We use Brillouin scattering to measure the longitudinal-phonon dispersion curves for a suspension of hard-sphere colloids. Two distinct propagating acoustic excitations are observed when the wavelength of the sound is comparable to, or smaller than, the size of the spheres. One excitation has a velocity intermediate between that of the fluid and the solid phases and is interpreted as an acoustic wave propagating through the composite medium of fluid and solid spheres. The second has a velocity slower than both that of the solid and that of the fluid phase and is interpreted as a coupled interfacial, or Stoneley, wave.

• Received 19 June 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.2602