Summary Context Lately underwater neutrino telescopes have become very important since it is a new and unique method to observe the Universe. The neutrinos are uncharged particles and interact weakly with matter. They can escape from sources which have produced them and arrive to the Earth unperturbed by magnetic fields and without interaction with other particles. This means that neutrinos can bring us astrophysical information that other messengers cannot and, then they can open a potential new window on the Universe. On the other hand, their low interaction cross section imposes to build very large detectors with dimensions of ~1 km3. Therefore, it is necessary the instrumentation of large volumes of water (or ice) with several optical sensors in order to detect characteristic signatures of high energy neutrino interactions. One way to detect neutrino interactions can be afforded through the detection of the Cherenkov light emitted by the muon generated after a neutrino interaction. This particle travels across the detector at a speed greater than the speed of light in water, so generating a faint blue luminescence called Cherenkov radiation, which can be detected through an array of optical sensors (photomultipliers). The arrival times of the light collected by photomultipliers can be used to reconstruct the muon track, and consequently that of the neutrino, which has produced it. The accuracy of the reconstruction of the muon track depends on the precision in the measurement of the light arrival time and on precise knowledge of the positions of the optical detectors. For this reason, in an underwater telescope an acoustic positioning system (APS) able to monitoring the position of the optical sensors with an accuracy of ~10 cm is necessary (about the length of the photomultipliers diameter). The studies in this thesis have been developed within the framework of acoustic position calibration in two European collaborations for the design, building and operation of an underwater neutrino telescope in the Mediterranean Sea: ANTARES (in operation phase) and KM3NeT (in preparatory phase for the construction). Summary In this thesis the work and the results for designing, developing, testing and characterizing a prototype of acoustic transceiver to be used in the APS of the future KM3NeT neutrino telescope are presented. Objectives The goals of this thesis can be summarized in the following aspects: - Design of the acoustic transceiver for the APS of KM3NeT. - Development of the acoustic transceiver prototype. - Characterization of the transceiver in the laboratory and in the sea. - Adapting the prototype for its integration in ANTARES and NEMO sites for the in situ test. Elements of the methodology to emphasize We would like to remark that the work of the thesis has been developed within the international consortium KM3NeT, funded with European and National funds. Due to the context and the nature of the activities done it has been necessary training in different fields: neutrino telescopes and astroparticles, but also in other field such as underwater acoustics or transducers. Moreover, different skills and abilities in different application fields have been developed: instrumentation, design and characterization of the acoustic system in water, data analysis, etc. Particularly, in order to design the transceiver, the acoustic transducer has been chosen according to the performance for the proposed application and afterwards specific electronics has been designed. Different configurations of the measurements have been set up to test and prove the prototype, the different measurements have been analyzed and the results and conclusions have been obtained. Finally, the tasks towards the integration in underwater neutrino telescopes have been done. Results Different studies for the development and testing of a prototype of transceiver for the proposed APS of KM3NeT have been done. The prototype consists of a transducer type Free Flooded Ring FFRSX30 with 20-40 kHz frequency range and an electronic board named SEB (Sound Emission Board), especially designed for it. The SEB is able to manage the emission and reception of different signals, as well as allowing the communication and configuration of the system. In a first step, the characterization of the hydrophones has been done in order to study the sensitivity and its dependence at high pressure (up to 440 bars). In a second step, the whole system (FFRSX30 plus SEB) has been tested and characterized in different conditions (tank, water pool and harbour of Gandia) in order to integrate it in the ANTARES, NEMO and KM3NeT neutrino detectors. The proposed transducers have good characteristics for their application for the KM3NeT APS and they can also be used as receivers, but in this case a good parameterisation of the sensitivity as a function of the frequency and angle for each transducer will be needed. The transducers have a very low intrinsic noise ~ -120 dB re V2/Hz (~ =Sea State 1) and they are quite stable at different pressures, that is with depth. For simplicity and due to limitations to the integration in both detectors, it was decided to test the transceiver only as emitter. The receiver functionality will be tested successively in other tests. The changes performed in the transceiver in order to integrate it in the different detectors show that the system is versatile and adaptable to the different conditions. The system, with low power consumption, is able to have a transmitting power above 170 dB re 1µPa@1m that combined with signal processing techniques allow to deal with the large distances involved in a neutrino telescope. In conclusion, the system has been integrated with success in ANTARES and NEMO neutrinos telescopes and, after being proved in situ, it will be implemented in the final configuration of the KM3NeT detector. Finally, we would like to remark that the acoustic system proposed is compatible with the different options for the receiver hydrophones proposed for KM3NeT and it is versatile, so in addition to the positioning functionality, it can be used for neutrino acoustic detection studies or for acoustic monitoring studies in deep-sea. Moreover, the transceiver (with slight modifications) may be used in other acoustic positioning systems or emitter-receiver systems, alone or combined with other marine systems, where the localization of the sensors is an issue. In that sense, the experience gained from this research can be of great use for other possible applications. Acknowledgments This work has been supported by the Ministerio de Ciencia e Innovación (Spanish Government), project references FPA2009-13983-C02-02, ACI2009-1067; and the European 6th and 7th Framework Programme, contract no. DS 011937 and grant no. 212525, respectively.