  Mechanisms of acoustic energy dissipation in heterogeneous solids attract
much attention in view of their importance for material characterization,
nondestructive testing, and geophysics. Due to the progress in measurement
techniques in recent years it has been revealed that rocks can demonstrate
extremely high strain sensitivity of seismo-acoustic loss. In particular, it
has been found that strains of order $10^{-8}$ produced by lunar and solar
tides are capable to cause variations in the seismoacoustic decrement on the
order of several percents. Some laboratory data (although obtained for higher
frequencies) also indicate the presence of very high dissipative nonlinearity.
Conventionally discussed dissipation mechanisms (thermoelastic loss in dry
solids, Biot and squirt-type loss in fluid-saturated ones) do not suffice to
interpret such data. Here, the dissipation at individual cracks is revised
taking into account the influence of wavy asperities of their surfaces quite
typical of real cracks, which can drastically change the values of the
relaxation frequencies and can result in giant strain sensitivity of the
dissipation without the necessity to assume the presence of unrealistically
thin (and, therefore, unrealistically soft) cracks. In particular, these
mechanisms suggest interpretation for observations of pronounced amplitude
modulation of seismo-acoustic waves by tidal strains.
