R. T. Lawrence
R. K. Yarber
R. M. Keolian
Dept. of Phys., Naval Postgraduate School, Monterey, CA 93943
Experimental investigation of a newly predicted propagating mode [Larraza et al., Phys. Rev. A 41, 3144 (1990)] on the surface of water, known as second sound, was performed in the laboratory. This collective mode propagates as an increase in energy density in the background spectrum of random waves. Individual waves of the background can be thought of as scattering ``particles'' forming a ``gas,'' and the second sound mode is a density wave in this gas of waves. In this experiment, a long tank was driven at one end by fans to provide different background spectra of random wind-driven waves and a paddle to provide variable wavelength coherent pulses. Wave height was measured with a four-wire probe at different locations along the tank. Thus far, energy transfer has been observed from shorter to longer wavelengths. Additionally, a rarefaction in the background spectral density at all frequencies is observed to propagate at a single speed---the group velocity of the spectral peak of the wind-generated background spectrum. [Work supported by ONR and NPS.]