  Based on the isospin-dependent Boltzmann-Uehling-Uhlenbeck(IBUU04) transport
model, we explored effects of in-medium $NN$ elastic scattering cross section
and nuclear symmetry energy on the sub-threshold pion production in
$^{132}Sn+^{124}Sn$ reaction. We find that with decrease of incident beam
energy, effects of in-medium $NN$ elastic scattering cross section on the
$\pi^{-}/\pi^{+}$ ratio are larger than that of the symmetry energy although
the latter may be also larger. While keeping the effect of symmetry energy, the
double ratio of $\pi^{-}/\pi^{+}$ from neutron-rich and neutron-poor reaction
systems (with the same mass number of system) $^{132}Sn+^{124}Sn$ and
$^{128}Pm+^{128}Pm$ almost fully cancels out the effects of in-medium $NN$
elastic scattering cross section.
