Design of high-efficiency, low frequency (<1000Hz) soundproof window or wall absorber which is transparent to airflow is presented. Due to the massive rise in human population and modernization, environmental noise has significantly risen globally. Prolonged noise exposure can cause severe physiological and psychological symptoms like nausea, headaches, fatigue, and insomnia. There has been continuous growth in building construction and infrastructure like offices, bus stops, and airports due to urban population. Generally, a ventilated window is used for getting fresh air into the room, but at the same time, unwanted noise comes along. Researchers used traditional approaches like noise barrier mats in front of the window or designed the entire window using sound-absorbing materials. However, this solution is not aesthetically pleasing, and at the same time, it is heavy and not adequate for low-frequency noise shielding. To address this challenge, we design a transparent hexagonal panel based on Sierpiński fractal triangle which is aesthetically pleasing, demonstrates normal incident sound absorption coefficient more than 0.96 around 700 Hz and transmission loss around 23 dB, while maintaining air circulation through triangular cutout. Next, we present a concept of fabrication of large acoustic panel for large-scale applications which lead to suppressing the urban noise pollution
Programmable acoustic metamaterials allow to manipulate sound waves at desirable frequency ranges intelligently, which makes it a viable and practical alternative to many noise cancellation applications. These kinds of metamaterials are amenable to intelligent programming and find place in a variety of real-life applications such as acoustic cloaking, noise cancellation, stealth applications, noise focusing, etc., simply with intelligent programming of the microstructure and the control of micro-movement of the various control boundaries instead of rebuilding new structures every time. Metamaterials are artificially designed materials that possess unique, unusual physical and mechanical properties, making them good lightweight candidates for various noise-shielding applications. However, many papers have been published on studying passive kinds of metamaterial structures whose material properties are fixed in space and time once designed and fabricated. This article is about a different class of metamaterials which are programmable and changeable through various control boundaries forming the structures for the applications pertaining to acoustic isolation (vibro-acoustics) and acoustic mitigation. The incorporation of shape memory polymers or smart polymers into the metamaterial structures enables fine and coarse tuning of these structures to the incoming noise frequency spectrum on a real-time basis. This article reviews some recent trends in progress made with smart materials and programmable metamaterials, their various programming techniques, the fundamental concept involved, various design strategies of such materials and their emergent applications in various fields of technology. The current fabrication challenges and future outlook in this promising field are also discussed.
In this work, we introduce fractal acoustic metamaterials (FAMs), in thicknesses ranging from 5 (λ/69) to 25 mm (λ/18), which are observed to provide multiple narrow-band low-frequency absorptions of acoustic signals. The fractal structures used in this work are carefully designed and fabricated using a side branch Helmholtz resonator design, making these structures easily tunable to multiple frequencies. Using different sizes of the side branches distributed in a fractally oriented configuration onto a plane rigid baseplate, the propagation velocity of acoustic waves is slowed down considerably. There is also a shifting resonating response of the structures toward lower frequencies (<1600 Hz). These FAM structures exhibit no dependence on the acoustic traverse length, as is otherwise commonly seen in coiled meta-structures and others. In order to achieve a near-perfect sound absorption behavior, the geometry of the structure is theoretically ascertained and validated numerically and experimentally. Significant emphasis has been placed on the associated physical mechanism modulating the loss of intensity of the incident acoustic signals. Moreover, with regression analysis performed on a response surface-based optimization scheme (using Design Expert 11 software), the geometric parameters are determined in a way that the absorption demonstrates a narrow-band characteristic at a frequency of 1 K Hz. We have shown in this work the tunability aspect of the various absorption frequency bands through appropriate designs of the FAM. It opens up wide application possibilities of multiple frequency sound absorptions (acoustic cloaking).
The recent emergence of acoustic metamaterials presents unparalleled possibilities for sound control across diverse scenarios. However, achieving both sound absorption and unrestricted airflow concurrently in a one-dimensional scenario poses a challenge. Most available acoustic metamaterials are designed in 2D or 3D, and in these configurations, a large portion of noise is reflected back. The key challenge, therefore, is how to allow maximum air to pass through the structure, make it tunable and noise be effectively damped within it. To address this, we propose four different types of configurations as KF (Kink Fiber), YAM (“Y” Type kink Fiber), CDAM (Converging Diverging Kink Fiber), and IRAM (Internal Resonator Kink Fiber, a unique one-dimensional design that simultaneously overcomes both challenges. The design subwavelength (7 cm) one-dimensional acoustic meta-structured blanket, demonstrating broadband sound absorption bandwidth of one octave with 0.5–0.9 absorption within the 500–1600 Hz range. In this study, we theoretically (through transfer matrix method), numerically (through FEM), and experimentally (through Impedance tube method) have showcased that this challenge can be surmounted by employing various configurations of kink fiber-based metamaterials. Following the concept developed in this article there have been efforts to develop blankets using hand lay-up method which can be deployed in real-world setup based on one-dimensional acoustic damping concept introduced in this work.
The recent emergence of acoustic metamaterials presents unparalleled possibilities for sound control across diverse scenarios. However, achieving both sound absorption and unrestricted airflow concurrently in a one-dimensional scenario poses a challenge. Most available acoustic metamaterials are designed in 2D or 3D, and in these configurations, a large portion of noise is reflected back. The key challenge, therefore, is how to allow maximum air to pass through the structure, make it tunable and noise be effectively damped within it. To address this, we propose four different types of configurations as KF (Kink Fiber), YAM (“Y” Type kink Fiber), CDAM (Converging Diverging Kink Fiber), and IRAM (Internal Resonator Kink Fiber, a unique one-dimensional design that simultaneously overcomes both challenges. The design subwavelength (7 cm) one-dimensional acoustic meta-structured blanket, demonstrating broadband sound absorption bandwidth of one octave with 0.5-0.9 absorption within the 500-1600 Hz range. In this study, we theoretically (through transfer matrix method), numerically (through FEM), and experimentally (through Impedance tube method) have showcased that this challenge can be surmounted by employing various configurations of kink fiber-based metamaterials. Following the concept developed in this article there have been efforts to develop blankets using hand lay-up method which can be deployed in real-world setup based on one-dimensional acoustic damping concept introduced in this work.
We present thin acoustic meta-structures with subwavelength dimensions through which almost perfect sound absorption is achieved in the low-frequency domain. Our overall strategy builds on the fact that the sound absorption capabilities of the meta-structures primarily depend on the geometric dimensions and can easily be reconfigured as per requirements through a change of geometry. To analyze various possibilities, we optimize the geometric structure through hybrid regression analysis using the genetic algorithm approach and finite element-based numerical simulations so that the geometry is tuned for high attenuation of acoustic signals over a broad range of frequencies. Both theoretical and experimental data show good parity and are able to establish the meta-structure nature of the assembly with respect to different frequency bands in the low frequency domain.
The proposed work enumerates a hybrid thin, deep-subwavelength (2 cm) acoustic metamaterials acting as a completely new type of sound absorber, showing multiple broadband sound absorption effects. Based on the fractal distribution of Helmholtz resonator (HRs) structures, integrated with careful design and construct hybrid cross micro-perforated panel (CMPP) that demonstrate broad banding approximately one-octave low-frequency sound absorption behavior. To determine the sound absorption coefficient of this novel type of metamaterial, the equivalent impedance model for the fractal cavity and the micro-perforated Maa's model for CMPP are both used. We validate these novel material designs through numerical, theoretical, and experimental data. It is demonstrated that the material design possesses superior sound absorption which is primarily due to the frictional losses of the structure imposed on acoustic wave energy. The peaks of different sound absorption phenomena show tunability by adjusting the geometric parameters of the fractal structures like cavity thickness 't', cross perforation diameter of micro perforated panel, etc. The fractal structures and their perforation panel are optimized dimensionally for maximum broadband sound absorption which is estimated numerically. This new kind of fractals cavity integrated with CMPP acoustic metamaterial has many applications as in multiple functional materials with broad-band absorption behavior etc.
Shape-memory polymers (SMP) are a class of materials that are better known as smart materials. The smartness of the SMP comes from their tunability properties allowing them to tweak into various shapes and orientations as per the desired applications. Many animals can change the stiffness of their skin, adopt the colors from the environment, change their appearance according to needs and in case of approaching danger, etc. Inspired by these above natural phenomena, many smart materials have been developed by researchers over the last decade and SMP is one of them. SMP polymers can easily be manipulated to the desired shape by changing their physical and chemical parameters such as temperature, light, ultrasound, chemical cross-linkage, crystallization, etc. SMP is a suitable candidate for designing novel metamaterials or programmable metamaterials in which the temporary shape can be retained and also imposed and controlled through programming such that the initial shape of the material can be recovered by an application of an external stimulus. The best characteristic of these polymers is the retain-ability of original or temporary shape in different stimuli fields prompting this material to have a memory effect for shape retainability. Due to such exceptional properties of the SMP, these smart materials find wide applications in Aerospace, Biomedical micro-devices, Robotics, Acoustics, Electronics, Textiles, Bionics engineering, and many other contemporary fields.This chapter discusses some fundamentals related to the fabrication strategies, emerging application aspects, and memory retention/ programming aspects of the shape memory polymers. The current state of the art and future directions has also been outlined to provide a roadmap for smart memory polymers-based technologies.
