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 $f_o$. This frequency is chosen large compared with the characteristic frequencies of bending waves. As a consequence, a range of conservative scales without energy flux on average exists for frequencies $f<f_o$. 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, amplitude, and of the plate characteristics.

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• Turbulence