  Nonequilibrium processes are probed by ultrasound waves in the spin-ice
material Dy2Ti2O7 at low temperatures. The sound velocity and the sound
attenuation exhibit a number of anomalies versus applied magnetic field for
temperatures below the "freezing" temperature of ~500 mK. These robust
anomalies can be seen for longitudinal and transverse acoustic modes for
different field directions. The anomalies show a broad hysteresis. Most notable
are peaks in the sound velocity, which exhibit two distinct regimes: an
intrinsic (extrinsic) one in which the data collapse for different sweep rates
when plotted as function of field strength (time). We discuss our observations
in context of the emergent quasiparticles which govern the low-temperature
dynamics of the spin ice.
