Sound Propagation in Liquid ⁴He

Measurements of the attenuation of sound in liquid ⁴He down to 0.1 K have been performed at 12, 30, 36, 60, 84, 90, 108, 132, 150, and 208 MHz. Measurements of the temperature dependence of the velocity of sound were made at 12, 36, 60, and 84 MHz. These data are compared with recent theoretical work, particularly that of Khalatnikov and Chernikova. The attenuation data agree well with theory in the vicinity of the peak in the attenuation near 1 K but do not agree elsewhere, the observed attenuation being greater than that predicted by theory. The temperature dependence of the velocity at low temperatures is found to be less than that predicted by theory, while the frequency dependence of the velocity (at finite temperature) is opposite to that predicted by theory.