Experiments on the attenuation of third sound in saturated superfluid helium films

Upper limits of the attenuation of third sound in saturated superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films have been measured in three separate experiments. Our observations at frequencies from 0.1 to 200 Hz indicate that the attenuation in these thick films is substantially lower than would be inferred from the only previous experiment done on saturated films. We observe the third-sound velocity to have the temperature dependence predicted by Bergman.