A method for calibrating an acoustic calibration tank providing the tanks with reference hydrophones positioned along the tank. The tank is filled with a fluid at a known temperature and pressure. A calibrated reverse hydrophone is positioned in the tank, opposite one of the reference hydrophones. A known acoustic wave is created in the tank using the calibrated reverse hydrophone and the opposite reference hydrophone output is measured. Free field voltage sensitivity is computed for the reference hydrophone. This gives the calibration factor for the reference hydrophone. The calibrated reverse hydrophone can be repositioned. An iterative process can be utilized for the other reference hydrophones, and other known pressures and temperatures.