Average Absorption Coefficient Research Articles

Polyacrylonitrile nanofiber reinforced polyimide composite aerogels with fiber-pore interpenetrating structures for sound absorption, air filtration and thermal insulation

Prolonged noise pollution and its induced vibration would not only lead to the failure of instruments and equipment, but also endanger human health. In this work, with water-soluble polyamide acid (PAA) as the matrix and polyacrylonitrile nanofiber (PANNF) as the reinforcement, PANNF reinforced polyimide (PANNF/PI) composite aerogels with fiber-pore interpenetrating structures are fabricated by the method of “homogeneous mixing, freeze-drying and heat treatment”. PANNF that intersperses among pores can effectively inhibit the shrinkage of PI aerogels and enhance sound absorption and mechanical properties. With PANNF dosage of 10 wt%, the composite aerogels own the excellent sound absorption performance, with noise reduction coefficient (NRC) of 0.398, and average sound absorption coefficient (SAC) of 0.913 in the frequency range of 2000∼6300 Hz. The composite aerogels also have excellent mechanical properties, with Young's modulus of 12.42 kPa, stress loss of 3.64% and energy loss coefficient of 0.203. 10 wt% PANNF/PI composite aerogels also possess satisfying air filtration performance (97.2% filtration efficiency for PM2.5, quality factor (QF) value of 0.0396 Pa-1; and 99.4% filtration efficiency for PM10, QF value of 0.0567 Pa-1) and thermal insulation performance (thermal conductivity coefficient as low as 0.036 W/(m·K)).

Sound-absorption and mechanical properties of multisheet Gyroid lattice structures by stereolithography

Sound-absorbing materials with high load-bearing capacities are particularly important in many engineering applications. However, the low load-bearing capacity of conventional sound-absorbing materials limits their engineering applications. In this study, a multisheet parallel design concept is employed to design multisheet sound-absorbing and load-bearing lattice structures. Results show that the multisheet structure outperform the single-sheet lattice structure. The former exhibits a maximum absorption coefficient of 1, a 1x increase in the maximum average absorption coefficient, and a 127x increase in the maximum frequency compared with the latter. Under compression, the multisheet structure exhibits elastic modulus and yield strength values that are 0.43 and 0.52 times those of the single-sheet structure, respectively. Compared with different sound-absorbing structures, the multisheet structure exhibits remarkably high peak absorption coefficients and significant sound-absorbing performance advantages in specific frequency bands. This study presents a new design approach for creating multifunctional structures with sound-absorbing and load-bearing capabilities.

Broadband quasi-perfect sound absorption by a metasurface with coupled resonators at both low- and high-amplitude excitations

This study presents an acoustic metasurface (AM) consisting of coupled Helmholtz resonators with extended necks (HRENs) to effectively absorb broadband sound waves at both low and high amplitudes. A nonlinear acoustic model is proposed to characterize the AM performance based on the equivalent medium theory and the transfer matrix method. The nonlinear effects due to the high-amplitude excitation are considered by a velocity-dependence equivalent density of the air in the neck. The acoustic characteristics of an AM with two coupled HREN units at the sound pressure level (SPL) ranging from 100dB to 140dB are analytically, numerically, and experimentally investigated. Results from the three approaches show good agreement. The AM achieves quasi-perfect absorption (absorption coefficient α>0.9) in a wide range of SPL, while each HREN unit alone only performs well at around 130dB. The superior absorption performance of the AM at relatively low-amplitude excitation originates from the coherent coupling effects between adjacent units. With the increase of the sound level, the coupling effects tend to deteriorate; fortunately, the sound-vortex conversions occur, contributing to additional acoustic dissipation and thereby sustaining the quasi-perfect absorption capability. Based on the absorption mechanisms, a broadband AM with multiple coupled resonators is designed and manufactured. The measured averaged absorption coefficient of the AM is larger than 0.91 at the SPL below 140 dB (α>0.97 at 100dB) within [700,1100]Hz, and the compact thickness is of 1/18 wavelength at the lowest operating frequency. The findings in this study offer a promising approach to achieve highly efficient broadband sound absorption in a wide-ranging noise level, which holds great potentials in practical applications for noise control.

Enhanced sound absorption performance of porous ceramics with closed-pore structure

To enhance the sound absorption performance of porous ceramics at low-to-medium frequencies, a novel porous ceramic with closed-pore structure was prepared through the addition of silicon carbide (SiC), which foamed at high temperature. The porous ceramic with closed-pore structure exhibited a better combination property than those without such pore structure. The results revealed a 69.6% improvement in the average sound absorption coefficient of porous ceramics with closed-pore structure compared to those without closed-pore structure at 200–1600 Hz. Furthermore, the sound insulation performance of porous ceramics with closed-pore structure exhibited a 150% improvement at 160–1600 Hz. Moreover, these ceramics featured a high open porosity from 65% to 82% when combined with sufficient compressive strength of 1.21–5.13 MPa and low thermal conduction of 0.126–0.198 W/m∙K, indicating a considerably promising combination of properties for noise control and reduction in corrosive environments at high temperature. These results indicated that the porous ceramic had sufficient strength, low thermal conduction, and good sound absorption performance, which was expected to be applied as the building insulation and acoustic damping material. Besides, this study provides a new method for enhancing the sound absorption performance of porous ceramics at low-to-medium frequencies.

Modeling, design, and optimization of a dielectric elastomer acoustic liner

The modeling, design, and optimization of an embedded dielectric elastomer (DE) membrane acoustic liner is considered. The acoustic impedance of the liner is modified when the DE is subjected to voltage, resulting in a reduction of the in-plane stress. A lumped element model of an embedded dielectric elastomer acoustic liner is derived and validated in a normal incidence impedance tube and is subsequently used to optimize its performance. The optimization cost functions include (1) maximization of the average absorption coefficient over a targeted frequency range, 400 – 1600 Hz and (2) maximization of the change in the liner fundamental resonance frequency when the membrane is activated. Good agreement between measured and predicted absorption is observed. Tuning of the resonant frequency requires a numerical solution for resonance using the imaginary part of the impedance, since a simple analytical expression for resonance cannot be derived due to the complex coupling between the acoustics of the liner and the electro-mechanics of the DE membrane. Nonetheless, resonant frequency shifts predicted with the lumped element model compare favorably to those measured with the activated liner sample, with a shift of 213 Hz.

Broadband low-frequency sound-absorbing metastructure based on an impedance-matching coiled-up cavity with continuously variable cross section

Proposed here is a metastructure based on a micro-perforated panel and an impedance-matching coiled-up cavity with continuously variable cross section, which achieves perfect absorption with a resonant frequency of 496 Hz and an absolute bandwidth (α ≥ 0.5) of 468 Hz. The structure thickness is ca. 1/13 of the operating wavelength λ in the deep subwavelength range. A relative bandwidth of 84.04%–111.67% is achieved through parametric studies. Physically, the continuous variation of the cavity cross section through which sound waves enter weakens the acoustic reflections generated by cross-sectional abruptness and enhances the impedance matching with the air. Furthermore, particle swarm optimization is coupled with a theoretical model to tailor the metastructure to realize the maximum absorption coefficient in the defined frequency range. It is shown theoretically that coherent coupling “weak resonance”—in which each unit individually exhibits imperfect absorption peaks—significantly improves the absorption performance in a broad frequency band through the coherent coupling effect. Finally, a hybrid metastructure using a parallel coupling sample is fabricated, and its acoustic properties are measured in an impedance tube. The average absorption coefficient of this metastructure is 0.934 in the quasi-perfect band (α > 0.9) from 400 to 650 Hz, and the thickness is only ca. λ/15. The unique innovation of a cavity with continuously variable cross section provides new ideas for designing broadband low-frequency sound-absorbing metastructures.

Low-frequency sound-absorbing metamaterial based on a series-parallel arrangement structure

Low-frequency noise has a long wavelength, decays very slowly, and is extremely penetrating. Traditional marine acoustic insulation materials have difficulty achieving effective control of this low-frequency noise. In this paper, a series-parallel arrangement structure of a low-frequency metamaterial is designed, which mainly comprises convoluted channels and Helmholtz cavities in series and a sandwich arrangement of multiple cells (as opposed to the traditional parallel arrangement). We use a numerical method to establish an acoustic-solid coupling model for the metamaterial, consider the influence of thermal and viscous losses on its sound absorption performance, and investigate the sound absorption characteristics and mechanisms of the single-cell and multicell structures. The metamaterial designed in this paper shows an average absorption coefficient of 0.97 in the range of 130–145 Hz. An experimental model was prepared by 3D printing, and the intended sound absorption effect was experimentally verified.

Study on a Hexagonal Acoustic Metamaterial Cell of Multiple Parallel-Connection Resonators with Tunable Perforating Rate.

The limited occupied space and various noise spectrum requires an adjustable sound absorber with a smart structure and tunable sound absorption performance. The hexagonal acoustic metamaterial cell of the multiple parallel-connection resonators with tunable perforating rate was proposed in this research, which consisted of six triangular cavities and six trapezium cavities, and the perforation rate of each cavity was adjustable by moving the sliding block along the slideway. The optimal geometric parameters were obtained by the joint optimization of the acoustic finite element simulation and cuckoo search algorithm, and the average sound absorption coefficients in the target frequency ranges of 650-1150 Hz, 700-1200 Hz and 700-1000 Hz were up to 0.8565, 0.8615 and 0.8807, respectively. The experimental sample was fabricated by the fused filament fabrication method, and its sound absorption coefficients were further detected by impedance tube detector. The consistency between simulation data and experimental data proved the accuracy of the acoustic finite element simulation model and the effectiveness of the joint optimization method. The tunable sound absorption performance, outstanding low-frequency noise reduction property, extensible outline structure and efficient space utilization were favorable to promote its practical applications in noise reduction.

Multifunctional elastic aerogels of nanofibrous metal−organic framework for thermal insulation and broadband low-frequency sound absorption

Generally, porous materials are good candidates for sound absorption and thermal insulation. However, low sound absorption coefficient under 1000 Hz, low mechanical stability and poor consistency of performance hinders the wide applications of porous materials. In this study, we develop a kind of aerogel (PAN@ZIF-8-Kevlar aerogel) with well-designed hierarchical and tortuous porous structures to consume the low-frequency sound, hinder thermal transmission. The sound absorption coefficient of the PAN@ZIF-8-Kevlar is as high as 0.99 at 500Hz; average sound absorption coefficient of the material at 250, 500, 750, and 1000 Hz reaches 0.73. In addition, the thermal conductivity of PAN@ZIF-8-Kevlar is similar to that of air, thermal conductivity is near that 24.61 mW m−1 K−1. Notably, the PAN@ZIF-8-Kevlar aerogels also exhibit durable and stable cycling performance. The material retains its structural stability and retains its original low frequency sound absorption properties after 500 compression cycles. The hydrophobic material provides them with good moisture resistance (the water contacts angle (WCA) of PAN@ZIF-8-Kevlar-90 wt% aerogel was 143.3°). These favorable properties indicate that the composite nanofibers aerogel has potential options for sound absorption, thermal insulation in vehicles, buildings, and indoor reverberation.

Reparation and characterization of carbon nanotubes coated on expanded perlite as sound absorption composite materials

A sound absorption perlite brick coated with carbon nanotubes (CNTs)/perlite slurry was prepared by two kinds of Chemical Vapor Deposition (CVD) methods (one-step method and two step method) to develop an efficient sound absorption composite material in full frequency. The characterization results indicated that CNTs delivered by two-step method are denser and purer than that delivered by one-step method. Moreover, the grain sizes played an important role in the acoustic performance of sound absorption perlite brick. The average sound absorption coefficient of sound absorption perlite bricks decreased steadily and significantly with incremental grain size. In addition, the effects of CNTs on the acoustic performance of the materials were explored. The results showed that the sound absorption coefficients of the bricks coated with CNTs/perlite slurry were all over 0.7 when the frequency was above 500 Hz, and the highest value reached 0.96, indicating a positive influence of the CNTs coatings.

Optimized membrane-type acoustic metamaterials for alleviating engineering fatigue damage via lightweight optimization

Currently, most acoustic metamaterials have been designed for ultra-thin sizes to achieve perfect and low-frequency sound absorption or insulation performance, with less attention paid to fatigue damage in practical engineering applications. In this study, a lightweight optimal design for the membrane-type acoustic metamaterial is proposed for alleviating fatigue damage. Normally, the elastic membrane was decorated with several metal platelets for tuning the natural frequencies in the membrane-type acoustic metamaterial. However, the sound waves may cause the fatigue damage to the membrane in the long-term engineering applications. The fatigue damage caused by sound can be significantly alleviated by the proposed lightweight optimization for the membrane-type acoustic metamaterial. To maintainthe low-frequency sound absorption performance while achieving the lightweight purpose, a multi-objective particle swarm optimization algorithm is applied to the lightweight optimization of the proposed structure. The fatigue simulation results showed that the corresponding minimum fatigue life is increased by 10.27% using the proposed lightweight approach. Furthermore, the experiments using the dual port measurement without backing were conducted to investigate sound absorption performance of the proposed structure before and after lightweight optimization. Both of theoretical and experimental results showed that the average sound absorption coefficients of the proposed structure after optimization are 41.6% and 22.2% larger than those before optimization, respectively. The total weight of the proposed structure after optimization is 25.9% lighter than that before optimization.

Porous ceramics based on high-thermal-stability Al2O3–ZrO2 nanofibers for thermal insulation and sound absorption applications

Al2O3 fibers are promising candidates for porous ceramics, but the sudden growth of grains in the fibers above 1200 °C will limit their applications for high temperature. Herein, we reported the successful fabrication of the Al2O3–ZrO2 nanofibers by electrospinning and the nanofiber-based porous ceramics by a combination of gel-casting, freeze-drying and high-temperature sintering. Results show that the addition of Zr could greatly improve the thermal stability (up to 1400 °C) of the Al2O3-based nanofibers, owing to the inhibition of the sudden growth of the grains in the fibers at high temperature. The Al2O3–ZrO2 nanofiber-based porous ceramics after sintering at 1100–1400 °C possessed a multi-level pore structure and exhibited high thermal stability, ultra-high porosity (97.79–98.04%), ultra-low density (0.075–0.091 g/cm3) and thermal conductivity (0.0474–0.0554 W/mK), and excellent sound absorption performance with the average sound absorption coefficient of 0.598–0.770. These porous ceramics are expected to be employed in the fields of high-temperature thermal insulation and sound absorption.

Sound Absorption Performance and Mechanism of Aluminum Foams with Double Main Pore‐Porous Cell Wall Structure

To enhance the sound absorption performance of open‐cell aluminum foam, the double main pores‐porous cell walls (DMP‐PCW) aluminum foams via infiltration casting of preforms mixed with two sizes of NaCl particles are prepared. The pore structure, sound absorption performance, and mechanism of DMP‐PCW aluminum foam are investigated. The pore structure consists of double‐sized main pores similar to the NaCl particles and the cell wall pores formed by the connections between NaCl particles. It is found that the static flow resistivity of DMP‐PCW aluminum foam reaches an optimum value of 28105 Pa.s m−2 when the volume proportion of small main pores increases, the size of cell wall pores decreases, and the number of cell wall pores per unit main pore surface area (NPPA) increases. At 800–6300 Hz, the average absorption coefficient is 0.89. In addition, the Wilson model predicts the sound absorption properties of DMP‐PCW aluminum foam. The predicted values agree well with the measured values. The finite‐element acoustic simulations and dynamic viscous‐thermal permeability calculations reveal that the improved sound absorption performance of DMP‐PCW aluminum foam is correlated to the enhanced sound transmission caused by increased NPPA and increased viscous‐thermal loss due to the double main pore structure.

PHYSICAL CHARACTERIZATION OF STARCH-COTTON FIBER COMPOSITE SHEETS

Biobased materials had been identified as replacements for materials produced with chemical binders. Panels made from starch-cotton fiber waste composite materials fit perfectly into the category of biobased materials. The aim of this study was to measure the physical properties of starch-cotton fiber composite sheets. For sheets manufactured in 300x300x100mm3, the density and dimensional stability properties (water content, absorption rate, dimensional and volumetric swelling) were calculated. Density was determined in accordance with NF EN 323. Swellings were determined in accordance with EN 317. Water content was measured in accordance with EN 322. The sheets were manufactured with starch/water and starch/cotton fiber mass ratios respectfully equal to 0.25 and 1. We obtained an average water content of 4.27% and an absolute densityγs =1.13g/cm3. The average absorption coefficient measured was 62.88%. The measurement of dimensional variations showed that the panels underwent little deformation in the length and width directions. The average was 2%. In the thickness direction, on the other hand, dimensional variation was very high. This averaged 44%. The latter reflects the porosity of the material.

Characterization of Chemical Components and Optical Properties of Toluene Secondary Organic Aerosol in Presence of Ferric Chloride Fine Particles

Iron ion is the common transition metal ion in atmospheric aerosol, which can affect the components and optics of secondary organic aerosol (SOA). In the current study, the atmospheric photooxidation of toluene to produce SOA in the presence of ferric chloride fine particles is simulated in a smog chamber; on-line and off-line mass spectrometry and spectroscopic instruments are used to characterize constituents and optics of SOA. Compare with SOA formed in the absence of fine particles, the laser desorption/ionization mass spectra of toluene SOA generated in the presence of ferric chloride fine particles show ion peaks of m/z = 163 and 178, the UV-Vis spectra of the extracting solution for toluene SOA have peaks near 400 and 700 nm, and the electrospray ionization mass spectra contain peaks at m/z = 248 and 300. Based on this spectral information, it is shown that gaseous methylcatechol formed from photooxidation of toluene may react with iron ion on the surface of fine particles by complexing and oxidation–reduction, resulting in methylbenzoquinone products and metallo-organic complex ions such as [Fe(III)(CH3C6H3OO)]+, [Fe(III)(CH3C6H3 OO)2]− and [Fe(III)(CH3C6H3OO)Cl2]−. These products have strong light absorption ability, resulting in an increase in the averaged mass absorption coefficient (<MAC>) in the 200~1000 nm range and the MAC at 365 nm (MAC365) for toluene SOA, while <MAC> and MAC365 progressively increase with an increasing concentration of ferric chloride fine particles. These results serve as experimental references for the study of the formation mechanism and optical properties of metallo-organic complexes in atmospheric aerosol particles in regions experiencing high levels of fine particles of metal and automobile exhaust pollution.

Superior broadband sound absorption in hierarchical ultralight graphene oxide aerogels achieved through emulsion freeze-casting

Ultra-lightweight aerogels have profound applications as advanced multi-functional sound-absorbing materials. At present, conventional aerogels are associated with pores with sizes too small for effective sound absorption in the audible range. In this work, we fabricated a hierarchically porous ultralight graphene oxide (GO) aerogel through a novel emulsion freeze-casting process of an air-in-water emulsion. Microstructural analysis of the aerogels reveals a hierarchical pore morphology consisting of highly inter-connected pores produced from the emulsion bubbles and the micro-pores produced by freeze-casting. Sound absorption measurements revealed that an aerogel with a surfactant-to-GO weight ratio of 1:1 displays an average absorption coefficient of up to 0.86 at a broadband frequency range between 250 Hz and 6300 Hz, a vast improvement by up to 58% as compared to conventional GO aerogels. Numerical microstructural models were developed to model the acoustic absorption phenomenon and understand the mechanisms behind enhanced sound absorption. These models identified the enhanced acoustic absorption as attributed to the increased air-wetting area as derived from the high reticulation rate in this hierarchical microstructure. Overall, we demonstrate the potential of leveraging the highly inter-dependent material-process-structure relationships of GO aerogel production to achieve hierarchical GO aerogels as advanced ultralight sound-absorbing materials.

A design method for accurate acoustic impedance matching under coherent coupling of sound-absorbing subunits

Acoustic metamaterials have garnered significant attention as an effective means to control low-frequency noise. However, the accurate design of complex structures composed of multiple subunits is still a challenge due to local coupling effects. To address this issue, in this work, a new design method is proposed that accurately achieves impedance matching at the target frequency when subunits are coupled in parallel. The method is demonstrated using six Fabry–Pérot (F–P) tubes to achieve perfect sound absorption in the continuous band of 405–445 Hz and the discontinuous bands of 400–410 and 430–440 Hz. Theoretical results show an average absorption coefficient of 99.3% in the target frequency band, which is verified through an impedance tube experiment. In addition, this paper explores the stability of this method under complex design conditions and discusses the mechanism of the influence of subunit parameters on sound-absorption performance from the perspective of impedance matching. Overall, the proposed design method offers a promising approach to achieving broadband sound absorption using multiple coupled subunits. The results of this study provide valuable insights for future research and the design of acoustic metamaterials.

Design and characterization of multi-functional ternary composites with a hierarchically porous and filled-microperforated plate structure

To effectively overcome the shortcomings of common polyurethane (PU) insulation materials in terms of flame retardancy, mechanical properties, and sound absorption, a new multi-functional insulation material needs to be developed. Here, simple materials such as PU and silica aerogel (SA) combined with a special three-dimensional spacer fabric were used to design an innovative SA/PU/spacer fabric (SPS) ternary composite. It integrates a “hierarchically porous” and “filled-microperforated plate” structure. The superior structure design endows the new type of composite with a remarkable performance while maintaining low density (0.198 g/cm3). Specifically, the SPS composite was difficult to ignite by alcohol lamps compared with PU and SA/PU, and its thermal conductivity and compression modulus can reach 0.0392 W/(m·k) and 1.5 MPa, respectively. Moreover, the average absorption coefficient of the SPS composite (7.5 mm thickness) is 0.62, and its noise reduction coefficient per unit thickness (0.37) exceeds most reported sound absorption materials. From these results, the SPS composite provides a convenient and low-cost method to improve the overall performance of traditional thermal insulation material, which is of great significance in the fields of construction and transportation.

An efficient underwater absorber using pentamode metamaterials for broadband frequency

To further design and develop broadband and efficient underwater sound absorption structure, we propose a hybrid acoustic structure composed of a rubber matrix layer, cylindrical cavity, pentamode metamaterial (PM) layer, impedance matching layer, and steel backing plate. Thanks to the special properties of the PMs, the hybrid acoustic structure obtains excellent broadband sound absorption performance (SAP) in the frequency range of 500–10000 Hz than that of the traditional cavity acoustic structure. To this end, we study the design principle of PM structure and its metal–water characteristics and analyze the sound absorption mechanism at the working frequency. In addition, the research shows that the material and geometric parameters of the structure have a significant impact on the SAP. By the optimization of structural material parameters, the average absorption coefficient at 500–10000 Hz can further increase to above 0.781. Finally, we also demonstrate that the structure still has good SAP under a wide range of oblique incidence angles. The research in this paper opens up bright perspectives for the design of underwater anechoic coatings for multifunctional applications.

Research on a Design Method of Generative Model for Cavity Section Shape of an Anechoic Coating

One of the main challenges of the low frequency, broadband and strong sound absorption performance of an anechoic coating is the design of the cavity section shape. A generative-based design method was investigated in this study. Two generative-based models were trained to design the cavity section shape of anechoic coatings with a highly efficient absorption coefficient. The design results were compared in terms of mode collapse and average sound absorption coefficients. The results show that the WGAN-GP model can stably generate 100 new designs in only 3 seconds, dramatically speeding up the design process.