  The detection of acoustic signals from ultra-high energy neutrino
interactions is a promising method to measure the tiny flux of cosmogenic
neutrinos expected on Earth. The energy threshold for this process depends
strongly on the absolute noise level in the target material. The South Pole
Acoustic Test Setup (SPATS), deployed in the upper part of four boreholes of
the IceCube Neutrino Observatory, has monitored the noise in Antarctic ice at
the geographic South Pole for more than two years down to 500 m depth. The
noise is very stable and Gaussian distributed. Lacking an in-situ calibration
up to now, laboratory measurements have been used to estimate the absolute
noise level in the 10 to 50 kHz frequency range to be smaller than 20 mPa.
Using a threshold trigger, sensors of the South Pole Acoustic Test Setup
registered acoustic pulse-like events in the IceCube detector volume and its
vicinity. Acoustic signals from refreezing IceCube holes and from anthropogenic
sources have been used to localize acoustic events. Monte Carlo simulations of
sound propagating from the established sources to the SPATS sensors have
allowed to check corresponding model expectations. An upper limit on the
neutrino flux at energies $E_\nu > 10^{11}$ GeV is derived from acoustic data
taken over eight months.
