  The parametric subharmonic instability in stratified fluids depends on the
frequency and the amplitude of the primary plane wave. In this paper, we
present experimental and numerical results emphasizing that the finite width of
the beam also plays an important role on this triadic instability. A new
theoretical approach based on a simple energy balance is developed and compared
to numerical and experimental results. Because of the finite width of the
primary wave beam, the secondary pair of waves can leave the interaction zone
which affects the transfer of energy. Experimental and numerical results are in
good agreement with the prediction of this theory, which brings new insights on
energy transfers in the ocean where internal waves with finite-width beams are
dominant.
