  We study experimentally the dynamics of long waves among turbulent bending
waves in a thin elastic plate set into vibration by a monochromatic forcing at
a frequency $f_0$. This frequency is chosen large compared with the
characteristic frequencies of bending waves. As a consequence, a range of
conservative scales, without energy flux in average, exists for frequencies
$f<f_0$. Within this range, we report a flat power density spectrum for the
orthogonal velocity, corresponding to energy equipartition between modes. Thus,
the average energy per mode $\beta^{-1}$ -- analogous to a temperature -- fully
characterizes the large-scale turbulent wave field. We present an expression
for $\beta$ as a function of the forcing frequency and amplitude, and of the
plate characteristics.
