Determination through an inverse method of the acoustic impedance and the propagation constant for some natural fibers

Natural materials are becoming a valid option for sound absorption treatments. Among natural materials, natural fibers are receiving significant attention since they also show good thermal insulation properties, no harmful effects on health, and are available in large quantities as a waste product of other production cycles. This paper reports the acoustical characterization of the following fibers: kenaf, wood, hemp, coconut, cork, cane, cardboard, and sheep wool. The absorption coefficient and the static flow resistance for natural fiber samples of different thickness have been measured. Moving from the Delany-Bazley model, this study then compares the impedance tube results with the theoretically predicted ones. An inversion approach allows to determine many physical parameters for each fiber. In particular, using a least-square fit procedure based on the Nelder-Mead simplex method, this paper reports the coefficients that best predict both the acoustic impedance and the propagation constant. An error analysis of the different relationships is finally discussed