Notes on the absorption of sound in high magnetic fields

We study the absorption of sound by a free electron-impurity system in a quantizing magnetic field and derive a formula for the absorbed power on the basis of linear response theory. In the approximation of constant relaxation time and in the limit of high temperatures (1/τ?kT) this formula is essentially equivalent to the formulas used by Gurevichet al. and by Svirskii. Without these restricting assumptions we obtain general conditions for the appearance of quantum oscillations. We discuss the influence of spin flip scattering on the shape of the 0⁺-absorption line. Furthermore, we show that the usual kinetic approach leads to the incorrect results given by Quinn.