Propagation of transverse sound waves in glasses at very low temperatures

For both longitudinal and transverse polarizations the variation of the velocity of sound has been measured in borosilicate glass BK7. The experiment has been carried out in the temperature range between 0.28 and 4.2 K and at frequencies between 30 and 150 MHz. The coupling between low energy two-level systems and phonons is found to be similar for both acoustic polarizations. Using our results we also calculate the magnitude of the thermal conductivity without additional assumptions and find complete agreement with existing experimental data.