Abstract
Measurements of the attenuation of fourth sound in a HeII filled porous solid always show anomalously high values at low temperatures [M. A. Kriss, UCLA thesis (1969); H. J. Lauter and H. Wiechert, J. Low Temp. Phys. 36, 139 (1979); S. R. Baker, UCLA thesis (1985)]. It is shown that this anomaly can be attributed to a reduction in the apparent normal fluid shear viscosity due to the finite mean free paths of phonons and rotons. Results of a model calculation of the mean free path correction to the attenuation of fourth sound in a HeII filled porous solid, which separately treats the average drift of phonons and rotons as the flow of an ideal gas through a porous solid [A. E. Scheidegger, The Physics of Flow Through Porous Media, Third Edition, 1974, p. 173], compare very well with measurements made in 1‐μ packed powder down to 1.1 K at SVP, where the observed anomaly amounts to more than an order of magnitude. [Work supported by ONR.]
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