Effect of pressure on the absorption of first sound in 3He–4He mixtures

We have measured the absorption coefficient for first sound at a frequency of 29.8 MHz in a 4% 3He–4He mixture for pressures in the range O ≤P ≤ 20 atm in the temperature interval 0.35–1.5 °K. It is shown that at elevated pressures the sound absorption in the temperature interval 0.3–0.7 °K basically stems from the viscosity of the 3He quasiparticles and from scattering by thermal phonons. In contrast to the case of pure 4He, three-phonon processes make practically no contribution to the sound absorption in the mixtures studied.

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