On the comparison of different techniques for measuring complex acoustical properties of fibrous and porous materials

The knowledge of the complex properties of fibrous and porous materials has become of paramount importance in the design of quieter environments, especially for enclosed spaces. Several techniques were developed for measuring the transmission coefficient, the characteristic impedance and the propagation constant of these materials. The present work focuses on the transfer matrix technique, that is, the sound field is firstly decomposed into an incident, a reflected and a transmitted wave. Then the complex pressure and particle velocity at each side of the sample material are calculated. From these quantities the transfer matrix is constructed and finally the complex acoustical properties are obtained. In this work the experimental results obtained for porous and fibrous sample materials are compared and discussed together with the two cavity technique in an impedance tube and the predictions obtained by means of the Delany-Bazley model