Acoustic Analysis of Additive Manufactured Multilayer Periodic Structures

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

Analytical investigation of propeller-wing interaction noise

This paper investigates the noise generated by propeller-wing configuration at take-off condition with the propeller mounted upstream. This study makes use of various axisymmetric noise models developed for contra-rotating propellers to estimate the noise generated by propeller-wing configuration and later integrate them to estimate total noise. First, using well-published theory, rotor-alone (loading, thickness, and self-noise) and interaction noise sources (viscous-wake, potential field, tip-vortex) including tonal and broadband components are estimated. Later, a systematic parametric study is carried out by changing the blade number and tip Mach, while maintaining the propeller thrust and blade solidity. The noise generated is represented by Overall Acoustic Sound Power Level (OSWLs), which is an integrated value over the emission angles and frequency range, in a matrix form for the range of blade number and tip Mach. This matrix shows the regions dominated by rotor-alone and interaction noise and found that the noise characteristics of a rotor in uninstalled conditions (rotor-alone) are significantly altered due to the presence of a wing (installed condition). Further, it is found that the balance between these regions shifts with the variation in separation distance between the propeller and the wing. These results are further discussed with the individual interaction noise source mechanism and their dominance at various blade numbers, tip Mach, and separation distances. In addition, the non-axisymmetric viscous-wake interaction noise is investigated for even and odd numbers of blades and found that viscous-wake interaction noise has considerable directivity in the azimuthal direction. The results presented in the study are preliminary findings of propeller-wing noise, however, it gives give a quantitative picture of the behaviour of various noise sources and their balance with respect to geometric and operating parameters. This study will help to understand the dominant noise sources involved in propeller-wing configuration and will provide a quick guide for designing a low-noise configuration

Acoustic characterization of additive manufactured perforated panel backed by honeycomb structure with circular and non-circular perforations

This paper studies the acoustic properties of an additive manufactured micro-perforated panel backed by a periodic honeycomb structure. Extrusion-based Fused Filament Fabrication (FFF) technique of Additive Manufacturing (AM) is used to fabricate the integrated honeycomb structures with a perforated face sheet. Normal absorption coefficient of the fabricated structure is measured in impedance tube using two microphone transfer function method. A generalized analytical formulation based on unit section analysis applicable to various cross sections of perforations has been proposed to predict the absorption coefficient, where shape dependent viscous effects in the perforation are incorporated by deriving effective complex density of the medium. To study the effect of perforation shape, three geometries viz., circular, triangular and square perforations are considered for analysis where triangular shape found to have more absorption coefficient and lower frequency of peak absorption. In addition, broadband absorption coefficient of proposed structure has been demonstrated by deploying hexagonal cells of different lengths in a unit section. The analytical results are compared with experimental results and a good agreement is observed between them. A parametric study is conducted to understand effect of perforated hole size and cell length on the absorption coefficient and peak frequency. Results show that the proposed structures can be tuned to desired frequency range by altering geometric parameters like cell length, shape and size of perforation hole. Technique and methodology presented in the current study gives an alternative way to design and fabricate honeycomb structures with perforations for acoustic applications such as aircraft cabins, ship structures and building acoustics

Acoustic characterization of additive manufactured perforated panel backed by honeycomb structure with circular and non-circular perforations

This paper studies the acoustic properties of an additive manufactured micro-perforated panel backed by a periodic honeycomb structure. Extrusion-based Fused Filament Fabrication (FFF) technique of Additive Manufacturing (AM) is used to fabricate the integrated honeycomb structures with a perforated face sheet. Normal absorption coefficient of the fabricated structure is measured in impedance tube using two microphone transfer function method. A generalized analytical formulation based on unit section analysis applicable to various cross sections of perforations has been proposed to predict the absorption coefficient, where shape dependent viscous effects in the perforation are incorporated by deriving effective complex density of the medium. To study the effect of perforation shape, three geometries viz., circular, triangular and square perforations are considered for analysis where triangular shape found to have more absorption coefficient and lower frequency of peak absorption. In addition, broadband absorption coefficient of proposed structure has been demonstrated by deploying hexagonal cells of different lengths in a unit section. The analytical results are compared with experimental results and a good agreement is observed between them. A parametric study is conducted to understand effect of perforated hole size and cell length on the absorption coefficient and peak frequency. Results show that the proposed structures can be tuned to desired frequency range by altering geometric parameters like cell length, shape and size of perforation hole. Technique and methodology presented in the current study gives an alternative way to design and fabricate honeycomb structures with perforations for acoustic applications such as aircraft cabins, ship structures and building acoustics

Acoustic characterization of additive manufactured micro-perforated panel backed by honeycomb structure

This paper studies the acoustic properties of an additive manufactured micro-perforated panel backed by a periodic honeycomb structure. Extrusion-based Fused Filament Fabrication (FFF) technique of Additive Manufacturing (AM) is used. Absorption coefficient of the proposed structure is measured using an Impedance tube. An analytical model is developed to predict the acoustic absorption coefficient. The analytical results are compared with the experimental results and a good agreement is observed between them. A parametric study is conducted to understand the effect of perforated hole size on the absorption coefficient and peak frequency

A Performance Study on Indirect Static Flow Resistivity Measurement Methods for Acoustic Materials

Various empirical models have emphasized the dependence of sound absorption coefficient on static airflow resistivity, and thus its measurement becomes essential. In this paper, the two-cavity and two-thickness indirect acoustic methods are implemented based on a standard impedance tube for evaluating the static flow resistivity of foam. A comparison is made between the resistivity results obtained by the two-cavity and two-thickness method , and later validated with results of an alternating air-flow test setup which is developed as per the ISO 9053 guidelines. Further, the empirical relations are utilized to estimate the absorption coefficientfrom measured values ​​of flow resistivity and are compared with measured absorption coefficient in an impedance tube . The results discussed in this study presents the feasibility and suitability of the indirect acoustic methods for evaluating the flow resistivity

Acoustic properties of additive manufactured narrow tube periodic structures

Quarter tube periodic resonators are used to attenuate the acoustic energy of discrete frequencies wherein their sizes can vary from narrow to large. This paper reports acoustical properties of additive manufactured, narrow tube, single and multi-periodic structures, both, theoretically and experimentally. Herein, multi-periodic structures are defined as periodically arranged unit sections of tubes, where each section is composed of periodically repeated unit cells of different sizes and shapes. Structures with hexagonal narrow tubes and octagonal narrow tubes with interfacial gaps are considered for the study, and normal absorption coefficients of these samples are measured by using impedance tube. the theoretical absorption coefficient of these structures is predicted using unit section analysis method and narrow tube theory, where shape dependent viscous and thermal losses are incorporated. Estimated theoretical absorption coefficients are in good agreement with measured results. The result shows that the frequency and amplitude of maximum absorption can be varied by altering the aperture ratio and/or the length of periodic structure. The proposed theoretical method gives an alternative approach for designing and manufacturing periodic narrow tubes for different applications such as absorbing panels, acoustic transducers, and engine filter elements

IMPLEMENTATION OF TWO-CAVITY METHOD FOR MEASURING THE FLOW RESISTIVITY OF ACOUSTIC MATERIAL

Acoustic materials are characterized according to their macroscopic and microscopic properties. The sound absorption co-efficient and the air-flow resistivity are of paramount importance among those used to describe the acoustic behaviour of materials. There are several methods developed for measuring the air-flow resistivity of acoustic material. The aim of this paper is to study the existing static flow resistivity measurement methods and then accordingly implement a suitable indirect method based on standard impedance tube. The flow resistivity measurements are carried out for additive manufactured ABS (Acrylonitrile butadiene styrene) sample, foam and glass fibre using the two-cavity method. There are certain similarities observed in their results. Further analysis of the raw impedance data is carried out and conclusions are drawn pertaining to the performance and feasibility of the implemented method