Velocity and Attenuation of First Sound near theλPoint of Helium

The velocity and attenuation of first sound have been measured to within microdegrees of the $\lambda$ transition of liquid helium at a frequency of 22 kc/sec. Relative sound-velocity measurements of 1 part in ${10}^5$ could be obtained. The attenuation reaches a maximum about 1 or 2 μdeg below the transition while the velocity minimum occurs 6 μdeg below $T_{\lambda }$. Above the transition, the sound velocity satisfies the necessary condition for the validity of the Pippard-Buckingham-Fairbank relations out to at least 6 mdeg from the $\lambda$ point. Below $T_{\lambda }$, the velocity shows an added temperature dependence beyond that expected from the Pippard-Buckingham-Fairbank relations, which is proportional to ${|T_{\lambda }-T|}^{0,9}$. If this temperature dependence persists, the velocity of sound above and below the transition approaches a common value of 217.3 m/sec at the $\lambda$ point. Except in the immediate neighborhood of $T_{\lambda }$, the attenuation below the transition is proportional to ${|T_{\lambda }-T|}^{-1\mathrm{}}$. This result is in agreement with the prediction of the Landau-Khalatnikov theory. Except in the immediate neighborhood of $T_{\lambda }$, the attenuation above the transition is proportional to ${|T_{\lambda }-T|}^{\frac{-1\mathrm{}}{2}}$. This temperature dependence is not understood theoretically at present. Velocity dispersion results above and below the $\lambda$ point are also presented.