  A new method of probing mechanical losses and comparing the corresponding
deposition processes of metallic and dielectric coatings in 1-100 MHz frequency
range and cryogenic temperatures is presented. The method is based on the use
of extremely high-quality quartz acoustic cavities whose internal losses are
orders of magnitude lower than any available coatings nowadays. The approach is
demonstrated for Chromium, Chromium/Gold and a multilayer tantala/silica
coatings. The ${\rm Ta}_2{\rm O}_5/{\rm Si}{\rm O}_2$ coating has been found to
exhibit a loss angle lower than $1.6\times10^{-5}$ near 30 {\rm MHz} at 4 {\rm
K}. The results are compared to the previous measurements.
