Phonon pulse propagation has been studied in fused silica at frequencies between 0.4 and 2.0 GHz for temperatures as low as 0.1 K. An increase in attenuation, observed as the incident acoustic intensity is decreased, is attributed to resonant scattering by two-level "tunneling states" intrinsic to glasses.
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Golding et al. (1973) studied this question.
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