CXXII. Ultrasonic attenuation in metals

Abstract The attenuation of ultrasonic waves by conduction electrons is analysed in terms of the free-electron model of a metal. At such low frequencies that the electronic free path is smaller than the wavelength, the result obtained agrees with that found by other authors using simpler concepts, but this agreement breaks down when the free path becomes comparable with or larger than the wavelength. At very high frequencies the attenuation of longitudinal waves increases in proportion to the frequency without limit, while that of transverse waves tends to a constant value. The drop in attenuation observed when a metal becomes superconducting is shown to present quite serious problems of interpretation, and tentative suggestions are made as to possible explanations.