The first part of this paper discusses the importance of porous materials in passive noise reduction, types of porous materials, various applications of these materials, relevant acoustical and non-acoustical parameters, and different methods to measure them based on international standards were explained. In this part of the paper, definitions and experimental methods used in measuring the acoustic properties of the absorption coefficient and the sound transmission loss based on international standards are presented. Then, while providing a comprehensive discussion of the sound absorption mechanisms in porous materials, the types of sound absorption mechanisms, including viscous, thermal, and structural losses, are described. The effect of different parameters on the variation of sound absorption of porous materials has been presented. Also, various models, including experimental, laboratory, and analytical models used for expressing the acoustical and mechanical properties of porous materials, are discussed. Finally, a brief review is present for recent research areas on porous materials.

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 highfrequency 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.

Introduction: Nowadays, the acoustic behavior analysis of natural fibers composites has received increasing attention by researchers. In this regard, the present study aimed to optimize and model the sound absorption behavior of composites made of Yucca Gloriosa (YG) fiber via using a mathematical modeling approach. Methodology: In this experimental cross-sectional study, in order to fabricate the natural acoustic composites, the alkaline treatment of the fibers was employed. In this study, the design of experiments and determination of the optimum amount of alkaline treatment parameters (NaOH concentration and immersion time) to improve the sound absorption was performed by Response Surface Methodology (RSM). Moreover, the sound absorption coefficient (SAC) of YG fiber was measured by an impedance tube system (ISO10534-2 standard). The applicability of Delany-Bazley (DB) and Miki analytical models for predicting the sound absorption coefficients of the natural composites by coding formulas in MATLAB software was investigated as well. Result: Comparison of the obtained SAC showed that this value of optimized was higher than untreated composite at all frequencies, and the sound absorption average (SAA) index increased by 18. 92%, particularly when compared to the raw composites. Also, good agreement was found between the results from the empirical models and the experimental results in the low and mid-frequency range of one-third octave band. Conclusion: the optimization of alkaline treatment and prediction of SAC by empirical model, with regard to the prominent benefits of natural fibers and the wide use of these fibers, is considered as an acceptable strategy for acoustic applications (industries and buildings).

Sound absorption behavior of A5083 open-celled foams produced by space holder method was investigated. Sound absorption coefficient of samples was investigated within the range of 500-16000 Hz, by the transfer function method. A5083 foams sound absorption was investigated by two cell sizes less than 106 and 106-250 microns. In addition, the effect of adding silicon carbide was studied with 10 vol. %. The effect of air gap depths on sound absorption coefficient was also investigated. Results indicated that A5083 foams sound absorption behavior with cell sizes less than 106 microns and 106-250 microns is very weak at most frequencies. Adding 10 vol. % of silicon carbide to A5083 foam with 106-250 microns cell sizes, improved the sound absorption properties of foams. The results of this research also indicated that using an air gap in A5083 foam with less than 106 microns cell size has no effect on sound absorption coefficient. In A5083 foam with 106-250 microns cell size and 20 mm air gap depths, at most frequencies, a better sound absorption was observed than the sample without air gap or the sample with 10 mm air gap depths.