  Turbulence modulation by inertial-range-size, neutrally-buoyant particles is
investigated experimentally in a von K\'arm\'an flow. Increasing the particle
volume fraction $\Phi_\mathrm{v}$, maintaining constant impellers Reynolds
number attenuates the fluid turbulence. The inertial-range energy transfer rate
decreases as $\propto\Phi_\mathrm{v}^{2/3}$, suggesting that only particles
located on a surface affect the flow. Small-scale turbulent properties, such as
structure functions or acceleration distribution, are unchanged. Finally,
measurements hint at the existence of a transition between two different
regimes occurring when the average distance between large particles is of the
order of the thickness of their boundary layers.
