Ultrasonic measurements in liquid helium

Measurements have been made of the velocity and absorption of ordinary sound in liquid helium at frequencies of 2, 6 and 12 Mc/s, over the temperature range from the normal boiling-point down to 0$\cdot $85 degrees K. The accuracy of the velocity measurements has been increased above that reported in an earlier paper, and the temperature and frequency ranges of those measurements have been extended. Particular attention has been paid to the attenuation in the neighbourhood of the $\lambda $-point and in the very low temperature region. The attenuation is found to be very high near the $\lambda $-point and at low temperatures, and is proportional to $\omega ^{2}$ between 1$\cdot $2 degrees K and the $\lambda $-point. At lower temperatures a change in the frequency dependence is observed, indicating the presence of a relaxation effect. The significance of these results is discussed in relation to a recent theory of Khalatnikov.