A brief review is given of the ultrasonic attenuation in metals arising from direct interaction of the elastic waves and conduction electrons, and the physical variables on which it depends. The drop in attenuation of longitudinal waves on entering the superconducting state is in good agreement with BCS theory, the various factors combining to make the relative attenuation, αs/αn, depend only on the energy gap; the measurements give evidence, however, for gap anisotropy or the presence of more than one gap. Recent measurements at Brown by Claiborne of shear wave attenuation in single-crystal Al, are in good agreement with a theory based on the Boltzmann and London equations. The steep drop at Tc is produced by shorting out of electromagnetic waves by supercurrents; the residual attenuation results from the effects of collision drag.
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