EXPERIMENTAL-DETERMINATION OF SOME NONLINEAR ELEMENTS OF 3RD SOUND DYNAMICS

The frequency shifts of third sound resonances during free decay are o bserved to have a quadratic dependence on the amplitude of the wave mo tion. An unambiguous measurement of amplitude allows for quantitative comparisons to predictions for the frequency shifts based on a variety of nonlinear influences acting within the superfluid He-4 film and re sonator. Dispersive terms play a minor role in this analysis. We concl ude that nonlinear terms dictated by a straight forward application of classical hydrodynamics are applicable to superfluid helium films.