###### Abstract  
Polyurethane foam (PUF) is an exceptionally adaptable product that has a variety of applications--it can be found almost everywhere. Due to such high utilization, the amount of polyurethane foam waste generated each year is growing over time. Rebonding polyurethane foam waste is a suitable way to progress towards a circular economy. In this paper, the prospect of using rebonded polyurethane foam (RPUF) in noise control applications is examined. An experimental study was carried out on RPUFs with various thicknesses and densities. The sound absorption coefficients at normal incidence and air resistivity were measured. The five-parameter Johnson-Champoux-Allard (JCA) model was adopted for the simulation of the porous layer. The remaining unknown parameters of the JCA model were estimated by inverse acoustic characterization based on fitting the transfer matrix method (TMM) model of an unbounded porous layer with rigid backing to the experimentally obtained sound absorption coefficients. Furthermore, sound absorption coefficients were calculated for a wide range of sample thicknesses, as well as for different air gap thicknesses between the wall and the porous layer. For some of the considered RPUFs, a sound absorption coefficient above 0.8 was achieved over a wide frequency range. rebonded polyurethane foam; sound absorption coefficient; airflow resistivity; impedance tube; transfer matrix method; finite element method +
Footnote †: journal: Medical Design  
1 Faculty of Civil Engineering, Ss Cyril and Methodius University in Skopje, 1000 Skopje, North Macedonia 1  
[PERSON]  
2M-Akustik, Cmiche No. 32, 1000 Skopje, North Macedonia 2  
[PERSON]  
2M-Akustik, Cmiche No. Since various polyurethane foams, both flexible and rigid, have been proven to have a high vibroacoustic potential [8; 9; 10; 11; 12; 13; 14], RPUFs can serve in sound adsorption applications. Several studies have investigated their noise abutment application, but further research is needed. [PERSON] et al. [15; 16] performed experimental characterization and empirical modelling on recycled polyurethane foams with different densities. [PERSON] et al. [17] conducted an experimental study on the sound absorption of recycled polyurethane foams with different thicknesses and also investigated the effect of included air gap. [PERSON] et al. [18] experimentally investigated the sound absorption coefficients of recycled polyurethane foams with and without perforation, as well as the effect of attached fabric to the sample. [PERSON] et al. [19] carried out an experimental investigation on RPUFs under a diffuse sound field in a reverberation chamber. [PERSON] et al. [20] conducted impedance tube absorption testing on floor coverings in combination with different materials as the underpad, among which RPUFs gave the best performance. [PERSON] and [PERSON] [10; 21] investigated the sound insulation impact of RPUFs through field measurements and static and dynamic tests. [PERSON] et al. [22] also studied the vibroacoustic parameters of RPUFs for their application in flooring systems. This paper aims to expand the knowledge of sound absorption by RPUFs through experimental and numerical investigation. RPUFs with various densities and thicknesses were produced by a local manufacturer for the purpose of the experimental study. Cylindrical specimens were cut from the fabricated RPUFs and subjected to impedance tube and airflow tests. The nature of the manufacturing process gives a somewhat uneven density distribution, and a variation of \(\pm 10\%\) can occur between samples. For the simulation of the porous layer, the widely used Johnson-Champoux-Allard (JCA) model was adopted. For the JCA model, five non-acoustic parameters need to be determined, namely: air flow resistivity (\(\sigma\)), porosity (\(\phi\)), tortuosity (\(\alpha_{\infty}\)), thermal characteristic length (\(\Lambda^{\prime}\)), and viscous characteristic length (\(\Lambda\)).