Showing 2 results for Impedance Tube
Ali Jafari, Mohammad Reza Monazzam, Ali Khavanin, Maede Lashgari, Seyed Ali Ghoreyshi,
Volume 13, Issue 1 (3-2023)
Abstract
Introduction: Wood-Wool Cement Panels (WWCPs) are environmentally friendly sound absorbers also used as heat, energy, and moisture insulators. WWCPs have suitable mechanical properties due to using Portland cement and wood strands as raw materials. In this study, the acoustic performance of WWCP absorbents will be investigated.
Material and Methods: The mixed raw materials were molded under pressure through a hydraulic press to fabricate the WWCP samples. Samples were demolded after 24 hours. Samples were created with two thicknesses of 2 and 4 cm and three bulk densities of 400, 500, and 600 kg/m3 to examine the impact of thickness and bulk density on the acoustic absorption coefficient. The sound absorption coefficients were determined as a function of frequency for two frequency ranges: low (63-500 Hz) and high (630-6300 Hz).
Results: In the low-frequency range, increasing the thickness from 2 to 4 cm increased the absorption coefficient at 500 Hz by 0.16 and 0.23 for densities of 400 and 500 kg/m3, respectively. Increasing the thickness added an absorption peak and increased the value of these absorption peaks to 0.9 in the high-frequency range. When the bulk density of the 4-cm-thick samples increased from 400 to 600 kg/m3, the low-frequency absorption peak increased by 0.33. In the high-frequency range, the same density change increased the absorption peak by 0.26 for the 2-cm-thick sample.
Conclusion: Increasing the thickness of WWCP improves both its high- and low-frequency acoustic absorption coefficients. In addition, increasing the bulk density to approximately 500 kg/m3 boosts the sound absorption efficiency in both frequency ranges.
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.
