Application of 2E-2U method for free-field underwater calibrations of hydrophones and projectors in a reverberant laboratory test tank

In this paper, application of a Two-Equations Two-Unknowns (2E-2U) method is described for calibration of hydrophones and projectors below 1 kHz in a laboratory test tank. At low frequencies, amplitude and phase measurements for the calibration of the hydrophones and projectors in the test tank are difficult to perform since echo-free time of the laboratory test tank is not large enough due to transducer initial transients and tank wall boundary reflections. To overcome these difficulties, the 2E-2U method is applied to received (windowed) signals obtained during calibration measurements. Thus, calibration measurements become possible at a frequency down to 250 Hz. These measurements in the test tank are performed for a hydrophone and a developed flextensional projector. First, the received sensitivities for the hydrophone are calculated and validated by comparisons with pressure calibration in a closed chamber. Good agreements are obtained between the two measurement platforms, with a maximum difference of 0.5 dB and uncertainty of 1.3 dB. Then, the transmitting voltage response (TVR) of the flextensional projector is calculated and compared with the calibration data obtained from the method defined in the relevant standards. Good agreements are obtained between the two TVR data with a maximum difference of 1.1 dB and uncertainty of 1.7 dB.