  The paper presents a description of the sound wave absorption in glasses,
from the lowest temperatures up to the glass transition, in terms of three
compatible phenomenological models. Resonant tunneling, the rise of the
relaxational tunneling to the tunneling plateau and the crossover to classical
relaxation are universal features of glasses and are well described by the
tunneling model and its extension to include soft vibrations and low barrier
relaxations, the soft potential model. Its further extension to non-universal
features at higher temperatures is the very flexible Gilroy-Phillips model,
which allows to determine the barrier density of the energy landscape of the
specific glass from the frequency and temperature dependence of the sound wave
absorption in the classical relaxation domain. To apply it properly at elevated
temperatures, one needs its formulation in terms of the shear compliance. As
one approaches the glass transition, universality sets in again with an
exponential rise of the barrier density reflecting the frozen fast Kohlrausch
t^beta-tail (in time t, with beta close to 1/2) of the viscous flow at the
glass temperature. The validity of the scheme is checked for literature data of
several glasses and polymers with and without secondary relaxation peaks. The
frozen Kohlrausch tail of the mechanical relaxation shows no indication of the
strongly temperature-dependent barrier density observed in dielectric data of
molecular glasses with hydrogen bonds. Instead, the mechanical relaxation data
indicate an energy landscape describable with a frozen temperature-independent
barrier density for any glass.
