Design and development of 3D-printed polymeric structures for acoustic absorption

• Main Author: Yang, Wenjing
• Other Authors: Zhou Kun
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/147846
• Institution: Nanyang Technological University

In acoustic engineering, noise control is of concern in various fields including industrial machines, vehicles, architectures and home appliances. Sound absorption is one of the major solutions to control noise and the efficiency of sound absorption can be evaluated by the acoustic absorption coefficient and the absorption frequency bandwidth. The development of novel acoustic absorbers is crucial in enhancing the efficiency of sound absorption and fulfilling various environmental requirements of acoustic absorption in different frequency ranges. The enhancement of functional properties of absorbers should also be considered by innovative designs. Additive manufacturing (AM) has been introduced to reduce the prototyping costs and lead time of sound absorbers, while the potential of AM for fabricating sound absorbers with complex structures has not been explored. This Ph.D. study designs and develops different types of novel acoustic absorbers with complex structures to enhance the efficiency of sound absorption in various frequency ranges by achieving wideband absorption. The development of the absorbers involves two different polymer AM processes owing to additive manufacturability of the complex designs as well as prototype cost. Three novel absorber prototypes with improved performances have been successfully developed. Firstly, for sound absorption enhancement in the range from low frequency to middle frequency, innovative multi-layer micro-perforated panels (MPPs) with adjustable geometries for tunable wideband absorption are designed and fabricated by selective laser sintering. The finite element method is applied to provide numerical predictions of the performances of the designed structures. The effects of structural parameters of multi-layer MPPs on absorption coefficients and frequency bandwidths are analyzed by theoretical predictions, numerical simulations and experiments. The results reveal that the absorption frequency bandwidths of MPPs are broadened by the multi-layer designs, while the absorption coefficients remain comparable or even higher. The frequency ranges can be tuned by varying the air gap distances and the inter-layer distances. An optimization method, in which the area under the sound absorption curve as the criterion to evaluate the sound absorption ability is maximized, is introduced to design acoustic absorbers with the most effective sound absorption. Secondly, for sound absorption enhancement in the range of high frequencies, triply periodic minimal surface (TPMS) structures that have shown potential absorption abilities but have not been experimentally validated are designed for characterization of the acoustic performances of innovative acoustic absorbers with multifunctionality. Three typical types of TPMS structures: Primitive (P), Gyroid (G) and Diamond (D) are designed and fabricated by stereolithography apparatus using polymeric resins. D structures exhibit superior absorption abilities in terms of high absorption coefficients and wide bandwidths. A series of D structures are designed to study the effects of the unit cell size, volume fraction and height on acoustic absorption. The analysis of the results provides an optimized design of geometric parameters with a small unit cell size and a large volume fraction for optimal acoustic absorption. Finally, for further enhancement of sound absorption of the TPMS absorbers, innovative micro-perforated TPMS absorbers are designed and investigated by combining MPPs and TPMS structures. Significant improvements on acoustic absorption are achieved by the micro-perforated TPMS structures, especially for P and G structures with lower absorption coefficients compared to those of D structures. In addition, the effects of the perforation ratio of P and G structures are studied. The increase of the perforation ratio in the observed range exhibits positive effects on improving the acoustic absorption of the structures. The fabrication materials are proved to have no major effects on the acoustic absorption performances of the TPMS structures and therefore can be selected according to the application environments and requirements. The study of innovative acoustic devices is significant in acoustic engineering and is facilitated by AM that efficiently manufactures the absorber prototypes with complex structures. AM is promising to be applied as an enabling technology that accelerates the development of acoustic engineering by fabricating innovative and customized designs.