Low-frequency compressional waves were observed in a suspension of strongly coupled $9.4\mu \mathrm{m}$ spheres in an rf Kr plasma. Both parts of the complex wave number were measured to determine the dispersion relation, which agreed with a theoretical model of damped dust acoustic waves, ignoring strong coupling, but not with a strongly coupled dust-lattice wave model. The results yield experimental values for the dust plasma frequency, charge, Debye length, and damping rate, and support the applicability of fluid-based dispersion relations to strongly coupled dusty plasmas, which has been a controversy.

• Received 21 June 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.3137