Frequency Dependence of the Superfluid Transition inHe4Films on Porous Vycor Glass

We have used an ultrasonic technique to measure the superfluid density and the dissipation in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on porous Vycor glass. The superfluid transition is quite sharp at the lowest frequency (5 MHz) but broadens and shifts to higher temperatures as the frequency is increased (up to 196 MHz). In contrast to earlier torsional-oscillator measurements on this system, we observe a dissipation peak at the transition. We discuss the information which these results provide about the role of two-dimensional vortices in the superfluid transition in films adsorbed on porous substrates.