Multibeam Echo Sounders (MBES) are designed primarily for high resolution bathymetry. Therefore, absolute sound pressure levels measurements have not been a high priority so far. There is, however, a growing demand for the characterization of, for example, species, sediments, man made objects, and oil pollution. Characterization is often based on physics based models, and consequently acoustic data should be obtained from calibrated system. In this paper steps taken towards a fully calibrated MBES are presented. The starting point is the GulfEx11 experiment conducted by the Applied Physics Laboratory of the University of Washingon offshore Panama City, Florida, USA, April 2011. Here, an SB-7125 has been applied at frequencies from 150 kHz up to 450 kHz in 50 kHz increments. In the laboratory receive array sensitivity has been measured over the specified range of frequencies with sound pressure levels ranging from 120 to 170 dB re. 1 µPa. Transmit voltage reponse at the various frequencies has also has bee...