Sound absorption of acoustic resonators with oblique perforations

Low-frequency airborne noise reduction is an issue of major concern in most practical cases due to the limiting space constraints. The applicability of acoustic resonators that not only work in this frequency range but can also be tuned is of great interest in many noise control applications such as muffler devices, noise barriers, or building isolation walls. This Letter studies the acoustic behavior of perforated panel absorbers with oblique perforations. Unlike more complex devices, the proposed absorber uses a simple concept that relies on the increase in the effective length of the panel by using perforations aligned obliquely with respect to the panel surface. In doing so, a shift of the resonance frequency toward low frequencies along with an increase in the sound absorption can be achieved provided that the geometrical characteristics of the absorber are properly chosen. A simple predictive model that relies on the fluid-equivalent theory was developed to investigate the acoustic properties of these absorbers, measurements in an impedance tube over additive manufactured samples serving to confirm the previous assertions. Preliminary results show the potential of these absorbers and encourage their further development for practical purposes.This work was supported by the COST (European Cooperation in Science and Technology) Action CA15125-DENORMS: “Designs for Noise Reducing Materials and Structures.

A mechanical property evaluation of graded density Al-Si10-Mg lattice structures manufactured by selective laser melting

Metal components with applications across a range of industrial sectors can be manufactured by selective laser melting (SLM). A particular strength of SLM is its ability to manufacture components incorporating periodic lattice structures not realisable by conventional manufacturing processes. This enables the production of advanced, functionally graded, components. However, for these designs to be successful, the relationships between lattice geometry and performance must be established. We do so here by examining the mechanical behaviour of uniform and graded density SLM Al-Si10-Mg lattices under quasistatic loading. As-built lattices underwent brittle collapse and non-ideal deformation behaviour. The application of a microstructure-altering thermal treatment drastically improved their behaviour and their capability for energy absorption. Heat-treated graded lattices exhibited progressive layer collapse and incremental strengthening. Graded and uniform structures absorbed almost the same amount of energy prior to densification, 6.3±0.26.3±0.2 MJ/m3 and 5.7±0.25.7±0.2 MJ/m3, respectively, but densification occurred at around 7% lower strain for the graded structures. Several characteristic properties of SLM aluminium lattices, including their effective elastic modulus and Gibson-Ashby coefficients, C1 and α, were determined; these can form the basis of new design methodologies for superior components in the future

Multifunctional Polyimide Aerogels with Tailored Nanostructure Assembly and Enhanced Properties

Aerogels are highly porous solid materials with more than 80% porosity and the cell size in mesoporous, 2-50 nm, range. The nanostructure assembly and high porosity provided the aerogels with unique electrical, thermal, physical, and mechanical properties. With respect the fast development of the modern technology and the need for the metamaterials with improved performance, recently the aerogels are received significant attention from the scientist. Based on the previous studies among the aerogel types, the polyimide aerogels presented the capability to achieve high mechanical flexibility in thin film geometries as well as moisture resistance. These along with the higher service temperature over the other organics, presented the potential of polyimide aerogels to be used in wide range of industrial applications. However, with respect to the very sensitive relations between the aerogel nanostructure configuration and its properties, the lack of control on aerogel nanostructure formation significantly reduced the possibility of tailoring polyimide aerogel properties to fit with in the industrial requirements. Given this background, the focus of this research is to design and develop robust methods to tailor the aerogels nanostructure configuration to achieve the aerogels with optimum performance. From the results four different successful strategies in tailoring the aerogel nanostructure assembly and controlling its properties are implemented in this work, and the properties of the fabricated aerogel prototypes are characterized through fully parametric studies. The capability of tailoring and improving the properties of polyimide aerogels may warrant their penetration in wide range of industrial applications.Ph.D

Design, Characterization and Modeling of Biobased Acoustic Foams

Polymeric open cell foams are widely used as sound absorbers in sectors such as automobile, aerospace, transportation and building industries, yet there is a need to improve sound absorption of these foams through understanding the relation between cell morphology and acoustic properties of porous material. Due to complicated microscopic structure of open cell foams, investigating the relation between foam morphology and acoustic properties is rather intricate and still an open problem in the field.The focus of this research is to design and develop biobased open cell foams for acoustic applications to replace conventional petrochemical based foams as well as investigating the link between cell morphology and macroscopic properties of open cell porous structures. To achieve these objectives, two industrially produced biomaterials, polylactide (PLA) and polyhydroxyalkanoate (PHA) and their composites were examined and highly porous biobased foams were fabricated by particulate leaching and compression molding. Acoustic absorption capability of these foams was enhanced utilizing the effect of co-continuous blends to form a bimodal porous structure. To tailor mechanical and acoustic properties of biobased foams, blends of PLA and PHA were studied to reach the desired mechanical and viscoelastic properties. To enhance acoustic properties of porous medium for having a broad band absorption effect, cell structure must be appropriately graded. Such porous structures with microstructural gradation are called Functionally Graded Materials (FGM). A novel graded foam structure was designed with superior sound absorption to demonstrate the effect of cell arrangement on performance of acoustic fixtures. Acoustic measurements were performed in a two microphone impedance tube and acoustic theory of Johnson-Champoux-Allard was applied to the fabricated foams to determine micro cellular properties such as tortuosity, viscous and thermal lengths from sound absorption impedance tube measurements using an inverse technique. As the next step towards in depth understanding of the relation between cell morphology and sound absorption of open cell foams, a semi-analytical model was developed to account for the effect of micro cellular properties such as cell wall thickness and reticulation rate on overall macroscopic and structural properties. Developed model provides the tools to optimize the porous structure and enhance sound absorption capability.Ph.D

Sound absorption of acoustic resonators with oblique perforations

© 2020 Author(s). Low-frequency airborne noise reduction is an issue of major concern in most practical cases due to the limiting space constraints. The applicability of acoustic resonators that not only work in this frequency range but can also be tuned is of great interest in many noise control applications such as muffler devices, noise barriers, or building isolation walls. This Letter studies the acoustic behavior of perforated panel absorbers with oblique perforations. Unlike more complex devices, the proposed absorber uses a simple concept that relies on the increase in the effective length of the panel by using perforations aligned obliquely with respect to the panel surface. In doing so, a shift of the resonance frequency toward low frequencies along with an increase in the sound absorption can be achieved provided that the geometrical characteristics of the absorber are properly chosen. A simple predictive model that relies on the fluid-equivalent theory was developed to investigate the acoustic properties of these absorbers, measurements in an impedance tube over additive manufactured samples serving to confirm the previous assertions. Preliminary results show the potential of these absorbers and encourage their further development for practical purposes.COST (European Cooperation in Science and Technology) Action CA15125-DENORM

Multi-layer perforated panel absorbers with oblique perforations

Many different solutions exist to improve the low-frequency sound absorption performance of acoustic resonators, extending or coiling up space into the resonator being some of the most widespread. In this context, modern additive manufacturing processes pose a new scenario in which these devices can be engineered to yield outstanding acoustic properties. In a recent work by the authors, a solution consisting of a perforated panel with oblique perforations was analyzed, results showing an enhanced sound absorption performance when compared to traditional perforated panel absorbers. This technical note aims to show the potential of these panels when used in multi-layer arrangements both to widen their effective sound absorption bandwidth and to improve their low-frequency performance. A simplified approach that relies on the fluid-equivalent theory was used together with the Transfer Matrix Method (TMM) to analyse different configurations, prediction results showing a good agreement when compared to experiments in an impedance tube over additive manufactured samples. Unlike other perforated-based solutions, the proposed system avoids addressing the cavity design while showing improved sound absorption features.This work was supported by the COST (European Cooperation in Science and Technology) Action CA15125 - DENORMS: ‘‘Designs for Noise Reducing Materials and Structures”

Switching Acoustic Propagation via Underwater Metasurface

© 2020 American Physical Society. Switching on and off acoustic waves through metamaterials has potential in noise canceling, underwater detection, and communication. However, traditional acoustic designs are challenging in manipulating underwater acoustic waves when the device thickness is less than wavelength. Here we report an alternative design of an underwater metasurface-based acoustic switcher to achieve this goal. The switching mechanism is revealed by combing acoustic diffraction of grating with mode conversion of double-layer PMMA plates. The device is tuned to control wave transmission by changing grating angle. Furthermore, we experimentally fabricate the metasurface acoustic switcher. The broadband-switching performance is realized to control underwater target detection and to produce binary digital encoding for acoustic waves. The proposed metasurface acoustic switcher offers the advantages of broadband performance and thin structure, which promises the opportunity for designing next-generation broadband-switching devices in underwater acoustic detection and communication