  We study response functions of integrable quantum impurity problems with an
external field at $T=0$ using non perturbative techniques derived from the
Bethe ansatz. We develop the first steps of the theory of excitations over the
new, field dependent ground state, leading to renormalized (or ``dressed'')
form-factors. We obtain exactly the low frequency behaviour of the dynamical
susceptibility $\chi''(\omega)$ in the double well problem of dissipative
quantum mechanics (or equivalently the anisotropic Kondo problem),and the low
frequency behaviour of the AC noise $S_t(\omega)$ for tunneling between edges
in fractional quantum Hall devices. We also obtain exactly the structure of
singularities in $\chi''(\omega)$ and $S_t(\omega)$. Our results differ
significantly from previous perturbative approaches.
