Hydrophone Calibration of the SMO acoustic antenna

Within the activities of KM3NeT Italia, a prototype neutrino detector was deployed the 23rd March 2013 and operated until August 2014. The prototype is a three dimensional “tower-like” mechanical structure, made of 8 stories (8 m horizontal length) interconnected by ropes, anchored on the sea bed (3500 m depth) and kept in tension by appropriate buoy, reaching a vertical extension of about 500 m above the seabed. The prototype tower hosts 4 optical modules and 2 hydrophones at each storey, two CTDs and one Doppler current sensor. All sensors are digitized underwater and data are sent continuously to shore via a vertical backbone and a 100 km-long electro optical cable, used also to provide power to the detector from the shore laboratory, in the harbour of Capo Passero (South East Sicily). The hydrophones form an acoustic antenna designed, built and installed within the SMO (Submarine Multidisciplinary Observatory) project. Innovative read-out electronics was developed to obtain underwater GPS time stamping of hydrophone data with accuracy better than 1µs. The SMO hydrophones fulfill three tasks: provide acoustic positioning to the detector, monitor the acoustic noise in the aim of searching for acoustic neutrino signals and detect biological signals of cetaceans. In this contribution, we present the calibrations of these acoustic sensors made through calibrated emitters.