  The infrared (IR) active phonons in multiferroic TbMn$_2$O$_5$ are studied as
a function of temperature. Most of the symmetry allowed IR modes polarized
along the \textit{a} and \textit{b} crystal axes are observed and the behavior
of several \textit{b} polarized phonons is correlated with the magnetic and
ferroelectric transitions. A high frequency \textit{b} polarized phonon only
Raman allowed in the paraelectric phase becomes IR active at the ferroelectric
transition. The IR strength of this mode is proportional to the square of the
ferroelectric order parameter and gives a sensitive measure of the symmetry
lowering lattice distortions in the ferroelectric phase.
