Mohammad Javad Sheikhmozafari, Zahra Hashemi, Ali Mohsenian,
Volume 14, Issue 4 (12-2024)
Abstract
Introduction: Micro-perforated panel (MPP) absorbers are emerging as next-generation absorbers due to their considerable advantages. However, their main drawback compared to other absorbers is their limited bandwidth. This study aims to investigate methods for enhancing the bandwidth of an MPP in the frequency range of 1 to 1500 Hz through simulation using the Finite Element Analysis (FEA) in COMSOL software.
Material and Methods: The modeling was conducted using FEA in COMSOL version 5.3a. To increase the bandwidth, techniques such as series-parallel configurations, symmetrical and asymmetrical air gap depths, and the incorporation of two porous absorbing materials in symmetric and asymmetric air gap layers were employed. In the initial phase, the best configuration was selected and retained for the subsequent stages.
Results: The optimal arrangement involved two upper MPPs having larger holes and a lower perforation percentage compared to the two lower MPPs. It was also found that increasing the depth difference between the air layers of the upper and lower MPPs led to a greater increase in bandwidth than when they were closer together. Furthermore, the use of fibrous porous materials in one of the layers resulted in a reduction of resonance peak while enhancing the bandwidth.
Conclusion: MPP absorbers exhibit diverse behaviors due to their Helmholtz structure and parametric design. If their constituent parameters are tailored to match the acoustic characteristics of the target sound, they achieve optimal efficiency. Additionally, employing numerical methods such as FEA serves as a suitable alternative to more costly laboratory methods.
Mohammad Javad Sheikhmozafari, Ebrahim Taban, Ali Mohsenian, Keith Attenborough, Mohammad Faridan,
Volume 15, Issue 4 (12-2025)
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 key design parameters like thickness and chopping level on sound absorption performance.
Material and Methods: Porous granular samples were fabricated from walnut shells at three chopping levels (minimally, moderately, finely) and four thicknesses (20, 40, 60, and 80 mm). The sound absorption coefficient 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 finely chopped samples, increasing thickness from 20 to 80 mm shifted the absorption peak from 2000 Hz to 630 Hz. At a constant 80 mm thickness, intensified chopping boosted the absorption coefficient at 630 Hz from 0.48 to 0.97. This improvement correlated directly with increased density, tortuosity, and airflow resistivity. 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 and sustainable sound-absorbing material. The chopping process optimizes the acoustic structure by activating the 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 for indoor noise control.
