  The AMADEUS (ANTARES Modules for the Acoustic Detection Under the Sea) system
which is described in this article aims at the investigation of techniques for
acoustic detection of neutrinos in the deep sea. It is integrated into the
ANTARES neutrino telescope in the Mediterranean Sea. Its acoustic sensors,
installed at water depths between 2050 and 2300 m, employ piezo-electric
elements for the broad-band recording of signals with frequencies ranging up to
125 kHz. The typical sensitivity of the sensors is around -145 dB re 1V/muPa
(including preamplifier). Completed in May 2008, AMADEUS consists of six
"acoustic clusters", each comprising six acoustic sensors that are arranged at
distances of roughly 1 m from each other. Two vertical mechanical structures
(so-called lines) of the ANTARES detector host three acoustic clusters each.
Spacings between the clusters range from 14.5 to 340 m. Each cluster contains
custom-designed electronics boards to amplify and digitise the acoustic signals
from the sensors. An on-shore computer cluster is used to process and filter
the data stream and store the selected events. The daily volume of recorded
data is about 10 GB. The system is operating continuously and automatically,
requiring only little human intervention. AMADEUS allows for extensive studies
of both transient signals and ambient noise in the deep sea, as well as signal
correlations on several length scales and localisation of acoustic point
sources. Thus the system is excellently suited to assess the background
conditions for the measurement of the bipolar pulses expected to originate from
neutrino interactions.
