Measurement of the velocity and attenuation of fourth sound in helium II

The velocity and the attenuation of fourth sound have been measured in liquid helium at temperatures between 0.8 K and the point, along the vapor pressure curve. The measurements were made using the resonance technique and the helium was contained between small pores in packed powder specimens. From the velocity, it could be determined that the sound propagates under adiabatic conditions. According to theory, the attenuation of fourth sound consists of two contributions: surface losses due to heat exchange with the resonator body and volume losses due to dissipative processes associated with the viscosity coefficients and ₃. The results of our attenuation measurements are in agreement with this theory; however, it appears that the attenuation is affected at low temperatures, if the mean free path of an elementary excitation becomes comparable to or larger than the mean pore diameter of the packed powder samples.This work was supported in part by the Deutsche Forschungsgemeinschaft. It is part of the dissertation (D77) of H.-J. Lauter.