  We measured the temperature dependent infrared reflectivity spectra of TbMnO3
with the electric field of light polarized along each of the three
crystallographic axes. We analyzed the effect, on the phonon spectra, of the
different phase transitions occurring in this material. We show that the
antiferromagnetic transition at TN renormalizes the phonon parameters along the
three directions. Our data indicate that the electromagnon, observed along the
a direction, has an important contribution to the building of the dielectric
constant. Only one phonon, observed along the c-axis, has anomalies at the
ferroelectric transition. This phonon is built mostly from Mn vibrations,
suggesting that Mn displacements are closely related to the formation of the
ferroelectric order.
