Objective As specific and demanding requirements are imposed by measuring the impedance of the anechoic coatings on the hull surface of an underwater vehicle using the traditional water-filled tube experiment, a method of sound absorption coefficient measurement based on a simplified impedance experiment is proposed, and the correction of non-planar wave reflection in the measurement is achieved.
Methods The velocity at different radii on the surface of the coating is deduced by simplifying the coating as the impedance matrix boundary and combining the fluid boundary conditions. The relationship between the pressure of the non-planar reflected wave and axial velocity is established via the Hankel transformation, and the formula of the sound absorption coefficient of the modified non-planar reflection wave is obtained. Numerical calculation and the impedance experiment are compared with theoretical calculation and the traditional acoustic tube experiment to validate the validity of the proposed method.
Results The numerical calculation shows that the results of solid coating are consistent with the theoretical results, but the results of coating with cylindrical cavities are obviously different from the theoretical results. The impedance experiment results have a large error compared with the acoustic tube results after planar wave processing, and are consistent with the acoustic tube results after correction.
Conclusions Cavities are among the causes of the reflection of the non-planar wave from the coating, and this needs to be corrected in impedance measurement. The sound absorption coefficient obtained by the modified impedance measurement method is consistent with that obtained by the acoustic tube experiment in the frequency range of interest, and the experiment is simplified, giving the proposed method a certain practical value.
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