We have measured by a new optical method the hypersonic attenuation in liquid ${\mathrm{He}}^4$ at a frequency of about 650 MHz. In this high-frequency range absorption data are presented between 1.2 and 4.2°K, including the vicinity of the $\lambda$ point. The results are compared with the previous low-frequency measurements.

• Received 17 December 1968

DOI:https://doi.org/10.1103/PhysRevLett.22.170