Nanoarchitectonics of RGO-Wrapped CNF/GO Aerogels with Controlled Pore Structures by PVA-Assisted Freeze-Casting Approach for Efficient Sound and Microwave Absorption.

Acoustic absorption materials play an important role in eliminating the negative effects of noise. Herein, a polyvinyl alcohol (PVA)-assisted freeze-casting was developed for controllably fabricating reduced graphene oxide wrapped carbon nanofiber (RGO@CNF)/graphene oxide composite aerogel. During the freeze-casting, PVA was used as an icing inhibitor to control the size of ice crystals. While the concentration of PVA increased from 0 to 15 mg ⋅ ml-1 , the average pore size of the aerogel was reduced from 154 to 45 µm. Due to the modulation of the pore size and composition, the propagation path and friction loss for sound were optimized, especially at low frequency. For instance, the normalized sound absorption coefficient of RGO@CNF/GO-10 achieves 0.79 (250-6300 Hz). The sample also exhibits a desirable microwave absorbing property whose maximum reflection loss is -47.3 dB (9.44 GHz, d=3.0 mm). Prospectively, this synthetic strategy can be extended to develop other forms of elastic aerogel with a controlled pore size.

Natural Fibre-Reinforced Polymer Composites (NFRP) Fabricated from Lignocellulosic Fibres for Future Sustainable Architectural Applications, Case Studies: Segmented-Shell Construction, Acoustic Panels, and Furniture.

Due to the high amounts of waste generated from the building industry field, it has become essential to search for renewable building materials to be applied in wider and more innovative methods in architecture. One of the materials with the highest potential in this area is natural fibre-reinforced polymers (NFRP), which are also called biocomposites, and are filled or reinforced with annually renewable lignocellulosic fibres. This would permit variable closed material cycles' scenarios and should decrease the amounts of waste generated in the building industry. Throughout this paper, this discussion will be illustrated through a number of developments and 1:1 mockups fabricated from newly developed lignocellulosic-based biocomposites from both bio-based and non-bio-based thermoplastic and thermoset polymers. Recyclability, closed materials cycles, and design variations with diverse digital fabrication technologies will be discussed in each case. The mock-ups' concepts, materials' compositions, and fabrication methods are illustrated. In the first case study, a structural segmented shell construction is developed and constructed. In the second case study, acoustic panels were developed. The final case studies are two types of furniture, where each is developed from a different lignocellulosic-based biocomposite. All of the presented case studies show diverse architectural design possibilities, structural abilities, and physical building characteristics.

Effects of Freeze-Drying Processes on the Acoustic Absorption Performance of Sustainable Cellulose Nanocrystal Aerogels.

Cellulose aerogels have great prospects for noise reduction applications due to their sustainable value and superior 3D interconnected porous structures. The drying principle is a crucial factor in the preparation process for developing high-performance aerogels, particularly with respect to achieving high acoustic absorption properties. In this study, multifunctional cellulose nanocrystal (CNC) aerogels were conveniently prepared using two distinct freeze-drying principles: refrigerator conventional freezing (RCF) and liquid nitrogen unidirectional freezing (LnUF). The results indicate that the rapid RCF process resulted in a denser CNC aerogel structure with disordered larger pores, causing a stronger compressive performance (Young's modulus of 40 kPa). On the contrary, the LnUF process constructed ordered structures of CNC aerogels with a lower bulk density (0.03 g/cm3) and smaller apertures, resulting in better thermal stability, higher diffuse reflection across visible light, and especially increased acoustic absorption performance at low-mid frequencies (600-3000 Hz). Moreover, the dissipation mechanism of sound energy in the fabricated CNC aerogels is predicted by a designed porous media model. This work not only paves the way for optimizing the performance of aerogels through structure control, but also provides a new perspective for developing sustainable and efficient acoustic absorptive materials for a wide range of applications.

Investigation of the Underwater Absorption and Reflection Characteristics by Using a Double-Layer Composite Metamaterial.

It is well-known that the acoustic stealth of an underwater vehicle composed of a non-watertight structure has been facing severe challenges. The origins of this effect are associated with the fact that the coupling between the water and the mechanical structure is not negligible because both sides are in the water. Along these lines, the idea of forward absorption and backward reflection was proposed in this work to address this issue. More specifically, a composite underwater acoustic metamaterial (AM) was designed based on different layers, namely a sound absorption layer and a sound insulation layer from the outside to the inside. The sound absorption layer was made of a soft rubber matrix with embedded steel scatterers (ESs) to enrich the coupled resonance effects, while the sound insulation layer was composed of hard rubber with a built-in cavity to improve the impedance mismatching between the AM and the water. The impact of the number and thickness of the embedded ESs on the acoustic performance of the AM was also thoroughly investigated via a finite element method (FEM). A fast non-dominated genetic algorithm (NAGA-II) with elite strategy was used to optimize the position and the size of the ESs. The optimization results revealed the high absorption at the forward incidence and the high reflection at the backward incidence. Thus, our work provides a novel and effective approach for improving the acoustic stealth of underwater vehicles composed of non-watertight structures.

Broadband impedance modulation via non-local acoustic metamaterials.

Causality of linear time-invariant systems inherently defines the wave-matter interaction process in wave physics. This principle imposes strict constraints on the interfacial response of materials on various physical platforms. A typical consequence is that a delicate balance has to be struck between the conflicting bandwidth and geometric thickness when constructing a medium with desired impedance, which makes it challenging to realize broadband impedance modulation with compact structures. In pursuit of improvement, the over-damped recipe and the reduced excessive response recipe are creatively presented in this work. As a proof-of-concept demonstration, we construct a metamaterial with intensive mode density that supports strong non-locality over a frequency band from 320 Hz to 6400 Hz. Under the guidelines of the over-damped recipe and the reduced excessive response recipe, the metamaterial realizes impedance matching to air and exhibits broadband near-perfect absorption without evident impedance oscillation and absorption dips in the working frequency band. We further present a dual-functional design capable of frequency-selective absorption and reflection by concentrating the resonance modes in three frequency bands. Our research reveals the significance of over-damped recipe and the strong non-local effect in broadband impedance modulation, which may open up avenues for constructing efficient artificial impedance boundaries for energy absorption and other wave manipulation.

A New Methodology Based on Cell-Wall Hole Analysis for the Structure-Acoustic Absorption Correlation on Polyurethane Foams.

Polyurethane foams with a hybrid structure between closed cell and open cell were fabricated and fully characterized. Sound absorption measurements were carried out in order to assess their acoustic performance at different frequency ranges. The cellular structure of these systems was studied in detail by defining some novel structural parameters that characterize the cell wall openings such as the average surface of holes (Sh), the number of holes (h), and the area percentage thereof (%HCW). Therefore, these parameters allow to analyze quantitatively the effect of different structural factors on the acoustic absorption performance. It has been found that the parameters under study have a remarkable influence on the normalized acoustic absorption coefficient at different frequency ranges. In particular, it has been demonstrated that increasing the surface of the holes and the percentage of holes in the cell walls allows increasing the acoustic absorption of these types of foams, a promising statement for developing highly efficient acoustic insulators. Additionally, we could determine that a suitable minimum value of hole surface to reach the highest sound dissipation for these samples exists.

Improvement of natural fiber's properties and evaluation of its applicability as eco-friendly materials in noise pollution control.

Purpose: The use of green and eco-natural fibers due to availability, biodegradability, reasonable cost, and non-toxic effects are known as potential acoustic materials in research. Nevertheless, in this area, the use of chemical treatment in natural fibers as an effective strategy to improve the overall properties and acoustic performance is faced with limitations. So that the current paper aims to investigate the effect of alkali treatment on the mechanical and acoustical properties of jute fibers and evaluation of its applicability in noise pollution control. Methods: To determine the effect of alkali treatment on the sound absorption behavior of jute fibers, a sufficient amount of both types of fibers (raw and treated) were used to the fabrication of acoustic samples with thicknesses of 30, 40 and 50 mm at a density of 200 kg/m3. The acoustic evaluation was measured by the Impedance tube system according to transfer function method using by the standard ISO 10534-2. The morphological and tensile properties of fibers were evaluated by the Scanning Electron Microscopy (SEM) and tensile test (ASTM C1557-14 standard). Moreover, Infrared Spectroscopy (FTIR) and X-ray diffraction (XRD) were used to study and compare the chemical properties of raw and treated fibers. Results: The result showed that the mean tensile strength and crystallinity index (CI) of treated fibers comparing with raw fibers increased by 61.66% and 3.26% respectively. The use of Alkali treatment helped to improve sound absorption performance of jute fibers with different thicknesses. Furthermore, noise reduction rate (NRC) in treated acoustic samples compared to untreated with a thickness of 50 mm increased from 0.66 to 0.69. Conclusion: Finally, the alkali treatment has enhanced the properties of jute fibers and confirmed the applicability of these fibers in acoustic absorption. Supplementary Information: The online version contains supplementary material available at 10.1007/s40201-022-00799-x.

Acoustic Performance and Flame Retardancy of Ammonium Polyphosphate/Diethyl Ethylphosphonate Rigid Polyurethane Foams.

Flame-retardant water-blown rigid polyurethane foams (RPUFs) modified by ammonium polyphosphate (APP) and diethyl ethylphosphonate (DEEP) were synthesized by a one-pot free-rising method. We performed scanning electron microscopy (SEM), compression strength tests, acoustic absorption measurements and thermogravimetric analysis, as well as limited oxygen index, vertical burning and cone calorimeter tests to investigate the mechanical properties, acoustic performance and flame retardancy of the foams. SEM confirmed that the open-cell structures of the foams were successfully constructed with the introduction of a cell-opening agent. Upon using 20 php APP, the average acoustic absorption coefficient of the foam reached 0.535 in an acoustic frequency range of 1500-5000 Hz. The results of thermogravimetric analysis demonstrated that the incorporation of APP and DEEP can effectively restrain mass loss of RPUFs during pyrolysis. In particular, the compressive strength of a foam composite containing 5 php APP and 15 php DEEP increased to 188.77 kPa and the LOI value reached 24.9%. In a vertical burning test and a cone calorimeter test, the joint use of APP and DEEP endowed RPUFs with a V-0 rating and they attained a THR value of 23.43 MJ/m2. Moreover, the addition of APP improved the acoustic absorption performance of the foam, verified by acoustic absorption measurements. Considering potential applications, the formulation containing 15 php APP and 5 php DEEP could be used in the preparation of a new flame-retardant acoustic absorption rigid polyurethane foam.

Experimental Investigation on Environmentally Sustainable Cement Composites Based on Wheat Straw and Perlite.

Environmentally sustainable cement mortars containing wheat straw (Southern Italy, Apulia region) of different length and dosage and perlite beads as aggregates were prepared and characterised by rheological, thermal, acoustic, mechanical, optical and microstructural tests. A complete replacement of the conventional sand was carried out. Composites with bare straw (S), perlite (P), and with a mixture of inorganic and organic aggregates (P/S), were characterised and compared with the properties of conventional sand mortar. It was observed that the straw fresh composites showed a decrease in workability with fibre length decrease and with increase in straw volume, while the conglomerates with bare perlite, and with the aggregate mixture, showed similar consistency to the control. The thermal insulation of the straw mortars was extremely high compared to the sand reference (85-90%), as was the acoustic absorption, especially in the 500-1000 Hz range. These results were attributed to the high porosity of these composites and showed enhancement of these properties with decrease in straw length and increase in straw volume. The bare perlite sample showed the lowest thermal insulation and acoustic absorption, being less porous than the former composites, while intermediate values were obtained with the P/S samples. The mechanical performance of the straw composites increased with length of the fibres and decreased with fibre dosage. The addition of expanded perlite to the mixture produced mortars with an improvement in mechanical strength and negligible modification of thermal properties. Straw mortars showed discrete cracks after failure, without separation of the two parts of the specimens, due to the aggregate tensile strength which influenced the impact compression tests. Preliminary observations of the stability of the mortars showed that, more than one year from preparation, the conglomerates did not show detectable signs of degradation.

3D Printing of Polymeric Multi-Layer Micro-Perforated Panels for Tunable Wideband Sound Absorption.

The increasing concern about noise pollution has accelerated the development of acoustic absorption and damping devices. However, conventional subtractive manufacturing can only fabricate absorption devices with simple geometric shapes that are unable to achieve high absorption coefficients in wide frequency ranges. In this paper, novel multi-layer micro-perforated panels (MPPs) with tunable wideband absorption are designed and fabricated by 3D printing or additive manufacturing. Selective laser sintering (SLS), which is an advanced powder-based 3D printing technique, is newly introduced for MPP manufacturing with polyamide 12 as the feedstock. The acoustic performances of the MPPs are investigated by theoretical, numerical, and experimental methods. The results reveal that the absorption frequency bandwidths of the structures are wider than those of conventional single-layer MPPs, while the absorption coefficients remain comparable or even higher. The frequency ranges can be tuned by varying the air gap distances and the inter-layer distances. Furthermore, an optimization method is introduced for structural designs of MPPs with the most effective sound absorption performances in the target frequency ranges. This study reveals the potential of 3D printing to fabricate acoustic devices with effective tunable sound absorption behaviors and provides an optimization method for future structural design of the wideband sound absorption devices.

Morphological, acoustical, mechanical and thermal properties of sustainable green Yucca (Y. gloriosa) fibers: an exploratory investigation.

BACKGROUND: The objective of this study was to evaluate the acoustical, morphological, mechanical and thermal properties of fibers extracted from the leaves of Yucca (Y. gloriosa) shrub. METHODS: Several tests were performed on either untreated or alkali-treated (5% NaOH) fibers. The chemical analysis of the fibers was performed to determine their lignin, cellulose, hemicellulose, wax and moisture content. Fourier transform infrared (FTIR) and thermogravimetric analysis were respectively employed to chemically and thermally analyze the fibers. The microscopic examination was also carried out using scanning electron microscope (SEM). INSTRON universal testing machine and an impedance tube system were employed for measuring the tensile properties of the fibers and the sound absorption coefficient values of the samples fabricated from the same fibers, respectively. RESULTS: The results from the experiments revealed that the fibers have low density (1.32 g/cm3) and higher cellulose content (66.36 wt.%,). The mechanical characterization of these fibers also confirmed they are similar to the other lignocellulose fibers used for the reinforcement of polymer matrix composites. The tensile strength test conducted on Yucca fibres showed that mechanical properties of alkali treated fibers are superior to the untreated fibers. The thermal analysis also demonstrated that the alkali treated fibres can thermally withstand temperatures of up to 364 °C which confirms the fact that the thermal stability of fibers was improved by alkali treatment. CONCLUSION: Fibers extracted from the leaves of Yucca demonstrated fair amounts of mechanical and thermal resistance and strength. The samples fabricated from Yucca fibers demonstrated fair levels of sound absorption coefficients particularly at higher frequency ranges which are typical to the other natural fibers. The possible use of Yucca fibers as either a reinforcing agent for composites or a sound absorbing medium is highly promising.

[The study of wideband acoustic immittance normative data of young people].

Objective:To explore the normative data of wideband acoustic absorption rate of young people with normal hearing, and the related factors of wideband acoustic immittance.Method:Eighty six cases of young people with normal hearing (43 cases of male, 43 cases of female) were collected in the clinical audiology centers of Beijing Tongren Hospital, Capital University of Medical. The average age is (22.0±1.5) years. Pure tone audiometry in the range of 250 to 8 000 Hz in each octave is carried out, and the air conduction thresholds are less than 25 dB HL; 226 Hz tympanometry were type A. For all subjects, the wideband acoustic immittance measurement was conducted at 226-8 000 Hz, and a total of 107 frequency points were carried out.Result:The test results of 172 ears were statistically analyzed.①In the range of 250-8 000 Hz,acoustic absorption rate in the low frequency range increases gradually, and the peak is found in the range of 1 000 to 3 000 Hz;getting along with the increase of frequency and acoustic absorption rate gradually decreases slowly.②The acoustic absorption rate in the range of 4 200-4 900 Hzin female subjects was significantly higher than that in male subjects.③There was no significant relationship between gender, compliance, peak pressure, and width in 226 Hz tympanometry (P=0.319, 0.831, 0.786), however there was a significant relationship between gender and the ear canal volume (P<0.01).④There was no significant relationship between gender and the resonance frequency (P>0.05).⑤There was a significant correlation between height/weight and ear canal volume (P<0.05), and there was a significant relationship between the ear canal volume and the resonance frequency (P<0.05).Conclusion:The acoustic absorption rate of young people in the low frequency range was lower, then increases gradually, and the peak was found in the range of mid-high frequency. The difference in the wideband absorption rate between genders was significant. The height and weight had influence on the results of wideband acoustic immittance.