Ultrasonic Propagation in Magnetically Cooled Helium

Measurements have been made of the velocity and attenuation of ordinary sound in liquid helium at a frequency of 12.1 Mc/sec, over the temperature range from 1^∘{}K down to 0.1^∘{}K. The velocity is essentially independent of temperature and has the value 240±{}5 m/sec. The attenuation passes through two closely spaced maxima near 0.9^∘{}K and then falls smoothly to zero as the absolute zero is approached. These results agree qualitatively with the theoretical predictions of Khalatnikov.