How universal are the low temperature acoustic properties of glasses?

We evaluated already published data on ultrasound absorption and velocity changes below 1 K for a set of 18 different glasses. By uniformly analysing these data according to the tunnelling model we have calculated the standard spectral density of two-level-systems (TLS) and their coupling energy ₁ and ₁ to longitudinal and transversal phonons. Contrary to literature, we have not observed any correlation between and the glass transition temperatureT _g for our set of 18 glasses. As far asT _g is concerned we only found a relation _min ^0.1K withn2.5 and where _min is the shortest TLS-phonon relaxation time at 0.1 K. Since the values for and _1,t scatter within half an order of magnitude, we conclude that the TLS parameters of a certain glass are only weakly dependent on bulk properties. We find that the ratio _t ² /C ₄₄ (whereC ₄₄ is the shear elastic constant) is a more sensitive parameter for tests on the variation of the TLS spectral density of glasses.