Measurements of the absorption coefficient of ultrasonic waves is in all cases larger than can be explained by viscosity and heat conduction. The benchmark paper by Herzfeld and Rice [Phys. Rev.31, 691 (1928)] introduced a third factor, namely vibrational relaxation, which for many substances accounted for most of the excess absorption. The remaining excess absorption has led to the concept of a volume viscosity, which has been historically associated with structural and other relaxation mechanisms. We have observed that when accurate absorption data is available for such diverse substances as gases, liquids, and polymers in which there is an absence of vibrational and structural (e.g., water) relaxation mechanisms, the ratio of volume to shear viscosity is exactly 2/3. We present numerous data to support this. The consistency of this ratio, and its temperature independence, suggests that the viscous counterpart to the Lamé constant λ″ is negligibly small. That is,ηv = λ″ + 2ηs/3, ηv/ηs≈ 2/3, therefore λ″ ≈ 0. Furthermore, if λ″ ≈ 0, α/αclass= 1.5 is to be expected, and therefore, in the many substances where this is observed there is no need to postulate a structural relaxation mechanism beyond that which leads to shear viscosity. Finally, when ηv/ηs>2/3, as in the case of water, λ″ is greater than zero. The meaining of λ″ as a measure of structural relaxation is explored.