  Fundamental mechanical loss mechanisms exist even in very pure materials, for
instance, due to the interactions of excited acoustic waves with thermal
phonons. A reduction of these losses in a certain frequency range is desired in
high precision instruments like gravitational wave detectors. Systematic
analyses of the mechanical losses in those low loss materials are essential for
this aim, performed in a highly sensitive experimental set-up. Our novel method
of mechanical spectroscopy, cryogenic resonant acoustic spectroscopy of bulk
materials (CRA spectroscopy), is well suited to systematically determine losses
at the resonant frequencies of the samples of less than 10^(-9) in the wide
temperature range from 5 to 300 K. A high precision set-up in a specially built
cryostat allows contactless excitation and readout of the oscillations of the
sample. The experimental set-up and measuring procedure are described.
Limitations to our experiment due to external loss mechanisms are analysed. The
influence of the suspension system as well as the sample preparation is
explained.
