Acoustic Analysis of Additive Manufactured Multilayer Periodic Structures

Akiwate, Deepak C and B, Venkatesham (2019) Acoustic Analysis of Additive Manufactured Multilayer Periodic Structures. PhD thesis, Indian Institute of Technology Hyderabad.

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Abstract

Traditional acoustic materials like glass wool, fiber glass, foams etc. are extensively used to attenuate acoustic energy or noise along the propagation path. To have maximum absorption along the path, acoustic material requires thickness of at least quarter of wavelength. Therefore, at lower frequencies it demands thicker acoustic materials where wavelength of acoustic wave is much higher. These requires more space to put these materials and ultimately adds weight to the system. The current study is focused on design and fabrication of periodic structures inspired from natural honeybee hive to improve low frequency absorption with relatively lower thickness, as an alternative to traditional acoustic materials. The main objective of this study is to understand the acoustic energy attenuation through periodic structures with central membrane. First part of work describes the standard techniques available for measurement of acoustic absorption coefficient, and effects of manufacturing technique on absorption coefficient of the structure. The proposed periodic structure has three distinct features namely; narrow tubes, periodicity and structural flexibility. In this stage, narrow tubes and periodicity excluding flexibility has been studied extensively. A generalized mathematical formulation to predict absorption coefficient for single (hexagonal) as well as multi-periodic (octagonal) structure has been developed where shape dependent viscous and thermal effects are included. The proposed method is based on unit section analysis which significantly reduces the complexity during analysis of periodic structures. Additive Manufacturing (AM) has been extensively used to fabricate periodic structures to examine effect of various cell parameters like cell size, shape and cell length. The estimated absorption coefficients using unit section have been corroborated with measured results in impedance tube. Second part of thesis deals with the influence of membrane flexibility on acoustic absorption coefficient of complete periodic structure. This part emphases on development of a mathematical formulation of membrane, perforated membrane, membrane backed by a cavity and membrane sandwiched between two periodic layers. A mathematical formulation of the perforated membrane has been rewritten by combining individual impedances of membrane and perforations with modified boundary conditions (velocity continuity at perforation circumference). A mathematical formulation based on transfer matrix method has been developed to estimate absorption coefficient of complete multilayer periodic structure (two periodic narrow tube layers with central membrane). The formulation is capable of handling membrane tension as well as perforations in the membrane. The measured results are correlated with predicted results. Third part of current work deals with improving low frequency absorption coefficient of periodic structures without incorporating flexibility in to it. The four different configurations are studied by reducing the cell size, providing impedance mismatch, increasing effective length of wave travel, and providing perforations to face sheets of hexagonal periodic structures. These proposed configurations are fabricated using Additive Manufacturing (AM) method. The tuning of these structures to narrow as well as broad band sound absorption coefficient has been discussed. The predicted results based on viscous and thermal effects are validated with measured results. Finally, the last part of thesis summarizes current work based on above findings. The results and methodology presented in this study helps to understand the acoustic energy attenuation through the periodic structures. This study also paves the basic framework to design and fabricate periodic structures for acoustic applications like automobile, aviation and building acoustics.

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