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The phonon-impurity system of dilute solutions of 3He in 4He is studied experimentally and theoretically using an acoustic technique. The sound velocity and absorption measurements make it possible to identify the theoretically predicted new mechanism of phonon relaxation with anomalous dispersion in the presence of impurities. A kinetic problem for arbitrary frequencies is solved, which also enables us to obtain all the dissipation coefficients of the solutions and to explain the experimental data on second-sound absorption, thermal conductivity, and heat pulse propagation. The relation is considered between relaxation processes in a phonon-impurity system and the phonon spectrum dispersion in 3He-4He solutions.

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The kinetic theory of first sound in a quantum liquid is presented. The results for velocity and sound attenuation are obtained and found to be valid over a wide range of frequencies, including hydrodynamic, kinetic, and intermediate regimes, and throughout the whole temperature range in which a quasiparticle description is applicable. Comparison of theory and experiments is carried out.