Measurements of sound propagation in HeII with partially locked normal component Free

We have measured the speed and attenuation of sound in HeII between 1.20 and 2.15 K in six porous solids with nominal grain diameters from 1–500μ. If δ_visc is the viscous penetration depth and a is a characteristic pore radius, the range of the experiment spans (δ_visc/a) = 10⁻² to 10². We have compared these measurements to the predictions of the two‐fluid theory of HeII, modified to account for the tortuosity and (normal fluid) flow resistance of the porous solid [S. Baker, D. Rudnick, and I. Rudnick, J. Acoust. Soc. Am. Suppl. 1 74 S59 (1983)]. We find excellent agreement between measured and predicted speed and attenuation provided the attenuation is dominated by the flow resistance. The three adjustable parameters determined in this experiment are the Biot parameters of the porous solid (tortuosity a, permeability k, and high‐frequency correction factor δ) [M. Biot, J. Acoust. Soc. Am. 28, 168–191 (1956)], and are the same for any fluid in the pores. [Work supported by ONR.]