The evaluation of intrinsic non-acoustic parameters of polyester fibrous materials by an optimization procedure

This paper discusses about the use of a parametric optimization method to assess nonacoustic parameters of porous materials. Fibrous materials obtained from recycled polyester have been analyzed. The parametric optimization method is based on the minimization of the difference between measured normal specific impedance at the surface of a fibrous layer backed by an impervious and rigid wall and the corresponding one calculated by available theoretical models. Impedance measurements have been carried out both in free-field and in an impedance tube. The material has been tested with different sample thicknesses. Numerical evaluations have been carried out by a single-parameter model (Delany and Bazley), a phenomenological one (Hamet) and a micro-structural model (Biot-Allard under both rigid and elastic assumption for the solid frame). It was noted that the porosity, the shape factor and the tortuosity resulted almost equal to unity for the tested material. The values of the resistivity assessed by the parametric optimization agree reasonably with the ones obtained by measurements and/or by fibrous materials resistivity models.