The operational requirements for naval and research vessels have seen recently an increasing demand for quieter ships either to comply the ship operational requirements or to minimize the influence of shipping noise on marine life. The preferred way to estimate the future radiated noise of a ship is to perform scale measurements in a facility as water tunnel of depressurized tank and to use scaling laws. With such a way to proceed, the knowledge of acoustic propagation features remains one of the key point to ensure reliability and accuracy of the global method. Evaluating the radiated noise from a confined environment (as a test section comparable to propagation in duct) with radiated noise in free field (as in ocean) remains a challenge. In water tunnel the impedance conditions at wall is unknown but could be supposed to be far from hard-wall conditions, contrary to air tunnel case. Therefore, direct application of demodulation techniques is difficult, partially due to the presence of evanescent and decayed modes. The paper presents a new method for demodulation developed with the double objective to estimate the modal magnitudes and the impedance conditions at wall positions. This method is based notably on Newton-Raphson algorithm to solve the transcendental eigen equation of dispersion required to compute the global acoustic field. Simulated data have been created to test the method. It appears that the method is efficient and converges against exact values without bias (for both impedance and complex magnitudes estimate). However, processing other simulated data computed introducing noise on measurements in magnitude and/or in phase (as it could happen in experimental data set) reveals that the method is sensitive to noise. Bias could be observed and quantified that allows in return to discuss about its application on experimental data.
