Evaluation of sound velocity inside underwater acoustic materials using test panel acoustic measurements

Acoustic materials can be used as external coating to reduce radiated noise or target strength of immersed structures, or to improve performance of sonar systems. In order to optimise the design and the use of such coatings, it is necessary to have a good estimate of their intrinsic properties, in particular the complex sound velocity inside the materials. The type of material studied here consists in a viscoelastic slab containing a given proportion of micro-voids. Determination of the complex longitudinal velocity is not an easy task, and direct methods or experimental set-ups do not give sufficient results. The method presented here consists in determining the complex longitudinal velocity by solving an inverse problem, from acoustic measurements of the reflection and transmission coefficients of a test panel in a water tank Best results, including attenuation, are obtained by over-determining the problem, using all data available. Results are consistent to the known physical behaviour of such materials, regarding influence of frequence and hydrostatic pressure.