Pistachio shell waste as a sustainable sound absorber: an experimental and empirical investigation

This study investigates the acoustic absorption properties of pistachio shell samples as eco-friendly alternatives to synthetic sound-absorbing materials. The samples were measured for thickness (3 cm and 5 cm) and forms (whole, semi crushed, fully crushed) using impedance tube measurements in the laboratory, followed by analysis using the Dunn–Davern (DD) model and optimal inverse method to predict the sound absorption performance. Additionally, the morphological tendencies of the samples were examined using the FESEM technique. The sound absorption mechanism in pistachio shell samples, akin to other porous materials, encompasses frictional dissipation between air molecules and material walls, along with viscosity-related losses due to air movement entrance to smaller voids. These processes facilitate the conversion of acoustic energy into heat. The findings reveal that increasing the thickness and density of the pistachio shell samples leads to higher absorption coefficients and a shift of the absorption peak toward lower frequencies. Furthermore, an increase in airflow resistivity resulting from enhanced tortuosity contributes to higher absorption coefficients and a similar peak shift toward lower frequencies. The degree of sample crushing positively influences the absorption coefficient by increasing the collision surface and component entanglement. However, it should be noted that the one-dimensional nature of the DD model restricts its accuracy, particularly for materials with non-uniform porosity. Conversely, the inverse DD model employing the NMS method proved highly effective in predicting the absorption coefficient of the samples. Overall, the results suggest that pistachio shell samples offer a viable alternative to conventional synthetic absorbers, demonstrating suitable absorption coefficients in middle and high frequencies, as well as possessing rigid and stable surface properties.