  Absorption of elastic waves in complex media often depends on frequency in a
linear way for both longitudinal and shear waves. This universal property
occurs in media such as rocks, unconsolidated sediments, and human tissue.
Absorption is due to relaxation processes and we argue that these processes are
thermally activated. Unusual for ultrasonics and seismics, we can therefore
express absorption as an integral over an activation energy landscape weighted
by an energy distribution. The universal power-law property corresponds to the
special case of a flat activation energy landscape. The derived distribution
agrees with results in soft glassy and glassy media.
