Abstract
Ultrasonic waves in liquid He II at temperatures below 0.6°K can be absorbed by thermal phonons in 3- or 4-phonon processes. For the case that the mean free path of thermal phonons is sufficiently long 3-phonon processes are generally excluded because energy and momentum cannot be conserved. The conservation requirements cannot be fullfilled because the group velocity of thermal phonons is lower than the ultrasonic velocity due to dispersion. — The purpose of this note is to point out that insmall samples of liquid the uncertainty in phonon energy prevents a violation of the conservation laws for 3-phonon processes. In small volumes of liquid, ultrasonic waves are therefore strongly absorbed in 3-phonon processes, while in larger volumes of liquid this contribution to the absorption is absent. — A similiar size dependent absorption is to be expected for longitudinal waves at low temperatures in ideal crystals.
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