Damping of fourth sound in4He due to normal fluid flow

Measurements of the attenuation of fourth sound were made in superfluid⁴He contained in several porous media with characteristic sizes sufficiently large to permit some normal fluid flow. The data are for pressures between vapor pressure and 25 bar and for temperatures between 1.4 K and the-line. The attenuation was dominated by viscous dissipation associated with the normal fluid flow. For a given medium, it varied by nearly two orders of magnitude over the pressure and temperature range of the experiment. Analysis of the data along isobars and isotherms in terms of an idealized model for the porous medium yielded a unique temperature- and pressure-independent characteristic size for each medium.