  To describe the interaction of the two level systems (TLSs) of an amorphous
solid with arbitrary strain fields, we introduce a generalization of the
standard interaction Hamiltonian. In this new model, the interaction strength
depends on the orientation of the TLS with respect to the strain field through
a $6\times 6$ symmetric tensor of deformation potential parameters, $[R]$.
Taking into account the isotropy of the amorphous solid, we deduce that $[R]$
has only two independent parameters. We show how these two parameters can be
calculated from experimental data and we prove that for any amorphous bulk
material the average coupling of TLSs with longitudinal phonons is always
stronger than the average coupling with transversal phonons (in standard
notations, $\gamma_l>\gamma_t$).
