  We study experimentally the interaction of nonlinear internal waves in a
stratified fluid confined in a trapezoidal tank. The set-up has been designed
to produce internal wave turbulence from monochromatic and polychromatic
forcing through three processes. The first is a linear transfer in wavelength
obtained by wave reflection on inclined slopes, leading to an internal wave
attractor which has a broad wavenumber spectrum. Second is the broad banded
time-frequency spectrum of the trapezoidal geometry, as shown by the impulse
response of the system. The third one is a nonlinear transfer in frequencies
and wavevectors via triadic interactions, which results at large forcing
amplitudes in a power law decay of the wavenumber power spectrum. This first
experimental spectrum of internal wave turbulence displays a $k^{-3}$ behavior.
