Abstract
Measurements are given on the velocity and attenuation of ultrasonic energy in liquid helium at a frequency of 15 megacycles per second as a function of temperature from 1.57\ifmmode^\circ\else\textdegree\fi{}K to 4.5\ifmmode^\circ\else\textdegree\fi{}K. The velocity at 15 Mc/sec. was found to agree with results obtained at 1.3 Mc/sec. by previous investigators, and hence there is no dispersion in this frequency range. The attenuation measurements exhibit three important features: (a) in the upper temperature range of He I the measurements agree very well with classical theory, (b) at the $\ensuremath{\lambda}$-point the attenuation coefficient rises abruptly, presumably to infinity, indicating complete absorption of the ultrasonic energy, (c) just below the $\ensuremath{\lambda}$-point the attenuation (first sound in He II) has its smallest value and with lowering temperature the attenuation increases.
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