Sound Absorption and Transmission Loss Characteristics of 3d Printed Porous Bio-Degradable Material

Abstract

Environmental concern pertaining to materials circular economy leads to the development of bio-degradable materials for various engineering applications. In recent years, noise pollution control and sound quality enhancement have been major concerns of industries, public places, automobiles, and even households. Noise pollution, especially at lower frequencies, causes a severe heath problem for humans, and it is difficult to control due to inadequate sound absorption capability of most of the existing material systems. Most of the materials currently used for sound absorption and transmission loss applications like glass wool, stone wool, plastics, synthetic foams, etc., are toxic and pose severe environmental and health issues. These materials leach toxins and are non-biodegradable, resulting in the production, usage, and disposal issues. Advancements in additive manufacturing motivated researchers to explore different eco-friendly materials and panels with unconventional geometric configurations that enhance their acoustic capabilities. The influence of perforations having arbitrarily varying cross-sections on the acoustic behaviour of 3D printed bio-degradable panels made of Poly Lactic Acid (PLA) and wood/PLA materials is presented in this study. Circular perforations having six different types of cross-sectional variations, namely convergent-divergent (CD), divergent-convergent (DC), convergent (C), divergent (D) with two different perforation diameters are realized using fused filament fabrication (FFF) based 3D printing. Sound absorption (SA) and sound transmission loss (STL) characteristics are estimated by the impedance tube method. Results revealed that perforated panels with varying cross-sections have better SA than the conventional cylindrical perforation for the specific frequency range. Among the different cross-sectional variations explored, DC and D perforation patterns exhibit comparable and lower transmission losses with respect to the panels with 1 mm cylindrical pores. The sound transmission results of all other five specimens were significantly higher than the typical perforated panel with 8 mm cylindrical pores and observed to be increasing with frequency. This study indicate that geometrical variations of perforations play a vital role while designing the soundproof panels. The experimental results compared with the numerical results are found to be in good agreement. Samples with spherical perforations are considered to investigate the effect of graded porosity on acoustic performance. In these samples, the spherical bubble perforations are distributed either uniformly or graded across the specimen thickness. A sample having typical cylindrical perforations is also considered for the comparative analysis. The results reveal that the SA of samples with different types of functionally graded (FG) perforations is higher at low frequencies. Better sound absorption (SA coefficient higher than 0.8) with wider bandwidth has been observed for the sample with small sized (2 mm diameter) uniform spherical perforations. The STL of sample is highest among the specimens, and the difference in STL increases significantly with frequency. FG perforations exhibited superior performance for both SA and STL. The proposed graded spherical porosity can be effectively utilized in soundproofing applications across building and transportation sectors. The impact of short wood fibers reinforcement on the 3D printed samples made of PLA for their acoustic characteristics is also investigated. Specimens having varying perforation geometries along with series connected uniform or graded size sphere perforations are analyzed. Results revealed that the addition of wood fibers in PLA improved the acoustic characteristics by significantly shifting the absorption range to the lower frequency side. Wood/PLA can be used in acoustic insulation for structural and transportation applications, especially where eco-friendliness and aesthetics are of major concern.