  We have studied the impact of the magnetic field on the electromagnon
excitations in TbMnO3 crystal. Applying magnetic field along the c axis, we
show that the electromagnons transform into pure antiferromagnetic modes,
losing their polar character. Entering in the paraelectric phase, we are able
to track the spectral weight transfer from the electromagnons to the magnon
excitations and we discuss the magnetic excitations underlying the
electromagnons. We also point out the phonons involved in the phase transition
process. This reveals that the Mn-O distance plays a key role in understanding
the ferroelectricity and the polar character of the electromagnons. Magnetic
field measurements along the b axis allow us to detect a new electromagnon
resonance in agreement with a Heisenberg model.
