The attenuation of 10-90 MHz longitudinal sound waves has been measured from 1.2K upwards in the superconducting mixed state of niobium near Bc2. The attenuation was determined as a function of the directly measured average induction, B, within single crystal specimens which had resistance ratios ranging from 83-3380. The specimens tended to the 'clean' limit (electron mean free path, l>> xi 0, the superconducting coherence length) in which there is a strong purity dependence of the relative attenuation. For the authors' purest crystals close to Bc2, the results are in reasonable agreement with the purity and field dependence predicted by microscopic theories, and agree over a much wider field range with a phenomenological modification of these theories. Measurements of attenuation and other transport properties in clean type II superconductors by other authors are discussed, and it is shown that many of the evident discrepancies can be attributed to neglect of the effects of magnetic irreversibility and crystalline anisotropy.