Sound Absorption Characterization of Natural Materials and Sandwich Structure Composites

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Article Sound Absorption Characterization of Natural Materials and Sandwich Structure Composites

Jichun Zhang 1, Yiou Shen 1,* ID , Bing Jiang 1 and Yan Li 1,2

1 School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, China; [email protected] (J.Z.); [email protected] (B.J.); [email protected] (Y.L.) 2 Key Laboratory of Advanced Civil Engineering Materials, Ministry of Education, Tongji University, Shanghai 200092, China * Correspondence: [email protected]; Tel.: +86-21-6598-5919

Received: 19 June 2018; Accepted: 5 July 2018; Published: 11 July 2018

Abstract: Natural fiber and wood are environmentally friendly materials with multiscale microstructures. The sound absorption performance of flax fiber and its reinforced composite, as well as balsa wood, were evaluated using the two-microphone transfer function technique with an impedance tube system. The microstructures of natural materials were studied through scanning electrical microscope in order to reveal their complex acoustical dissipation mechanisms. The sound absorption coefficients of flax fiber fabric were predicted using a double-porosity model, which showed relatively accurate results. The integrated natural materials sandwich structure was found to provide a superior sound absorption performance compared to the synthetic-materials-based sandwich structure composite due to the contribution of their multiscale structures to sound wave attenuation and energy dissipation. It was concluded that the natural-materials-based sandwich structure has the potential of being used as a sound absorption structure, especially at high frequency.

Keywords: plant ﬁber; balsa; sound absorption; microstructures; sandwich structures

1. Introduction Over the last couple of decades, there has been an increasing demand for improving the interior noise and indoor noise of aircraft, railway, automobile, and building compartments. Interior noise impairs people’s health and causes a decrease in passenger comfort and active safety performance [1]. The use of sound-absorbing materials is one of the present effective noise control technologies. The conventional industrial sound absorption and insulation materials are mineral ﬁbers, foam, and their composites. However, the usage of such materials is expensive, energy-consumptive, and adds more weight to the structures, which impacts their structural integrity. Sandwich structure composites have been extensively employed in aerospace, transportation, construction, and new energy ﬁelds as load-bearing components due to their high strength-to-weight ratio. Sandwich structure composites can be designable and multifunctional, and more functions can be provided besides bearing with proper design, such as impact tolerance, thermal isolation, radiation resistance, acoustic absorption, etc. Nowadays, growing attention has focused on environmentally friendly materials that can be recycled or need less energy for production and contribute minimum greenhouse gas emissions. Utilizing natural materials as face sheets or core materials for sandwich structures would provide superior load-bearing and fatigue properties. Moreover, this kind of structure provides good sound absorption properties compared to the conventional metallic and laminated composite structural members [2]. These natural materials and sandwich structures have the potential of replacing synthetic materials in applications with demands of weight constraints or

Aerospace 2018, 5, 75; doi:10.3390/aerospace5030075 www.mdpi.com/journal/aerospace Aerospace 2018, 5, 75 2 of 13 environmental friendliness, such as interior panels or floors in the aerospace industry, high-speed trains, and automobiles. Zheng and Li [3] found using plant-fiber-reinforced composite (PFRC) as skins of sandwich structures processes better sound absorption properties than using glass-fiber-reinforced composite (GFRC) as skins. The damping properties of flax-fiber-reinforced composite (FFRC) face sheets and GFRC face-sheet-based sandwich structures were compared by Petrone et al. [4,5]. Their research results indicated that FFRC face sheet sandwich panels have outperformed energy dissipation ability compared to the synthetic fiber face sheet ones. Sarginis et al. [6] investigated the sound and vibration damping characteristics of balsa-core-based sandwich beams. They found that the coincidence frequency of sandwich beams with natural fiber facing sheet and balsa core was tripled compared to a synthetic-core-based sandwich beam. Plant fiber is a kind of natural material, and it has been widely used as reinforcement in composite structures for the last decades due to their rich source, low price, specific strength, high specific modulus, and so on. Plant fiber has a complex, multiscale, hollow structure compared with synthetic fiber [7,8]. For example, flax fiber is extracted from the stalk of the plant, and the characteristics of its natural growth result in its multiscale structure, which can be simplified at two scales. At the mesoscopic scale, a bundle contains 10 to 40 elementary fibers (12–16 µm in diameter, 2–5 cm in length) which are linked together mainly by pectin. At the microscopic scale, different from the solid structure of synthetic fiber, each elementary fiber is made up of concentric cell walls called the primary cell wall (0.2 µm) and the secondary cell wall. The secondary cell wall is composed of three layers, namely the S1 layer (0.08–0.2 µm), the S2 layer (1–10 µm), and the S3 layer (0.1 µm). Most of the cellulose microfibrils are located in the S2 layer, which is a type of polymer and possesses viscoelastic properties. The S2 layer of flax fiber consists of numerous cellulose microfibril sublayers (L1, L2 ... Ln) which run parallel to each other and form a microfibril angle with the fiber direction. At the center of the elementary fiber, there is a 2–5-µm diameter small open channel called lumen, as revealed in Figure1a. It can be seen from Figure1b that hundreds of elementary fibers twist together to form a continuous yarn, and a gap of dozens of microns is between each elementary fiber. The unique microstructure brings many distinctive advantages for plant fiber, such as superior energy absorption [9], acoustic [10] and damping [11] performances, etc. Natural fiber was found to be a good acoustically absorbent material due to the hollow structures which efficiently convert acoustic energy into mechanical and heat energy [12]. The investigations on the acoustic absorption properties of natural-fibrous-material-reinforced composites are limited; however, several scholars have experimentally studied the sound absorption coefficients of plant-fiber-reinforced composites (PFRC) and found their sound absorption coefficients (SAC) were much higher than those of the synthetic-fiber-reinforced composites at 100–2000 Hz frequency level [12,13]. Yang and Li [14] also found that the Garai–Pompoli model and Delany–Bazley model have good agreement with the experimental results. However, the acoustical dissipation mechanisms of plant fiber materials were not plenarily analyzed in these studies and the multiscale hollow structure of plant fibers was not reflected by these two models. Therefore, the multiscale structure of plant fiber should be considered during sound absorption mechanisms analysis as well as the acoustic properties prediction. Olny and Boutin [15] theoretically studied the acoustic wave propagation in media with two interconnected networks of pores of very different characteristic sizes. In this study, flax fiber yarn was considered as a double-porosity media, and this double-porosity model was employed to predict the acoustic properties of flax fiber. The relatively high mechanical properties and low density of balsa make it attractive for cores in sandwich panels, and more importantly, sustainable. To date, there are no engineered materials suitable for sandwich panel cores with a similar combination of mechanical properties and low density [16]. Balsa wood can be considered as a kind of cellular material; it is composed of different size cell fibers, rays, and vessels, and there are also many micropores connecting each grain. It can be seen from Figure2a,b that the fibers are long prismatic cells, resembling a hexagon in cross-section, ranging from 20 to 40 µm in diameter. The rays are brick-like parenchyma cells ranging from 20 to 40 µm Aerospace 2018, 5, 75 3 of 13 in cross-section. The vessels are long, tubular structures that run axially along the trunk of the tree, and theAerospace diameter 2018, 5, x of FOR the PEER vessels REVIEW is 200–350 µm. Tiny micropores with a diameter of a few 3 of microns 13 generallyAerospace exist 2018 on, 5, thex FOR cell PEER wall REVIEW as seen in Figure2c, which make it a kind of semi-open cellular 3 structure. of 13 generally exist on the cell wall as seen in Figure 2c, which make it a kind of semi-open cellular The thickness of the double cell wall is about 0.8 to 3 µm in fibers as shown in Figure2d. Balsa wood generallystructure. existThe thickness on the cell of thewall double as seen cell in wall Figure is about 2c, which 0.8 to make3 μm init afibers kind as of shown semi-open in Figure cellular 2d. also possessesstructure.Balsa wood aThe multiscalealso thickness possesses structure, of athe multiscale double and cell the structure, cellwall wallis aboutand is similarthe 0.8 cell to wall3 to μm that is in similar of fibers natural asto shownthat fiber, of whichnaturalin Figure consistsfiber, 2d. of a primaryBalsawhich layerwood consists andalso of threepossesses a primary secondary a layermultiscale layers,and three structure, the secondary S1, S2,and and the layers, S3,cell and wallthe S1, theis similarS2, S2 and layer to S3, isthat and generally of the natural S2 layer the fiber, thickest is layerwhichgenerally [17]. The consists the cell thickest wallof a primary of layer wood [17].layer is a The compositeand cell three wall secondary structure of wood layers, inis a which composite the S1, cellulose S2, structure and microfibrils S3, andin which the S2 are cellulose layer reinforced is with hemicellulosesgenerallymicrofibrils the are thickest reinforced and lignin.layer with [17]. Therefore, hemicelluloses The cell wall this andof structure wood lignin. is Therefore, lowersa composite the this densitystructure structure and inlowers which has the a cellulose competitive density mechanicalmicrofibrilsand has performance a competitive are reinforced withmechanical with commercial hemicelluloses performance close-cell and with lignin. foam.commercial Therefore, Many close-cell researchersthis structure foam. have lowersMany investigated researchersthe density the mechanicalandhave has investigated performance a competitive the ofmechanical mechanical balsa [18 performance, 19performance] and balsa-based of with balsa commercial [18,19] sandwich and close-cell balsa-based structures foam. sandwich [20 Many,21], andresearchers structures they found the balsahave[20,21], coreinvestigated and sandwiches they the found mechanical were the balsa stiffer performance core than sandwiches traditional of balsa were [18,19] polymer-based stiffer and balsa-based than traditional sandwiches sandwich polymer-based and structures have better energy[20,21],sandwiches absorption and and they capacity. have found betterOnly the energy balsa a few absorption core researchers sandwiches capacity. have were focusedOnly stiffer a few on thanresearchers the acoustic traditional have absorption polymer-basedfocused on properties the sandwichesacoustic absorption and have properties better energy of this absorption natural cellular-material-based capacity. Only a few researchers sandwich havestructure, focused and on they the of this natural cellular-material-based sandwich structure, and they believed that balsa, with a high acousticbelieved absorptionthat balsa, withproperties a high ofvolume this natural of pores, cellular-material-based also has great potential sandwich as noise absorptionstructure, and material they volumebelieved[6]. of pores, that alsobalsa, has with great a high potential volume asof pores, noise also absorption has great material potential [ 6as]. noise absorption material [6].

Figure 1. (a) Cross-section of an elementary flax fiber and (b) flax fiber yarns. Figure 1. (a) Cross-section of an elementary flax fiber and (b) flax fiber yarns. Figure 1. (a) Cross-section of an elementary flax fiber and (b) flax fiber yarns.

Figure 2. SEM micrographs of balsa wood (a) cross-section; (b) longitudinal view; (c) micropores on Figurefibers; and2. SEM (d) multilayermicrographs cell of wall. balsa L, wood R, and (a T) cross-section;refer to the longitudinal, (b) longitudinal radial, view; and (tangentialc) micropores axes on of Figure 2. SEM micrographs of balsa wood (a) cross-section; (b) longitudinal view; (c) micropores on fibers;symmetry. and (d) multilayer cell wall. L, R, and T refer to the longitudinal, radial, and tangential axes of ﬁbers; and (d) multilayer cell wall. L, R, and T refer to the longitudinal, radial, and tangential axes symmetry. of symmetry.Considering the complicated multiscale structure of natural materials, different acoustic absorptionConsidering mechanisms the complicated may be found multiscale for the structure natural materials of natural and materials, their composite different structures acoustic absorption mechanisms may be found for the natural materials and their composite structures

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Considering the complicated multiscale structure of natural materials, different acoustic absorption mechanisms may be found for the natural materials and their composite structures compared to the synthetic fiber and reinforced composites. However, the above studies have not focused on this point, thus the sound absorption mechanism of natural materials and its composite structures is still unclear. In this paper, the sound absorption performance of flax fiber, reinforced composite, balsa wood, and the integrated sandwich structure composites were evaluated using the impedance tube method. The acoustical dissipation mechanisms were analyzed on the point of view of their unique multiscale microstructures. With a purpose of comparison, commonly used sandwich composites with glass fiber face sheets and polyethylene terephthalate (PET) foam core were also investigated.

2. Materials and Experiments

2.1. Materials Flax-fiber-fabric-reinforced epoxy composite was used as skins and balsa wood was used as the core material for Flax-Balsa sandwich structures. Likewise, Flax-PET and Glass-PET sandwich structures were prepared for comparison. The unidirectional flax fabric was provided by Lineo Co. Ltd., Saint Martin Du Tilleul, France, and the unidirectional glass fabric was provided by Zhejiang Mengtai Reinforced Composite Material Co. Ltd., Jiaxing, China. These two fabrics had similar areal density, however, the thickness of the glass fiber fabric was only half of the flax fiber fabric due to the density reason. The NPEL-128 epoxy was supplied by Shanghai Zhongsi Industry Co. Ltd., Shanghai, China. The balsa wood and the AIREX® T92.100 close-cell PET foam were supplied by Zhuhai Dechi Technology Co. Ltd., Zhuhai, China and 3A Composites (China) Ltd., Shanghai, China, respectively. The detail parameters of the flax and glass fiber fabric used for fabricating the sandwich structure skins are listed in Table1. Balsa wood and PET foam panel with a thickness of 10 mm were prepared as core materials to fabricate the sandwich structure. The basic parameters of these two types of core material are shown in Table2. Balsa wood and PET foam with similar density and porosity were selected in this study for comparison.

Table 1. The details of ﬁber fabrics used in manufacturing sandwich structures skins.

Speciﬁc Speciﬁc Fiber Density Mean Fiber Mean Yarn Areal Mean Materials Strength Modulus (g/cm3) Diameter (µm) Diameter (µm) Density (g/m2) Thickness (mm) (MPa/g/cm3) (GPa/g/cm3) Flax fabric 1.5 15 200 213.3 0.2 1034 55 Glass fabric 2.55 11 100 207.5 0.1 980 30

Table 2. The parameters of core materials used in manufacturing sandwich structures.

Density Cellular Axial Compression Transverse Compression Materials Porosity (kg/m3) Diameter (µm) Modulus (MPa) Modulus (MPa) Balsa wood 125 0.920 35–200 280–320 30–80 PET foam 112 0.933 500–700 90 90

2.2. Fabrication The epoxy resin, curing agent, and accelerating agent were mixed at the weight ratio of 100:80:1. Initially, 12 layers of flax fiber fabrics and 24 layers of glass fiber fabrics were impregnated with the resin system and laid inside of a steel mold in stacking sequence of [0/90/0]2s and [02/902/02]2s, respectively. In addition, 6 layers of impregnated flax fiber fabrics were laid on the top and bottom of the core material with stacking sequence of [0/90/0]s in order to fabricate flax-fiber-reinforced composite (FFRC) skin-based sandwich panels. A similar process was used for GFRP skin-based sandwich panels (12 layers on each side, [02/902/02]s). The resulting laminates and sandwich prepreg Aerospace 2018, 5, 75 5 of 13 systems were then respectively placed in a hot press machine under pressure of 1 MPa and temperature ◦ Aerospaceof 120 C2018 for, 5,2 x hFOR for PEER complete REVIEW curing. Two types of composite laminate, being flax-fiber-reinforced 5 of 13 Aerospace 2018, 5, x FOR PEER REVIEW 5 of 13 composite (FFRC) and glass-fiber-reinforced composite (GFRC), with mean thickness of 4 mm and mmfiber and volume fiber fractionvolume fraction of 50% wereof 50% manufactured. were manufactured. Three typesThree oftypes sandwich of sandwich structures, structures, Flax-Balsa, Flax- Balsa,Flax-PET, Flax-PET, and Glass-PET, and Glass-PET, with 2 mmwith thickness2 mm thickness and 50% and fiber 50% volume fiber volume fraction fraction of composite of composite skin on skineach on side each were side manufactured, were manufactured, in which in Flax-Balsawhich Flax-Balsa and Flax-PET and Flax-PET denote denote sandwich sandwich structures structures based basedon FFRC on skinsFFRC andskins balsa and orbalsa PET or foam PET cores,foam respectively.cores, respectively. The thickness The thickness of the balsaof the panel balsa waspanel on was the ontangential the tangential direction direction of the balsaof the wood. balsa wood. The Glass-PET The Glass-PET denotes denotes a sandwich a sandwich structure structure with GFRCwith GFRC skins skinsand PET and foam PET core,foam ascore, illustrated as illustrated in Figure in Figure3. 3.

Figure 3. Schematic diagram of sandwich structures.

2.3. Acoustic Property Measurement Sound absorption measurement was performed using the impedance tube method. The sound absorption coefficientcoefficient (SAC) of sandwich composites was measured through a transfer function technique with the impedance tube tube facilities facilities manufactured manufactured by by BSWA BSWA Technology Co. Ltd., Ltd., Beijing, Beijing, China. The The SAC SAC is is defined defined as as the the ratio ratio of of the the absorbed absorbed sound sound energy energy and and the the incident incident sound sound energy. energy. The measurements were conducted according to ISO10534-2 standard at medium to high frequencies ◦ from 250 to 10,000 Hz at 25 °CC and 60% relative humidity, and at least three specimens were tested for each each group. group. The The measurements measurements were were composed composed of SW422, of SW422, SW477, SW477, and SW499 and SW499 impedance impedance tubes withtubes diameters with diameters of 100, of30, 100,and 30,16 mm and for 16 measuring mm for measuring frequencies frequencies from 250–2000, from 800–6300, 250–2000, and 800–6300, 2500– 10,000and 2500–10,000 Hz, respectively. Hz, respectively. Samples were Samples backed were up backed by a rigid up bywall a rigidwhich wall reflected which all reflected the incoming all the soundincoming energy. sound Two energy. microphones Two microphones were mounted were mounted at the at wall the wall of the of the tube tube to to measure measure the the sound sound pressure. Therefore,Therefore, if if the the incident incident sound sound energy energy was was known known and and the transmissionthe transmission sound sound energy energy could couldbe measured be measured with the with aid the of aid the of transfer the transfer function, function, the sound the sound energy energy absorbed absorbed by the by materials the materials could couldbe obtained, be obtained, as illustrated as illustrated in Figure in Figure4. 4. The SACs of 12 layers of flaxflax fiberfiber fabric and 24 layers of glass fiberfiber fabric with same thickness were initially measured at frequencyfrequency rangingranging from 250250 toto 40004000 Hz.Hz. The composite laminates and sandwich structures were carefully cut to the required size and wrapped with Teflon Teflon tape provided by 3M Company, Maplewood, MN, USA to prevent air leaks betweenbetween the sample and tube. All the ◦ samples were put in the oven at 120 °CC for 4 h to remove moisture.

Figure 4. Instrumentation for the transfer function method of measuring sound absorption Figure 4. 4.Instrumentation Instrumentation for for the transfer the transfer function function method method of measuring of measuring sound absorption sound coefficients. absorption coefficients. 3. Results and Discussions 3. Results and Discussions 3.1. Sound Absorption Performance of Flax Fiber and Its Reinforced Composite 3.1. Sound Absorption Performance of Flax Fiber and Its Reinforced Composite The tested sound absorption coefficients of unidirectional flax fiber fabric and glass fiber fabric are presentedThe tested in sound Figure absorption5 as a function coefficients of frequency. of unidirectional The results showflax fiber that fabric in the and test frequencyglass fiber rangefabric are presented in Figure 5 as a function of frequency. The results show that in the test frequency range of 250–4000 Hz, flax fiber fabric has superior acoustic absorption ability compared to glass fiber fabric. It can be seen from Figure 5 that the SACs of flax fiber fabric were higher than 0.5 when the sound wave frequency was over 1000 Hz, which means over half of the incident sound energy was absorbed by the fabric. In this study, the critical frequency of flax fiber fabric was 3200 Hz, which means the

Aerospace 2018, 5, 75 6 of 13 of 250–4000 Hz, flax fiber fabric has superior acoustic absorption ability compared to glass fiber fabric. It can be seen from Figure5 that the SACs of flax fiber fabric were higher than 0.5 when the sound waveAerospace frequency 2018, 5, x FOR was PEER over REVIEW 1000 Hz, which means over half of the incident sound energy was absorbed 6 of 13 by the fabric. In this study, the critical frequency of flax fiber fabric was 3200 Hz, which means the SACSAC reachedreached aa peakpeak valuevalue ofof 0.960.96 atat 32003200 HzHz andand showedshowed aa relativelyrelatively stablestable soundsound absorptionabsorption abilityability afterwards.afterwards. Clearly,Clearly, thethe maximummaximum SACSAC ofof glassglass isis muchmuch lowerlower thanthan thatthat ofof thethe flaxflax fiber,fiber, onlyonly 0.580.58 inin thethe testtest frequencyfrequency range,range, andand thethe criticalcritical frequencyfrequency isis higherhigher thanthan thatthat ofof thethe flaxflax fiberfiber (>4000(>4000 Hz).Hz). ThisThis meansmeans flaxflax fibersfibers processprocess notnot onlyonly outstandingoutstanding soundsound absorptionabsorption capabilitycapability butbut alsoalso widerwider soundsound absorbingabsorbing frequencyfrequency range.range.

1.0

0.8

0.6

0.4

0.2 SoundAbsorptionCoefficient Flax Glass

0.0 0 1000 2000 3000 4000 Frequency (Hz)

FigureFigure 5.5. TheThe experimentalexperimental soundsound absorptionabsorption coefficientscoefficients ofof unidirectionalunidirectional flaxflax andand glassglass fiberfiber fabricsfabrics withwith frequency.frequency.

It is well known that the mechanism of sound attenuation for fibrous materials is due to the It is well known that the mechanism of sound attenuation for fibrous materials is due to the interaction between acoustic waves and fiber assemblies, and the resonant frequency of sound wave interaction between acoustic waves and fiber assemblies, and the resonant frequency of sound wave absorption could be mainly attributed to the vibration of fibrous materials [12]. For natural fibrous absorption could be mainly attributed to the vibration of fibrous materials [12]. For natural fibrous material, the acoustic absorption mechanism is more complicated compared to the synthetic fibrous material, the acoustic absorption mechanism is more complicated compared to the synthetic fibrous material due to its complex microstructure. As the mechanism illustrated in Figure 6 shows, the material due to its complex microstructure. As the mechanism illustrated in Figure6 shows, the hollow hollow and multiscale structure of flax fiber acts on the reduction and dissipation of sound energy in and multiscale structure of flax fiber acts on the reduction and dissipation of sound energy in several several manners. Firstly, the incident sound wave arouses the vibration of air molecules between manners.elementary Firstly, fibers the and incident within soundthe lumens, wave which arouses causes the vibration interaction of airbetween molecules air and between the fiber elementary cell wall. fibers and within the lumens, which causes interaction between air and the fiber cell wall. The resulting The resulting viscous resistance transfers this part of acoustic energy to thermal energy. Meanwhile, viscousthermal resistanceexchange transfersbetween neighboring this part of particles acoustic energyof fiber toalso thermal resultsenergy. in acoustic Meanwhile, attenuation. thermal Most exchange between neighboring particles of fiber also results in acoustic attenuation. Most importantly, importantly, differing from the solid structure of glass fiber, vibration and friction of microscale or differingeven nanoscale from the microfibrils solid structure in elementary of glass fiber, flax vibration fiber also and have friction an effect of microscale on the extra or even dissipation nanoscale of microfibrilssound energy in elementarybesides vibration flax fiber and also friction have an between effect on elementary the extra dissipation fibers, leading of sound to a energy more efficient besides vibrationconversion and from friction sound between energy elementary to thermal fibers,energy. leading It can be to aseen more from efficient Figure conversion 1a that there from existed sound a energylarge number to thermal of gaps energy. between It can adjacent be seen S2 from sublayers Figure in1a some that thereregions. existed In this a largecase, interfacial number of friction gaps betweenbetween adjacent sublayers S2 sublayers may in also some dissipate regions. energy. In this Moreover, case, interfacial the large frictionfiber yarn between diameter adjacent of flax sublayersfiber fabric may causes also bigger dissipate interspace energy. Moreover,between adjacent the large fibers, fiber yarnwhich diameter leads to ofthe flax better fiber mobility fabric causes of air biggerin fabric interspace and smaller between surface adjacent impedance fibers, which of the leads composite. to the better Therefore, mobility the of airsound in fabric wave and can smaller easily surfacetransmit impedance into the flax of thefiber composite. fabric and Therefore, be absorbed the or sound dissipated, wave can consequently easily transmit enhancing into the the flax sound fiber fabricabsorption and beof absorbedthe materials. or dissipated, This explains consequently the high sound enhancing absorption the soundcoefficients absorption of flax fiber of the in materials.Figure 5. This explains the high sound absorption coefficients of flax fiber in Figure5.

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FigureFigure 6.6. AcousticAcoustic energyenergy dissipationdissipation mechanismsmechanisms ofof ﬂaxflax ﬁberfiberyarns. yarns.

The superior sound absorbing ability of plant fiber is essentially attributed to its multiscale The superior sound absorbing ability of plant fiber is essentially attributed to its multiscale micromorphology. However, the multiscale hollow structure of plant fibers was not reflected by the micromorphology. However, the multiscale hollow structure of plant fibers was not reflected by the Garai–Pompoli model or Champoux–Allard model in predicting the SACs of plant fibers. In this Garai–Pompoli model or Champoux–Allard model in predicting the SACs of plant fibers. In this study, study, the lumen of flax fiber was considered in predicting its acoustic properties. Firstly, an idealized the lumen of flax fiber was considered in predicting its acoustic properties. Firstly, an idealized flax flax elementary fiber was assumed as a 15-μm diameter tubular cell with a 5-μm diameter lumen elementary fiber was assumed as a 15-µm diameter tubular cell with a 5-µm diameter lumen inside. inside. The effective density ρeq and the effective modulus Keq of unidirectional flax fiber fabric were The effective density ρ and the effective modulus K of unidirectional flax fiber fabric were predicted predicted according toeq the double-porosity model eqdeveloped by Olny and Boutin [15] as shown in according to the double-porosity model developed by Olny and Boutin [15] as shown in Equations (1) Equations (1) and (2): and (2): 1 1 − ∅ −1 휌(휔) = 1+ 1 − ∅ p (1) ρeq(ω) =휌(휔) 휌+(휔) (1) ρp(ω) ρm(ω) 1 (1 − ∅ )퐹 (휔, 휔 ) − 푃 ( ) ! 1 (2) 퐾 휔 = 1+ 1 − ∅p Fd(ω,ωd), 휔 ≈ P0 ( ) =퐾(휔) + 퐾(휔) ∅휎≈푙 Keq ω , ωd 2 (2) Kp(ω) Km(ω) ∅mσmlp where subscripts ‘p’ and ‘m’ present the mesoscopic scale (between fibers) and microscopic scale where subscripts ‘p’ and ‘m’ present the mesoscopic scale (between fibers) and microscopic scale (within (within the fiber). lp and lm present the characteristic length in these two scales. ρp(ω), ρm(ω), and Kp(ω) thewere fiber). obtainedlp and bylm pluggingpresent the the characteristic basic mesoscopic length and in microscopic these two scales. scalesρ pacoustic(ω), ρm( ωparameters), and Kp(ω into) were the obtained by plugging the basic mesoscopic and microscopic scales acoustic parameters into the Champoux–Allard model, Equations (3) and (4). Fd is a pressure diffusion function. In this study, the Champoux–Allard model, Equations (3) and (4). F is a pressure diffusion function. In this study, test frequency ω < ωd, Fd ≈ 1 and voids at both scales participatedd in the attenuation of acoustic energy; thethe testsound frequency pressureω between< ωd, Fd fibers≈ 1 and and voids lumens at both are the scales same. participated in the attenuation of acoustic energy; the sound pressure between fibers and lumens are the same. 휌 훼 휎∅ 2훼 (3) 휌(휔) = 1− 푖 1s+ 푖휔휌휂  ∅  휔휌훼 휎∅훬 2 ρ0α∞  σ∅ 2α∞  ρeq(ω) = 1 − i 1 + iωρ0η  (3) ∅ ωρ0α∞ σ∅Λ 1 ∅ 8휂 푖휔휌푃 훬′ (휔) = 훾 − (훾 − 1) 1 − 푖 1 + (4) 퐾 훾푃 휔휌 푃 훬 휂 4   s −1  0 2  1 ∅  8η iωρ0Pr Λ  (ω) = γ − (γ − 1)1 − i 0 1 +  (4) where ω is the Kangular frequency,γP ω = 2πf, f is theωρ incidentP Λ 2 sound waveη frequency,4 ø is the porosity, eq 0  0 r  and α∞ is the tortuosity, which characterizes the buckling of channels in the material. For ideal fiber wherematerials,ω is α the∞ = angular1.34. Λ and frequency, Λ’ are viscousω = 2π fcharacteristic, f is the incident length sound and wavethermal frequency, characteristicø is the dimension, porosity, andwhichα∞ characterizeis the tortuosity, the average which characterizesdiameter of unit the length buckling for ofviscous channels loss inand the thermal material. loss, For respectively. ideal fiber materials,Λ and Λ’ areα∞ defined= 1.34. Λ asand Λ =Λ 1’/2 areπrL viscous, Λ’ = 2Λ characteristic, in which L = length 4ρf/πa and2ρb, r thermal = a/2, a is characteristic fiber diameter dimension, and ρb is whichthe fabric characterize density. the ρ0 average and γ arediameter air density of unit and length specific for viscous heat loss ratio, and respectively, thermal loss, and respectively. P0 is the 2 Λatmosphericand Λ’ are pressure. defined as PrΛ is= the1/2 PrandtlπrL, Λ’ number, = 2Λ, in defined which L as = P4rρ =f/ Cπpa/kρ, whereb, r = a/ C2,p isa isobaricis fiber diameter heat capacity, and μ is viscosity, and k is thermal conductivity. The Prandtl number characterizes the effect of the physical characteristic of air on the heat transfer process. The porosity can be obtained by Equation (5)

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ρb is the fabric density. ρ0 and γ are air density and speciﬁc heat ratio, respectively, and P0 is the atmospheric pressure. Pr is the Prandtl number, deﬁned as Pr = Cp/k, where Cp is isobaric heat capacity, µ is viscosity, and k is thermal conductivity. The Prandtl number characterizes the effect of the physical characteristic of air on the heat transfer process. The porosity can be obtained by Equation (5)

ρb ∅ = 1 − (5) ρ f

Aerospace 2018, 5, x FOR PEER REVIEW 8 of 13 where ρf is the fiber density. The resulting ρ and K were then substituted휌 into Equation (6) to get the acoustic characteristic eq eq ∅ = 1 − (5) 휌 impedance, Zeq, and wave number, keq, of flax fiber fabric. The sound absorption coefficients can then be obtainedwhere ρ usingf is theEquation fiber density. (9) The resulting ρeq and Keq were then substituted into Equation (6) to get the acoustic characteristic impedance, Zeq, and wave number, keq, of flax fiber fabric. Thes sound absorption coefficients can then q ρeq be obtained using Equation (9) Zeq = ρeqKeq, keq = ω (6) Keq 휌 푍 = 휌퐾, 푘 = 휔 (6) 2 퐾 Z − 1 Zeq α n = 1 − , Zn = −i cot h keqt (7) 푍 −Z1n + 1 푍 ρ0c0 α = 1 − , 푍 = −푖 coth (푘푡) (7) 푍 + 1 휌푐 where c0 is the sound velocity in air, t is the thickness of material. It can be observed from Equationswhere (3)–(7)c0 is the thatsound the velocity acoustic in air, parameters t is the thickness of material of material. are It related can be toobserved its physical from Equations parameters, including(3)–(7) fiber that the diameter, acoustic fiber parameters density, of material fabric density, are related and to fabricits physical thickness. parameters, It indicates including that fiber these physicaldiameter, parameters fiber density, of material fabric playdensity, a critical and fabric role thickness. on the sound It indicates absorption that these properties. physical parameters Itof can material be found play a from critical Figure role 7on that the thesound double-porosity absorption properties. model showed more accuracy predicting It can be found from Figure 7 that the double-porosity model showed more accuracy predicting results compared to the Champoux–Allard model [22], and the critical frequency of flax fiber fabric was results compared to the Champoux–Allard model [22], and the critical frequency of flax fiber fabric accuratelywas accurately estimated. estimated. This indicates This thatindicates the main that factorsthe main affecting factors the affecting sound the absorption sound absorption properties of plantproperties fiber materials of plant are fiber not onlymaterials fiber are diameter, not only density, fiber diameter, and fabric density, density, and butfabric the density, lumen diameterbut the is also anlumen important diameter parameter. is also an Theimportant double-porosity parameter. The model double-porosity can be used tomodel predict can thebe used sound to predict absorption coefficientsthe sound of plant absorption fibers coefficients effectively. of plant fibers effectively.

1.0

0.8

0.6

0.4 Flax-experiment

Flax-Allard's model 0.2 SoundAbsorptionCoefficient Flax-Double porosity model 0.0 0 1000 2000 3000 4000 Frequency (Hz) Figure 7. Comparison of experimental and theoretical sound absorption coefficients of unidirectional Figure 7. Comparison of experimental and theoretical sound absorption coefficients of unidirectional flax fiber fabrics with frequency. flax fiber fabrics with frequency. Figure 8 compares the experimental results on SACs of flax fiber and glass-fiber-reinforced Figurecomposites.8 compares FFRC laminate the experimental provides superior results sound on SACs absorption of flax capacity fiber and to glass-fiber-reinforcedGFRC especially at composites.higher frequency, FFRC laminate as expected. provides However, superior the soundSACs of absorption the composites capacity are obviously to GFRC much especially lower atthan higher frequency,the fiber as expected. fabric and However, the effective the SACs sound of absorption the composites frequency are obviouslyrange (SACs much value lower over than 0.2) the was fiber fabricreduced. and the This effective is due sound to the reason absorption that when frequency an incident range sound (SACs wave value reaches over the 0.2) surface was reduced. of material, This is due topartial the reason sound thatenergy when is reflected. an incident The reflected sound waveenergy reaches depends the on surfacethe surface of material,acoustic impedance, partial sound Z, which can be described using Equation (8). (8) 푍 = 퐸 ∙ 휌

where E is the modulus of the materials (sound wave incident direction) and ρ is the density of the materials. The sound wave can be easily reflected when Z is high and more sound energy is reflected.

Aerospace 2018, 5, 75 9 of 13 energy is reflected. The reflected energy depends on the surface acoustic impedance, Z, which can be described using Equation (8). p Z = E·ρ (8) where E is the modulus of the materials (sound wave incident direction) and ρ is the density of the materials. The sound wave can be easily reflected when Z is high and more sound energy is reflected. In contrast,Aerospace a 2018 substantial, 5, x FOR PEER part REVIEW of sound energy enters into the materials when Z is low. Apparently, 9 of 13 more sound energy was reflected by the composite and this limited its sound absorption ability. In contrast, a substantial part of sound energy enters into the materials when Z is low. Apparently, Furthermore,more sound the microstructureenergy was reflected of the by material the composite structure and hadthis alimited major its effect sound on absorption energy dissipation ability. at high frequencyFurthermore, due the to microstructure the very short of the wavelength material structure of sound had wave, a major which effect on made energy it easily dissipation eliminated at whenhigh transmitted frequency through due to the the very media. short Thewavelength SACs ofof puresound epoxy wave, arewhich very made low, it generallyeasily eliminated below 0.2. Thus,when the sound transmitted absorption through performance the media. The of compositeSACs of pure laminate epoxy are is mainlyvery low, dependent generally below on the 0.2. sound absorptionThus, the characteristics sound absorption of fiber. performance Previous of work composite [23] has laminate found is that mainly the multilayerdependent on structure the sound of flax fiber providedabsorption more characteristics internal interfacesof fiber. Previous (primary work cell [23] wall, has found S1, S2, that and the S3 multilayer layers of structure the secondary of flax cell wall, andfiber S2 provided layer consist more internal of numerous interfaces cellulose (primary microfibril cell wall, S1, sublayers) S2, and S3 and layers paths of the inside secondary of flax cell fiber to wall, and S2 layer consist of numerous cellulose microfibril sublayers) and paths inside of flax fiber dissipate energy compare to the solid synthetic fiber. Therefore, the damping property of flax fibers to dissipate energy compare to the solid synthetic fiber. Therefore, the damping property of flax fibers was much better and resulted in a higher damping ratio for its reinforced composite. This indicates was much better and resulted in a higher damping ratio for its reinforced composite. This indicates that thethat FFPC the FFPC possesses possesses higher higher energy energy dissipation dissipation andand acoustic–thermalacoustic–thermal conversion conversion capacity, capacity, and andit it also evidencesalso evidences its better its better acoustic acoustic absorption absorption performance performance than than that that of of GFRP. GFRP. Prabhakaran Prabhakaran et al. et [13] al. [13] also foundalso found FFRC FFRC process process better better SACs SACs and and damping damping factors factors compared to to GFRC. GFRC.

1.0 FFRP GFRP 0.8

0.6

0.4

0.2 Sound absorption coefficientSoundabsorption

0.0 0 2000 4000 6000 8000 10000 Frequency (Hz) Figure 8. The experimental sound absorption coefficients of FFRC and GFRC with frequency. Figure 8. The experimental sound absorption coefﬁcients of FFRC and GFRC with frequency. 3.2. Sound Absorption Performance of Balsa Wood 3.2. Sound Absorption Performance of Balsa Wood Figure 9 illustrates the SACs of balsa and PET foam, which indicate that these two types of core Figurematerials9 illustrates possess similar the SACs sound of absorption balsa and ability PET foam,along the which test frequencies. indicate that The these peak two value types of SACs of core materialsbelow possess 10,000 similarHz for these sound two absorption materials are ability 0.4 and along 0.42, the respectively. test frequencies. The maximum The peak SAC value of these of SACs belowtwo 10,000 porous Hz formaterials these twoat 10,000 materials Hz are are also 0.4 very and close, 0.42, being respectively. 0.85 and The 0.87. maximum The similar SAC density of these and two porousporosity materials of theseat 10,000 two Hz materials are also lead very to close, their essentially being 0.85 identical and 0.87. trend The similaron the sound density absorption and porosity of thesebehavior. two materials The cell wall lead of balsa to their is composed essentially of polymer identical materials, trend onand the the soundvisco-inertial absorption and thermal behavior. damping dominates the sound absorption behavior for polymeric foams. In low frequency stage, The cell wall of balsa is composed of polymer materials, and the visco-inertial and thermal damping thermal exchange is dominated in energy dissipation, whereas in high frequency, viscous resistance dominates the sound absorption behavior for polymeric foams. In low frequency stage, thermal is the main energy consuming method. The loss of sound energy is mainly due to the contribution of exchangethe porous is dominated sound absorption in energy dissipation,mechanism and whereas resonance in high absorption frequency, mechanism. viscous resistanceThe air molecules is the main energyperiodically consuming vibrate method. under The the loss influence of sound of sound energy waves; is mainly meanwhile, due to thethe contributionfriction between of thethe porousair soundmolecules absorption and mechanism cell wall cause and frictional resonance heat. absorption On the other mechanism. hand, the Thecompression air molecules and expanding periodically vibratedeformation under the inﬂuenceof air inside of of sound the cell waves; occurs; meanwhile, this transfers the partial friction sound between energy the into air thermal molecules energy. and cell wall causeIn terms frictional of balsa, heat. vibration On the and other friction hand, of cellulose the compression microfibrils and in the expanding cell wall deformationand interfacial of friction air inside of thebetween cell occurs; adjacent this transferssublayers partialcaused sound energy energy dissipation, into thermal which energy. is similar In terms to the of flax balsa, fiber. vibration In addition, the micropores in the cell wall of balsa consumed sound energy due to this complex inner geometry providing a tortuous sound path through the foam. It can be found that natural material has a complicated acoustical energy dissipation mechanism due to its natural growth multiscale structure which reveals a satisfactory sound absorption performance.

Aerospace 2018, 5, 75 10 of 13 and friction of cellulose microfibrils in the cell wall and interfacial friction between adjacent sublayers caused sound energy dissipation, which is similar to the flax fiber. In addition, the micropores in the cell wall of balsa consumed sound energy due to this complex inner geometry providing a tortuous sound path through the foam. It can be found that natural material has a complicated acoustical energy dissipation mechanism due to its natural growth multiscale structure which reveals a satisfactory soundAerospace absorption 2018, 5,performance. x FOR PEER REVIEW 10 of 13

1.0 Balsa 0.8 PET

0.6

0.4

0.2 Sound absorption coefficientSoundabsorption

0.0 0 2000 4000 6000 8000 10000 Frequency (Hz) Figure 9. The experimental sound absorption coefficients of balsa and PET foam with frequency. Figure 9. The experimental sound absorption coefficients of balsa and PET foam with frequency. 3.3. Sound Absorption Performance of Sandwich Structures 3.3. Sound Absorption Performance of Sandwich Structures An effective way to improve the acoustic absorption performance of composite laminate is Aninducing effective a porous way material to improve as the the core acoustic materialabsorption to cooperateperformance a sandwich structure. of composite Meanwhile, laminate the is inducingsound a wave porous can material be reflected as repeatedly the core materialby the stiff to composite cooperate skins a sandwichback to the interior structure. of the Meanwhile, cellular the soundcore material wave can for be energy reflected exchange repeatedly and dissipation. by the stiff Figure composite 10 shows skins the back variation to the of interiorSACs with of the cellularfrequency core material of Flax-Balsa, for energy Flax-PET, exchange and Glass-PET and dissipation. sandwich structures Figure 10 and shows their thecorresponding variation ofcore SACs materials. It can be seen from Figure 10 that the SACs increase with frequency for all the samples, with frequency of Flax-Balsa, Flax-PET, and Glass-PET sandwich structures and their corresponding reaching at least 0.8 at 10,000 Hz. For flax-fiber-reinforced composite skin-based sandwich structures, core materials. It can be seen from Figure 10 that the SACs increase with frequency for all the the SACs of the sandwich panel are higher than those of the core materials in all frequency levels. samples,The reachingSACs are clearly at least improved 0.8 at 10,000 for the Hz. balsa For if flax-fiber-reinforcedcooperated with FFRC compositeskins from 1600 skin-based Hz. The sound sandwich structures,absorption the SACs peak of of the the sandwich Flax-Balsa panel sandwich are higher panel isthan 0.65, those which of theis approximately core materials 63% in all and frequency 86% levels.enhancement The SACs arecompared clearly to improvedthat of the balsa for the and balsa FFRC, if respectively. cooperated On with the FFRCother hand, skins the from SACs 1600 of Hz. The soundPET foam absorption have been peak distinctly of the enhanced Flax-Balsa over sandwich40% at high panel frequency is 0.65, from which 4000 Hz is when approximately cooperated 63% and 86%with enhancement FFRC skins. It comparedcan be concluded to that that of flax the fiber balsa skin-based and FFRC, sandwich respectively. structures On provide the other better hand, the SACssound of absorption PET foam performance have been than distinctly their corresponding enhanced overcore materials 40% at high at a large frequency range of from frequency 4000 Hz whenband, cooperated and the with composite FFRC skins. skins Itdefinitely can be concluded play a significant that flax role. fiber The skin-based dashed line sandwich represents structures the effective sound absorption peak width (SACs value over 0.2) which has been enlarged from 4000– provide better sound absorption performance than their corresponding core materials at a large 8000 Hz (balsa) to 3150–8000 Hz (Flax-Balsa). Nevertheless, the SACs of the sandwich structure with range of frequency band, and the composite skins definitely play a significant role. The dashed line flax-fiber-reinforced skins are overall higher compared to the sandwich structures with glass fiber representsskins,the especially effective at soundhigher absorptionfrequency due peak to widththe high (SACs surface value acoustic over impedance 0.2) which and has beenlow sound enlarged from 4000–8000absorption ability Hz (balsa) of GFRC to 3150–8000skins. Thus, Hzchoosing (Flax-Balsa). the appropriate Nevertheless, fiber type the for SACscomposite of the skins sandwich will structureimprove with the flax-fiber-reinforced acoustic performance skins of sandwich are overall structures higher effectively. compared to the sandwich structures with glass fiberIt skins, is worth especially noting that at higherwhen cooperated frequency with due flax to thefiber high reinforced surface skins, acoustic the SACs impedance of Flax-Balsa and low soundsandwiches absorption are ability clearly of higher GFRC than skins. those Thus, of Flax-PET choosing despite the appropriate their core materials fiber type possessing for composite similar skins will improveacoustic thebehavior acoustic with performance the same skins. of This sandwich is due to structures the fact that effectively. PET foam has larger cell size ranging Itfrom is worth 500 to noting 700 μm, that whereas when the cooperated cell size of with balsa flax ranges fiber from reinforced 35 to 200 skins, μm. The the epoxy SACs resin of Flax-Balsa filled the layer of cells that contacted the skin during fabrication. After curing, the solid layer of epoxy resin sandwiches are clearly higher than those of Flax-PET despite their core materials possessing similar in Flax-PET sandwich structures was much thicker than that of Flax-Balsa; this limited the sound wave acousticentering behavior into the with core the materials same skins. and resulted This is duein its to relatively the fact lower that PETSACs. foam It is hasclear larger from Figure cell size 10 that ranging from 500the Flax-Balsa to 700 µm, sandwich whereas structure the cell sizehas the of best balsa sound ranges absorption from 35 behavior to 200 µ m.over The most epoxy of the resin frequency filled the layer ofband cells compared that contacted to the other the skintwo sandwich during fabrication. structures and After their curing, core materials. the solid The layer superior of epoxy acoustic resin in Flax-PETperformance sandwich of structuresthis environmentally was much friendly thicker sandwich than that structure of Flax-Balsa; plays a thisgreat limited role in thetheir sound unique wave multiscale microstructure advantages.

Aerospace 2018, 5, 75 11 of 13 entering into the core materials and resulted in its relatively lower SACs. It is clear from Figure 10 that the Flax-Balsa sandwich structure has the best sound absorption behavior over most of the frequency band compared to the other two sandwich structures and their core materials. The superior acoustic performanceAerospace 2018, of5, x this FORenvironmentally PEER REVIEW friendly sandwich structure plays a great role in their unique 11 of 13 multiscaleAerospace microstructure 2018, 5, x FOR PEER advantages. REVIEW 11 of 13 1.0 1.0 Flax-Balsa Flax-Balsa Flax-PET Flax-PET 0.8 0.8 Glass-PETGlass-PET BalsaBalsa PETPET 0.60.6

0.40.4

0.2

0.2coefficientSoundabsorption Sound absorption coefficientSoundabsorption

0.0 0.0 0 2000 4000 6000 8000 10000 0 2000Frequency 4000 (Hz) 6000 8000 10000 Frequency (Hz) FigureFigure 10. 10.The The experimental experimental sound sound absorption absorption coefficientscoefficients of of sandwich sandwich structures structures andand their their Figurecorresponding 10. The experimentalcore materials. sound absorption coefficients of sandwich structures and their corresponding core materials. corresponding core materials. Figure 11 illustrates the sound absorption mechanism of natural-material-based sandwich FigurestructureFigure 11 11composites. illustrates illustrates When the the sound sound sound absorptionenergy absorption (Ei) vertically mechanism mechanism incidents of of natural-material-based from natural-material-based one side of the structure, sandwich sandwich structurepartial composites. sound energy When is reflected sound (E energyr) by the (surfaceE ) vertically of the composite incidents skin. from The one incident side sound of the energy structure, structure composites. When sound energy (Ei i) vertically incidents from one side of the structure, partialis sound initially energy absorbed is reflected (Ea1) by ( E ther) by FFRC the surface skin through of the composite viscous resistance, skin. The thermal incident exchange, sound energy and is partial sound energy is reflected (Er) by the surface of the composite skin. The incident sound energy damping of fibers or microfibrils and multilayer cell walls. Partial energy transmits through top skin initially absorbed (Ea ) by the FFRC skin through viscous resistance, thermal exchange, and damping is initially absorbed1 (Ea1) by the FFRC skin through viscous resistance, thermal exchange, and (Et1) and some energy reflects by the interface of skin and core (Er1). Balsa core absorbs sound energy of fibers or microfibrils and multilayer cell walls. Partial energy transmits through top skin (Et1) damping(Ea2) mainly of fibers through or microfibrils visco-inertial and and multilayer thermal damping, cell walls. multilayer Partial energycell wall transmits damping, throughand air flow top skin and some energy reflects by the interface of skin and core (Er ). Balsa core absorbs sound energy (Et1) throughand some micropores energy reflects also causing by the sound interface energy of dissipation. skin and core The 1 residual(Er1). Balsa sound core energy absorbs was soundreflected energy (Ea2) mainly through visco-inertial and thermal damping, multilayer cell wall damping, and air flow (Ea2) (mainlyEr2), absorbed through (Ea3 ),visco-inertial and transmitted and by thermal the bottom damping, skin (Et 1multilayer). cell wall damping, and air flow throughthrough micropores micropores also also causing causing sound sound energy energy dissipation. dissipation. The The residual residual sound sound energy energy was was reflected reflected (Er ), absorbed (Ea ), and transmitted by the bottom skin (Et ). (E2r2), absorbed (E3a3), and transmitted by the bottom skin (Et11).

Figure 11. Schematic diagram of sound energy absorption mechanism for sandwich structure.

4. Conclusions FigureFigureSound 11. 11. absorptionSchematic Schematic properties diagram diagram of of sound soundnatural energy energy materials absorption absorption and their mechanism mechanism integrated for for sandwich sandwich sandwich structure structure. structure. were characterized and compared with synthetic materials. It was found that the multiscale structure of 4.4. Conclusions Conclusionsnatural materials was the reason for their outstanding sound energy absorption performance and complicated energy dissipation mechanism. The double-porosity model was applied for predicting SoundtheSound sound absorption absorption absorption properties properties coefficients of ofof natural naturalplant fiber materials materials yarns and and and it theirshowed their integrated integrated more accurate sandwich sandwich results structure comparedstructure were were characterizedcharacterizedto the Champoux–Allard and and compared compared model. with withsynthetic This synthetic suggested materials. materials. that the It Itunique was was found foundmultiscaled that that the hollowthe multiscale multiscale structure structure structurehas an of of naturalnaturaleffect materials materials on the wasacoustic was the the behavior reason reason for offor planttheir their outstandingfiber. outstanding The multilayer sound sound sandwichenergy energy absorption absorption structure is performance conduciveperformance to and and complicatedcomplicatedeffective energy andenergy gradual dissipation dissipation dissipation mechanism. mechanism. of sound The The energy double-porosity double-porosity compared modelto the model individual was was applied applied core for material predicting for predicting or the composite laminates, especially at high frequency. With proper design, the natural-materials-based soundthe sound absorption absorption coefﬁcients coefficients of plant of ﬁberplant yarns fiber andyarns it showedand it showed more accurate more accurate results comparedresults compared to the sandwich structure has the potential for being utilized as a load-bearing and sound-absorbing to the Champoux–Allard model. This suggested that the unique multiscaled hollow structure has an multifunctional structure, especially at high frequency, which would be very beneficial for effect on the acoustic behavior of plant fiber. The multilayer sandwich structure is conducive to effective and gradual dissipation of sound energy compared to the individual core material or

composite laminates, especially at high frequency. With proper design, the natural-materials-based sandwich structure has the potential for being utilized as a load-bearing and sound-absorbing multifunctional structure, especially at high frequency, which would be very beneficial for

Aerospace 2018, 5, 75 12 of 13

Champoux–Allard model. This suggested that the unique multiscaled hollow structure has an effect on the acoustic behavior of plant ﬁber. The multilayer sandwich structure is conducive to effective and gradual dissipation of sound energy compared to the individual core material or composite laminates, especially at high frequency. With proper design, the natural-materials-based sandwich structure has the potential for being utilized as a load-bearing and sound-absorbing multifunctional structure, especially at high frequency, which would be very beneﬁcial for aeronautical applications due to weight restriction and the high sound frequency service environment in order to increase the comfort of the passengers.

Author Contributions: Y.S. and Y.L. conceived and guided the project and study; J.Z. was responsible for the manufacturing of the sandwich structures and preparing samples; J.Z. and B.J. processed the experimental data and performed the calculation; J.Z. prepared the writing-original draft and Y.S. made the writing-review & editing. Funding: This paper was supported by National Natural Science Foundation (11302151) and the Fundamental Research Funds for the Central Universities. Acknowledgments: The authors are thankful to Lineo Co. Ltd. (Saint Martin Du Tilleul, France) and BSWA Technology Co. Ltd. (Beijing, China) for supplying the natural flax fibers and the impedance tube facilities for measuring SACs respectively. Conflicts of Interest: The authors declare no conflict of interest.

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