  We perform an experimental study of granular impact, where intruders strike
2D beds of photoelastic disks from above. High-speed video captures the
intruder dynamics and the local granular force response, allowing investigation
of grain-scale mechanisms in this process. We observe rich acoustic behavior at
the leading edge of the intruder, strongly fluctuating in space and time, and
we show that this acoustic activity controls the intruder deceleration,
including large force fluctuations at short time scales. The average intruder
dynamics match previous studies using empirical force laws, suggesting a new
microscopic picture, where acoustic energy is carried away and dissipated.
