Ultrasonic Velocity and Dispersion in Liquid Helium II from 0.15 to 1.8°K

From the measurement of changes in the delay experienced by an ultrasonic pulse traversing a fixed path, the temperature dependence of the velocity of sound in liquid helium under its saturated vapor pressure has been determined from 0.15 to 1.8°K at the frequencies 1.00, 3.91, and 11.9 Mc/sec. A phase-comparison technique made it possible to resolve velocity changes amounting to a few mm/sec. With increasing temperature, the velocity $u_1$ increases above its asymptotic limit $u_{10}$ at 0°K, passes through a maximum near 0.7°K, and decreases rapidly at higher temperatures. The velocity difference over any temperature interval that extends from below 0.2 to above 1.1°K is independent of frequency, but within the temperature range indicated $u_1$ increases with frequency and the position of its maximum shifts to higher temperatures. At (1.00, 3.91, 11.9) Mc/sec the maximum velocity lies (5, 7, 14) cm/sec above $u_{10}$ and occurs at (0.65, 0.70, 0.72)°K. The dispersion is greatest in the neighborhood of 0.9°K, where the attenuation coefficient goes through a maximum. Within experimental error, the theory of Khalatnikov and Chernikova accounts for the separation between the velocity-versus-temperature curves at the three frequencies. Combination of the velocity changes measured in these experiments with values of the absolute velocity obtained by other workers above 1°K yields $u_{10}={238.3}_0\pm {0.1}_3$ m/sec.