloidal spheres with viscous coupling

We measure attenuation and phase velocity of sound waves in suspensions of colloidal silica spheres. As the concentration of colloids increases, the d ependence of the attenuation changes from linear to at least quadratic in v olume fraction. This change is attributed to a viscous coupling of neighbor ing scatterers. When the wavelength is comparable to the sphere size, the s ound velocity decreases with increasing volume fraction and falls below tha t of the pure liquid.