  Magnetic and magnetoelectric excitations in the multiferroic TbMnO_3 have
been investigated at terahertz frequencies. Using different experimental
geometries we can clearly separate the electro-active excitations
(electromagnons) from the magneto-active modes, i.e. antiferromagnetic
resonances (AFMR). Two AFMR resonances were found to coincide with
electromagnons. This indicates that both excitations belong to the same mode
and the electromagnons can be excited by magnetic ac-field as well. In external
magnetic fields and at low temperatures distinct fine structure of the
electromagnons appears. In spite of the 90^o rotation of the magnetic
structure, the electromagnons are observable for electric ac-fields parallel to
the a-axis only. Contrary to simple expectations, the response along the c-axis
remains purely magnetic in nature.
