  In this paper we describe an acoustic transceiver developed for the KM3NeT
positioning system. The acoustic transceiver is composed of a commercial free
flooded transducer, which works mainly in the 20-40 kHz frequency range and
withstands high pressures (up to 500 bars). A sound emission board was
developed that is adapted to the characteristics of the transducer and meets
all requirements: low power consumption, high intensity of emission, low
intrinsic noise, arbitrary signals for emission and the capacity of acquiring
the receiving signals with very good timing precision. The results of the
different tests made with the transceiver in the laboratory and shallow sea
water are described, as well as, the activities for its integration in the
Instrumentation Line of the ANTARES neutrino telescope and in a NEMO tower for
the in situ tests.
