Metamaterials for Acoustic Noise Filtering and Energy Harvesting.

Artificial methods for noise filtering are required for the twenty-first century's Factory vision 4.0. From various perspectives of physics, noise filtering capabilities could be addressed in multiple ways. In this article, the physics of noise control is first dissected into active and passive control mechanisms and then further different physics are categorized to visualize their respective physics, mechanism, and target of their respective applications. Beyond traditional passive approaches, the comparatively modern concept for sound isolation and acoustic noise filtering is based on artificial metamaterials. These new materials demonstrate unique interaction with acoustic wave propagation exploiting different physics, which is emphasized in this article. A few multi-functional metamaterials were reported to harvest energy while filtering the ambient noise simultaneously. It was found to be extremely useful for next-generation noise applications where simultaneously, green energy could be generated from the energy which is otherwise lost. In this article, both these concepts are brought under one umbrella to evaluate the applicability of the respective methods. An attempt has been made to create groundbreaking transformative and collaborative possibilities. Controlling of acoustic sources and active damping mechanisms are reported under an active mechanism. Whereas Helmholtz resonator, sound absorbing, spring-mass damping, and vibration absorbing approaches together with metamaterial approaches are reported under a passive mechanism. The possible application of metamaterials with ventilation while performing noise filtering is reported to be implemented for future Smart Cities.

[Analysis of noise reduction measures in a noise workshop handover control room].

Objective: To explore the sound insulation, sound absorption and other noise reduction transformation methods in a noise workshop handover control room. Methods: In December 2021, through the occupational health investigation and on-site testing of the handover control room of a noise workshop, the causes of excessive noise were analyzed, and the transformation design scheme to reduce noise was proposed and the effect was analyzed. Results: Before the transformation, the peak frequency band noise intensity of the noise workshop handover control room was 112.8 dB (A), and the peak frequency was 1000 Hz. After noise reduction, the theoretical calculated control value was 61.0 dB (A), and the measured noise intensity was 59.8 dB (A) . Conclusion: The noise intensity of the handover control room is reduced after noise reduction, which is in line with the contact limit requirements of the control room in GBZ 1-2010 "Hygienic Standards for the Design of Industrial Enterprises", and has reference significance for noise control engineering.

[Analysis on the effect of sound insulation reconstruction in operation room of steel rolling production line in a steel plant].

Objective: To investigate the effect of sound insulation improvement on the noise exposure of workers in the operation room of hot rolling line for wide and heavy plate. Methods: From September 2019 to September 2017, based on the occupational health Survey, the data of 25 fixed operation rooms and workers in operation rooms of a steel rolling production line were collected retrospectively, the noise exposure levels before and after the improvement of sound insulation were statistically analyzed. Results: The noise exposure value of the workers, the qualified rate of 0 Grade 8 hours equivalent noise (L(EX, 8 h)) ≤85 dB (A) and the qualified rate of the design limit value of the operation room were all higher than those before the modification, the difference was statistically significant (P<0.01) , after the renovation, the Class II and above noise hazards were eliminated, the equivalent continuous a sound level (L(Aeq, 8 h)) >75 dB (A) of the workers in the operation room was 8h, and the noise level in the operation room still did not meet the Ergonomics limit standard. Conclusion: The improvement of sound insulation can effectively improve the working environment of noise workplace operating room and reduce the workers'noise exposure level.

Low-Frequency Sound-Insulation Performance of Labyrinth-Type Helmholtz and Thin-Film Compound Acoustic Metamaterial.

This paper presents a type of acoustic metamaterial that combines a labyrinth channel with a Helmholtz cavity and a thin film. The labyrinth-opening design and thin-film combination contribute to the metamaterial's exceptional sound-insulation performance. After comprehensive research, it is observed that in the frequency range of 20-1200 Hz, this acoustic metamaterial exhibits multiple sound-insulation peaks, showing a high overall sound-insulation quality. Specifically, the first sound-insulation peak is 26.3 Hz, with a bandwidth of 13 Hz and giving a transmission loss of 56.5 dB, showing excellent low-frequency sound-insulation performance. To further understand the low-frequency sound-insulation mechanism, this paper uses the equivalent model method to conduct an acoustic-electrical analogy, construct an equivalent model of the acoustic metamaterial, and delve into the sound-insulation mechanism at the first sound-insulation peak. To confirm the validity of the theoretical calculations, physical experiments are carried out by 3D printing experimental samples. The analysis of the experimental data has yielded results that are consistent with the simulation data, providing empirical evidence for the accuracy of the theoretical model. The material has significant practical application value. Finally, various factors are studied in depth based on the established equivalent model, which can provide valuable insights for the design and practical engineering application of acoustic metamaterials.

Study on the Preparation and Performance of Lightweight Wallboards from MSWIBA Foam Concrete.

To reduce land use and avoid further pollution, incineration for power generation has become the main method for municipal solid waste treatment. This research focused on the potential for transforming Municipal Solid Waste Incineration Bottom Ash (MSWIBA) into a finely ground powder. The impact of the powder's fineness and the amount of water used on its effectiveness was analyzed using a method called grey theory. MSWIBA was used as a partial substitute for cement in making MSWIBA foam concrete and lightweight wall panels. By modifying the fineness and water utilization of the recycled micro-powder, its maximum activity index can be increased to 90.1. This study determined the influence of factors including apparent dry density, water-cement ratio, foaming agent dilution ratio, and admixture dosage on the strength of the recycled foam concrete, and established the optimal mix ratio. This study employed a combination of physical experiments and numerical simulations to elucidate the impact of panel material, core layer thickness, and layer sequence on sound insulation performance. The simulation results were in close agreement with the experimental findings.

Thermal and Sound Insulation Properties of Organic Biocomposite Mixtures.

Sustainable building materials with excellent thermal stability and sound insulation are crucial for eco-friendly construction. This study investigates biocomposites made from cellulose pulp reinforced with beeswax, fir resin, and natural fillers like horsetail, rice flour, and fir needles. Eight formulations were obtained, and their thermal resistance, oxidation temperature, and acoustic properties were evaluated. Biocomposites exhibited significant improvements compared to conventional materials. Oxidation temperature onset increased by 60-70 °C compared to polyurethane foam or recycled textiles, reaching 280-290 °C. Sound absorption coefficients ranged from 0.15 to 0.78, with some formulations exceeding 0.5 across mid-frequencies, indicating good sound-dampening potential. These findings demonstrate the promise of these biocomposites for sustainable construction, offering a balance of thermal and acoustic performance alongside environmental and health benefits.

Assessing the Sound and Heat Insulation Characteristics of Layered Nonwoven Composite Structures Composed of Meltblown and Recycled Thermo-Bonded Layers.

Sound and heat insulation are among the most important concerns in modern life and nonwoven composite structures are highly effective in noise reduction and heat insulation. In this study, three layered nonwoven composite structures composed of a recycled polyester (r-Pet)-based thermo-bonded nonwoven outer layer and meltblown nonwovens from Polypropylene (PP) and Polybutylene terephthalate (PBT) as inner layers were formed to provide heat and sound insulation. Fiber fineness and cross-section of the thermo-bonded outer layer, fiber type (PP/PBT), areal weight (100/200 g/m2) and process conditions (calendared/non-calendared) of the meltblown inner layer were changed systematically and the influence of these independent variables on thickness, bulk density, air permeability, sound absorption coefficient and thermal resistance of composite structures were analyzed statistically by using Design Expert 13 software. Additionally, the results were compared with composite structures including an electrospun nanofiber web inner layer and with structures without an inner layer. It was concluded that comparable or even better sound absorption values were achieved with the developed nonwoven composites containing meltblown layers compared to nanofiber-included composites and the materials in previous studies.

Development of a vibration-damping, sound-insulating, and heat-insulating porous sphere foam system and its application in green buildings.

With the development of green buildings, people pay more attention to the quality of the indoor sound environment. The air sound insulation performance of floors and exterior walls plays a key role in today's green buildings. The thermal performance of the enclosure structure's floor and exterior wall heat transfer resistance is an important factor in reducing building carbon emissions in green buildings. The aim of this paper is to study the efficiency of the acoustic and thermal insulation of a foaming system with porous carbon balls and the combination of different structural ways of construction boards and external walls. The acoustic and thermal parameters of different sound insulation and thermal insulation systems designed with porous carbon sphere foam and inserted into the floors and exterior walls are compared to highlight the optimal structure. The theoretical and experimental tests showed that to improve the sound insulation performance of the floor, a sound insulation system needs to be placed on the surface of the floor in contact with the impact object and inlaid in the vertical gap in contact with the floor and the wall. Furthermore, it has been determined that the surface of the foam particle acoustic ball with micropores has good sound absorption performance. Finally, the high-quality building thermal insulation material with low thermal conductivity in any combination with the floor slabs and the external wall structure improves the thermal insulation performance.

Experimental Investigation on the Acoustic Insulation Properties of Filled Paper Honeycomb-Core Wallboards.

Honeycomb plates, due to their multi-cavity structure, exhibit excellent mechanical properties and sound insulation. Previous studies have demonstrated that altering the cell size and arrangement of honeycomb structures impacts their acoustic performance. Based on these findings, this study developed a wallboard structure with enhanced sound insulation by filling the cavities with paper fiber/cement facesheets and designing a stacked core structure. Through the reverberation chamber-anechoic chamber sound insulation experiment under 100-6300 Hz excitation and conducting orthogonal experiments from three dimensions, it was found that: (1) Compared to no filling, the filling with straw and glazed hollow bead can increase the sound transmission loss (STL) by more than 50% in the frequency bandwidth above 2000 Hz. This indicates that both types of fillings can significantly enhance the sound insulation performance of the honeycomb structure without a significant increase in economic costs. (2) The increase in paper fiber/cement facesheets improves the STL across the entire experimental bandwidth, with a maximum improvement exceeding 70%. This structural design not only offers superior sound insulation performance but also better suits practical engineering applications. (3) Increasing the number of core stacking units (from one to three), taking straw-filled paper honeycomb-core wallboards as an example, effectively increased the STL bandwidth. (4) This test enriches the application of honeycomb plates in sound insulation. Introducing fiber paper fiber/cement facesheets and eco-friendly, low-cost straw improves sound insulation and enhances the strength of honeycomb, making them more suitable for construction, particularly as non-load-bearing structures.

Sound insulation dataset of 30 wooden and 8 concrete floors tested in laboratory conditions.

In a Finnish-Swedish consortium project, a large amount of sound insulation tests was conducted for several intermediate floors in laboratory conditions to serve various scientific research questions. The dataset contains 30 wooden and 8 concrete constructions which are commonly used between apartments in multistorey buildings. Impact sound insulation was determined according to ISO 10140-3 standard using both tapping machine and rubber ball as standard sound sources. Airborne sound insulation was determined according to the ISO 10140-2 standard. The data are special since they have a broad frequency range: 20-5000 Hz. Data are reported in 1/3-octave frequency bands and the single-number values of ISO 717-1 and ISO 717-2 are also reported. Detailed construction drawings are available for all reported constructions. The data are highly valuable for research, education, and development purposes since all data were obtained in the same laboratory (Turku University of Applied Sciences, Turku, Finland), and all the constructions were built by the same installation team.

Effect of the Laying Order of Core Layer Materials on the Sound-Insulation Performance of High-Speed Train Carbody.

The design of sound-insulation schemes requires the development of new materials and structures while also paying attention to their laying order. If the sound-insulation performance of the whole structure can be improved by simply changing the laying order of materials or structures, it will bring great advantages to the implementation of the scheme and cost control. This paper studies this problem. First, taking a simple sandwich composite plate as an example, a sound-insulation prediction model for composite structures was established. The influence of different material laying schemes on the overall sound-insulation characteristics was calculated and analyzed. Then, sound-insulation tests were conducted on different samples in the acoustic laboratory. The accuracy of the simulation model was verified through a comparative analysis of experimental results. Finally, based on the sound-insulation influence law of the sandwich panel core layer materials obtained from simulation analysis, the sound-insulation optimization design of the composite floor of a high-speed train was carried out. The results show that when the sound absorption material is concentrated in the middle, and the sound-insulation material is sandwiched from both sides of the laying scheme, it represents a better effect on medium-frequency sound-insulation performance. When this method is applied to the sound-insulation optimization of a high-speed train carbody, the sound-insulation performance of the middle and low-frequency band of 125-315 Hz can be improved by 1-3 dB, and the overall weighted sound reduction index can be improved by 0.9 dB without changing the type, thickness or weight of the core layer materials.

Acoustic Performance of Floors Made of Composite Panels.

Airborne and impact sound insulation of composite panels arranged in different configurations were investigated in this study. The use of Fiber Reinforced Polymers (FRPs) in the building industry is growing; however, poor acoustic performance is a critical issue for their general employment in residential buildings. The study aimed to investigate possible methods of improvement. The principal research question involved the development of a composite floor satisfying acoustic expectations in dwellings. The study was based on the results of laboratory measurements. The airborne sound insulation of single panels was too low to meet any requirements. The double structure improved the sound insulation radically at middle and high frequencies but the single number values were still not satisfactory. Finally, the panel equipped with the suspended ceiling and floating screed achieved adequate level of performance. Regarding impact sound insulation, the lightweight floor coverings were ineffective and they even enhanced sound transmission in the middle frequency range. Heavy floating screeds behaved much better but the improvement was too small to satisfy acoustic requirements in residential buildings. The composite floor with a dry floating screed and a suspended ceiling appeared satisfactory with respect to airborne and impact sound insulation; the single number values were Rw (C; Ctr) = 61 (-2; -7) dB, and Ln,w = 49 dB, respectively. The results and conclusions outline directions for further development of an effective floor structure.

How to hide your voice: noise-cancelling bird photography blind.

Getting close to birds is a great challenge in wildlife photography. Bird photography blinds may be the most effective and least intrusive way if properly designed. However, the acoustic design of the blinds has been overlooked so far. Herein, we present noise-cancelling blinds which allow photographing birds at close range. First, we conducted a questionnaire in the eco-tourism centre located in Yunnan, China. Thus, the birders' expectations of the indoor sound environment are determined. We then identify diverse variables to examine the impact of architectural and acoustic decisions on noise propagation. Finally, the acoustic performances of the blinds by considering the birds' hearing threshold are examined. The numerical simulations are performed in the acoustics module of Comsol MultiPhysics. Our study demonstrated that photography blinds require a strong and thorough acoustic design for both human and bird well-being.

Formulation and Performance of NBR/CR-Based High-Damping Rubber Composites for Soundproof Using Orthogonal Test.

Multiple functional-material-filled nitrile butadiene rubber/chloroprene rubber (NBR/CR) acoustic composites were extensively studied and prepared. According to the orthogonal test table L25 (56), 25 groups of samples were prepared by using a low-temperature one-time rubber mixing process. With tensile strength, average transmission loss, and damping peak as indexes, the influence degree of different factors and levels on the properties of acoustic composites was quantitatively discussed and analyzed. The matrix weight analysis was employed to optimize the material formula of rubber composites, and the corresponding influence weight was given. Results showed that the acoustic composite with blending ratio of 70/30 for NBR/CR matrix had preferable mechanical and acoustic properties; adding mica powder (MP) and montmorillonite (MMT) in matrix contributed to improve all above three indexes owing to their specific lamellar structures; hollow glass beads (HGB) had a positive influence on improving acoustic property due to its hollow microcavities, however, it had a negative impact on damping property because of the smooth spherical surfaces. Accordingly, the optimal formulation was found to be NBR/CR blending ratio of 70/30, MP of 10 phr (per hundred rubber), HGB of 4 phr, and MMT of 10 phr.

Study on Sound-Insulation Performance of an Acoustic Metamaterial of Air-Permeable Multiple-Parallel-Connection Folding Chambers by Acoustic Finite Element Simulation.

In order to achieve a balance between sound insulation and ventilation, a novel acoustic metamaterial of air-permeable multiple-parallel-connection folding chambers was proposed in this study that was based on Fano-like interference, and its sound-insulation performance was investigated through acoustic finite element simulation. Each layer of the multiple-parallel-connection folding chambers consisted of a square front panel with many apertures and a corresponding chamber with many cavities, which were able to extend both in the thickness direction and in the plane direction. Parametric analysis was conducted for the number of layers nl and turns nt, the thickness of each layer L2, the inner side lengths of the helical chamber a1, and the interval s among the various cavities. With the parameters of nl = 10, nt = 1, L2 = 10 mm, a1 = 28 mm, and s = 1 mm, there were 21 sound-transmission-loss peaks in the frequency range 200-1600 Hz, and the sound-transmission loss reached 26.05 dB, 26.85 dB, 27.03 dB, and 33.6 dB at the low frequencies 468 Hz, 525 Hz, 560 Hz, and 580 Hz, respectively. Meanwhile, the corresponding open area for air passage reached 55.18%, which yielded a capacity for both efficient ventilation and high selective-sound-insulation performance.

Research on the Sound Insulation Performance of Composite Rubber Reinforced with Hollow Glass Microsphere Based on Acoustic Finite Element Simulation.

The composite rubber reinforced with hollow glass microsphere (HGM) was a promising composite material for noise reduction, and its sound insulation mechanism was studied based on an acoustic finite element simulation to gain the appropriate parameter with certain constraint conditions. The built simulation model included the air domain, polymer domain and inorganic particles domain. The sound insulation mechanism of the composite material was investigated through distributions of the sound pressure and sound pressure level. The influences of the parameters on the sound transmission loss (STL) were researched one by one, such as the densities of the composite rubber and HGM, the acoustic velocities in the polymer and inorganic particle, the frequency of the incident wave, the thickness of the sound insulator, and the diameter, volume ratio and hollow ratio of the HGM. The weighted STL with the 1/3 octave band was treated as the evaluation criterion to compare the sound insulation property with the various parameters. For the limited thicknesses of 1 mm, 2 mm, 3 mm and 4 mm, the corresponding optimal weighted STL of the composite material reached 14.02 dB, 19.88 dB, 22.838 dB and 25.27 dB with the selected parameters, which exhibited an excellent sound insulation performance and could promote the practical applications of the proposed composite rubber reinforced with HGM.

Sound Insulation Performance of Furfuryl Alcohol-Modified Poplar Veneer Used in Functional Plywood.

Plywood has poor sound insulation due to its insufficient areal density, which cannot satisfy the demands of an indoor acoustic environment. This report proposed to use furfuryl alcohol to impregnate poplar veneer as a raw material for plywood and explored the sound insulation potential of furfuryl alcohol-modified poplar veneer. The effect of different formulations on the sound insulation performance of modified veneers was discussed, such as furfuryl alcohol concentrations, catalyst categories, and solvent categories. The weight percent gain (WPG) and areal density (AD) were used to evaluate the impregnation effectiveness of furfuryl alcohol modification. The sound insulation was measured by the impedance tube method. The results showed that the WPG of the furfuryl alcohol-modified veneers was evident, and the AD was effectively improved. Furthermore, the average sound insulation of furfuryl alcohol-modified poplar veneer was 25.68~40.10 dB, which increased by 10.8~19.1% compared with that of unmodified veneer. The modified veneer with 50% furfuryl alcohol concentration, taking isopropanol as a solvent, and maleic anhydride as a catalyst, had the optimal sound insulation performance. At the same time, the cell microstructure and chemical components were characterized by scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET) theory to explain the sound insulation mechanism further. The results showed that the distortion of cell walls was improved, suggesting a change in the mechanical properties of the cell wall. At the same time, more micropores formed since the filling of furfuryl alcohol resin, yielding a tortuous propagation pathway, so the sound insulation performance improved. Finally, it demonstrated the potential of furfuryl alcohol-modified poplar veneer as raw material to prepare plywood with excellent sound insulation.

Thermal, Mechanical, and Acoustic Properties of Polydimethylsiloxane Filled with Hollow Glass Microspheres.

Polydimethylsiloxane (PDMS) is the most widely used silicon-based polymer due to its versatility and its various attractive properties. The fabrication of PDMS involves liquid phase cross-linking to obtain hydrophobic and mechanically flexible material in the final solid form. This allows to add various fillers to affect the properties of the resulting material. PDMS has a relatively low Thermal Conductivity (TC), in the order of 0.2 W/mK, which makes it attractive for thermal insulation applications such as sealing in construction. Although a further decrease in the TC of PDMS can be highly beneficial for such applications, most research on the thermal properties of PDMS composites have focused on fillers that increase the TC rather than decrease it. In the present work, we propose a simple and reliable method for making a PDMS-based composite material with significantly improved thermal insulation properties, by adding hollow glass microspheres (HGMs) to the mixture of the liquid base and the cross-linker (10:1 ratio), followed by degassing and heat-assisted crosslinking. We obtained a 31% reduction of thermal conductivity and a 60% increase in the elastic modulus of samples with HGM content of 17% by weight. At the same time, the sound insulation capacity of the PDMS-HGM composite is slightly decreased in comparison to pure PDMS, as a result of its lower density. Finally, the wettability of the samples had no dependence on HGM content.

Sound Insulation Performance of Composite Double Sandwich Panels with Periodic Arrays of Shunted Piezoelectric Patches.

The existing sandwich structure of the aircraft cabin demonstrates a good sound insulation effect in medium and high frequency bands, but poor in the low frequency band. Therefore, we propose an infinite new lightweight broadband noise control structure and study its sound transmission loss (STL). The structure is an orthogonally rib-stiffened honeycomb double sandwich structure with periodic arrays of shunted piezoelectric patches, and demonstrates lighter mass and better strength than the existing sandwich structure. The structure is equivalent according to Hoff's equal stiffness theory and the effective medium (EM) method. Using the virtual work principle for a periodic element, two infinite sets of coupled equations are obtained. They are solved by truncating them in a finite range until the solution converges. The correctness and validity of the model are verified by using simulation results and theoretical predictions. Eventually, a further study is performed on the factors influencing the STL. All the results demonstrate that the STL in low-frequency can be improved by the structure, and the sound insulation bandwidth is significantly broadened by adding shunted piezoelectric patches. The structure can provide a new idea for the design of broadband sound insulation.

Broadband Sound Insulation and Dual Equivalent Negative Properties of Acoustic Metamaterial with Distributed Piezoelectric Resonators.

Aiming at the unsatisfactory sound transmission loss (STL) of thin-plate structures in the low-mid frequency range, this paper proposes an acoustic insulation metamaterial with distributed piezoelectric resonators. A complete acoustic prediction model is established based on the effective medium method and classical plate theory, and the correctness is verified by the STL simulation results of the corresponding acoustic-structure fully coupled finite-element model. Moreover, the intrinsic relationship between the dual equivalent negative properties and STLs is investigated to reveal the insulation mechanisms of this metamaterial. Then, the influence of the geometric and material parameters on the double equivalent negative characteristics is studied to explore the broadband STL for distributed multi-modal resonant energy-dissipation modes in the frequency band of interest. The results show that the two acoustic insulation crests correspond to the dual equivalent negative performances, and the sound insulation in the low-mid frequency range is improved by more than 5 dB compared with that of the substrate, even up to 44.49 dB.