Development and Fabrication of a Natural and Eco-Friendly Sound Absorber from Walnut Shell Waste: A Mathematical Modeling and Acoustic Performance Evaluation Approach

Mohammad Javad  SheikhMozafari; Ebrahim  Taban; Ali  Mohsenian; Keith  Attenborough; Mohammad  Faridan

doi:10.18502/jhsw.v15i4.21459

Mohammad Javad SheikhMozafari Department of Occupational Health Engineering, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran
Ebrahim Taban Department of Occupational Health Engineering, School of Health, Mashhad University of Medical Sciences, Mashhad, Iran
Ali Mohsenian Department of Occupational Health and Safety Engineering, School of health, Shiraz University of Medical Sciences, Shiraz, Iran
Keith Attenborough School of Engineering and Innovation, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
Mohammad Faridan Environmental Health Research Center, Department of Occupational Health and Safety at Work Engineering, Lorestan University of Medical Sciences, Khorramabad, Iran

DOI: https://doi.org/10.18502/jhsw.v15i4.21459

Keywords: Walnut shell waste, Impedance Tube, Slanted slit, Non-uniform pore size distribution

Abstract

Introduction: Environmental and health concerns regarding synthetic sound absorbers necessitate natural,sustainable alternatives. Agricultural waste like walnut shells is promising due to its inherent porosity.This study evaluates the acoustic properties of walnut shell composites, investigating the influence of keydesign parameters like thickness and chopping level on sound absorption performance.

Material and Methods: Porous granular samples were fabricated from walnut shells at three choppinglevels (minimally, moderately, finely) and four thicknesses (20, 40, 60, and 80 mm). The sound absorptioncoefficient was measured via the impedance tube method. Field Emission Scanning Electron Microscopy(FESEM) analyzed the material’s morphology, and results were validated with Slanted Slit (SS) and Non-uniform Pore Size Distribution (NUPSD) mathematical models.

Results: Both increased thickness and chopping level significantly enhanced sound absorption. For finelychopped samples, increasing thickness from 20 to 80 mm shifted the absorption peak from 2000 Hz to630 Hz. At a constant 80 mm thickness, intensified chopping boosted the absorption coefficient at 630Hz from 0.48 to 0.97. This improvement correlated directly with increased density, tortuosity, and airflowresistivity. Model predictions showed the best agreement for the most finely chopped samples.

Conclusion: Walnut shell waste, especially after intensive mechanical processing, is a highly effective andsustainable sound-absorbing material. The chopping process optimizes the acoustic structure by activatingthe material’s inherent micro-porosity, yielding excellent performance in the speech frequency range(500-2000 Hz). This material shows significant potential as a green alternative to synthetic absorbers forindoor noise control.