  Temperature (5--250 K) and magnetic field (0--70 kOe) variations of the
low-energy (1--10 meV) electrodynamics of spin excitations have been
investigated for a complete set of light-polarization configurations for a
ferroelectric magnet DyMnO$_3$ by using terahertz time-domain spectroscopy. We
identify the pronounced absorption continuum (1--8 meV) with a peak feature
around 2 meV, which is electric-dipole active only for the light $E$-vector
along the a-axis. This absorption band grows in intensity with lowering
temperature from the spin-collinear paraelectric phase above the ferroelectric
transition, but is independent of the orientation of spiral spin plane ($bc$ or
$ab$), as shown on the original $P_{\rm s}$ (ferroelectric polarization)
$\parallel c$ phase as well as the magnetic field induced $P_{\rm s}\parallel
a$ phase. The possible origin of this electric-dipole active band is argued in
terms of the large fluctuations of spins and spin-current.
