  The South Pole Acoustic Test Setup (SPATS) is located in the upper part of
the optical neutrino observatory IceCube, currently under construction. SPATS
consists of four strings at depths between 80 m and 500 m below the surface of
the ice with seven stages per string. Each stage is equipped with an acoustic
sensor and a transmitter. Three strings (string A-C) were deployed in the
austral summer 2006/07. SPATS was extended by a fourth string (string D) with
second generation sensors and transmitters in 2007/08. One second generation
sensor type HADES (Hydrophone for Acoustic Detection at South Pole) consists of
a ring-shaped piezo-electric element coated with polyurethane. The development
of the sensor, optimization of acoustic transmission by acoustic impedance
matching and first in-situ results will be discussed.
