  Tunneling-two-level-system (TTLS) model has successfully explained several
low-temperature glass universal properties which do not exist in their
crystalline counterparts. The coupling constants between longitudinal and
transverse phonon strain fields and two-level-systems are denoted as $\gamma_l$
and $\gamma_t$. The ratio $\gamma_l/\gamma_t$ was observed to lie between
$1.44$ and $1.84$ for 18 different kinds of glasses. Such universal property
cannot be explained within TTLS model. In this paper by developing a
microscopic generic coupled block model, we show that the ratio
$\gamma_l/\gamma_t$ is proportinal to the ratio of sound velocity $c_l/c_t$. We
prove that the universality of $\gamma_l/\gamma_t$ essentially comes from the
mutual interaction between different glass blocks, independent of the
microscopic structure and chemical compound of the amorphous materials. In the
appendix we also give a detailed correction on the coefficient of non-elastic
stress-stress interaction $\Lambda_{ijkl}^{(ss')}$ which was obtained by
Joffrin and Levelut\cite{Joffrin1976}.
