释压法混合吸声系统中多孔材料厚度优化研究

研究释压法主被动混合吸声结构吸声性能与多孔吸声材料参数的关系,利用释压法对材料厚度敏感的特性,选择合适的多孔材料厚度提高系统的低频吸声性能。提出一种释压法主被动混合双层吸声结构,将误差传声器置入两层吸声结构之间的空气层中,分别优化各层的厚度,在较宽频带内获得较好吸声。对流阻系数为15000NSm^-4的一类典型的玻璃棉数值仿真,发现选择优化厚度分别为2.8 cm和6 cm,中间空腔长为2 cm,可使有效吸声频带向下大幅扩展。实验结果验证了所提宽带吸声结构。     

软质聚氨酯泡沫塑料的低温热导率

讨论了软质聚氨酯泡沫塑料对流判据的低温传热机理.根据软质聚氨酯泡沫塑料结构简图,将各种参数和低温数据带入对流判据公式,得到对流判据L的计算结果.计算了软质聚氨酯泡沫塑料的低温热导率.为了与计算的低温热导率比较,采用测试硬质泡沫塑料热导率的稳态热流双试样保护热板装置,测试了软质聚氨酯泡沫塑料的低温热导率,最后给出了低温热导率与温度关系曲线.根据试验,讨论了低温热导率的影响因素如温度、密度、吸水率等.     

Improving the Morphological Parameters of Aluminum Foam for Maximum Sound Absorption Coefficient using Genetic Algorithm

Fabricating of metal foams with desired morphological parameters including pore size, porosity and pore opening is possible now using sintering technology. Thus, if it is possible to determine the morphology of metal foam to absorb sound at a given frequency, and then fabricate it through sintering, it is expected to have optimized metal foams for the best sound absorption. Theoretical sound absorption models such as Lu model describe the relationship between morphological parameters and the sound absorption coefficient. In this study, the Lu model was used to optimize the morphological parameters of Aluminum metal foam for the best sound absorption coefficient. For this purpose, the Lu model was numerically solved using written codes in MATLAB software. After validating the proposed codes with benchmark data, the genetic algorithm (GA) was applied to optimize the affecting morphological parameters on the sound absorption coefficient. The optimization was carried out for the thicknesses of 5 mm to 40 mm at the sound frequency range of 250 Hz–8000 Hz. The optimized parameters ranged from 50% to 95% for porosity, 0.1 mm to 4.5 mm for pore size, and 0.07 mm to 0.6 mm for pore opening size. The result of this study was applied to fabricate the desired Aluminum metal foams for the best sound absorption. The novel approach applied in this study, is expected to be successfully applied in for best sound absorption in desired frequencies.     

Experimental study of a smart foam sound absorber

This article presents the experimental implementation and results of a hybrid passive/active absorber (smart foam) made up from the combination of a passive absorbent (foam) and a curved polyvinylidene fluoride (PVDF) film actuator bonded to the rear surface of the foam. Various smart foam prototypes were built and tested in active absorption experiments conducted in an impedance tube under plane wave propagation condition at frequencies between 100 and 1500 Hz. Three control cases were tested. The first case used a fixed controller derived in the frequency domain from estimations of the primary disturbance at a directive microphone position in the tube and the transfer function between the control PVDF and the directive microphone. The two other cases used an adaptive time-domain feedforward controller to absorb either a single-frequency incident wave or a broadband incident wave. The non-linearity of the smart foams and the causality constraint were identified to be important factors influencing active control performance. The effectiveness of the various smart foam prototypes is discussed in terms of the active and passive absorption coefficients as well as the control voltage of the PVDF actuator normalized by the incident sound pressure.     

An acoustic chamber for sound intensity measurements

The construction and performance of an acoustic chamber suitable for sound intensity measurements is described. The walls and the ceiling of the room are treated with glass wool sheets, air gap and pleated carpet in that order for sound absorption. The final testing of the room shows that good sound absorption is obtained down to low frequencies. The sound absorption coefficient for the room varies between 0.83 and 0.91 for different frequencies.     

键合型PU/Eu(Cit)Phen荧光纳米纤维制备与表征

合成了含-OH(羟基)的Eu(Cit)Phen(铕-柠檬酸-邻菲咯啉)三元配合物与端-NCO(异氰酸酯)基PU(聚氨酯)预聚物,通过一OH与一NCO间加成反应及l,3.丙二醇扩链反应制备键合型PU/Complex稀土高分子材料,将其溶液经静电纺丝制得具有荧光性能的纳米纤维.采用FT-IR、SEM、XRD、DSC和PL对...     

Analysis of coir fiber acoustical characteristics

Coir fiber from coconut husk is an important agricultural waste in Malaysia. Acoustic absorption coefficient of the fiber as a porous material is studied in this paper. Two types of fiber are investigated, fresh from wet market and industrial prepared mixed with binder. Moreover two analytical models, namely; Delany–Bazley and Biot–Allard are used for analysis. Experimental measurements in impedance tube are conducted to validate the analytical outcomes. Results show that fresh coir fiber has an average absorption coefficient of 0.8 at f > 1360 Hz and 20 mm thickness. Increasing the thickness is improved the sound absorption in lower frequencies, having the same average at f > 578 Hz and 45 mm thickness. Delany–Bazley technique can be used for both types of fiber while Biot–Allard method is compensated for the industrial prepared fiber considering the binder additive. This form generally shows poor acoustical absorption in low frequencies. Inevitably, fiber has to be mixed with additives in commercial use to enhance its characteristics such as stiffness, unti-fungus and flammability. Hence other approaches such as adding air gap or perforated plate should be used to improve the acoustical properties of industrial treated coir fiber.     

Noise absorption by woven fabrics

The paper describes an investigation of the relationship between the noise absorption coefficients of a cover made of woven fabric and its intrinsic parameters. The parameters considered were: fibre content, yarn count, cover factor, the air gap behind the fabric, the frequency of the impinging sound wave, and the influence of washing on the sound absorption capacity of the woven fabric. The results show a potential of obtaining NRC ranging between 0·09 and 0·22, which may be useful when coating rigid surfaces. The absorption coefficient is much higher at higher frequencies, say, 4000 Hz.     

Piezoelectricity in polymers

In the last few years it has been shown that polyvinylidene fluoride, a polymer which can.readily be formed into very thin, flexible, and transparent films, is a ferroelectric material. The availability of a ferroelectric material in this unusual form has led to a large number of possible new applications, and the Japanese company, Pioneer Electronic Corporation, has already developed a commercial product.¹ They are producing headphones in which the active element is an 8-pm-thick film of polyvinylidene fluoride. Indicative of the kind of creativity this new material permits, the headphones do not use the change in thickness of the film at all. The polymer film is stretched over a flexible polyurethane foam and application of a voltage causes a change in the area of the film so that the foam, functioning like a spring, is either compressed or allowed to expand, depending on the polarity of the applied voltage.     

Sound insulation property of Al–Si closed-cell aluminum foam sandwich panels

Al–Si closed-cell aluminum foam sandwich panels (1240 mm × 1100 mm) of different thicknesses and different densities were prepared by molten body transitional foaming process in Northeastern University. The experiments were carried out to investigate the sound insulation property of Al–Si closed-cell aluminum foam sandwich panels of different thicknesses and different densities under different frequencies (100–4000 Hz). Results show that sound reduction index (R) is small under low frequencies, large under high frequencies; thickness affects the sound insulation property of material obviously: when the thicknesses of Al–Si closed-cell aluminum foam sandwich panels are 12, 22, and 32 mm, the corresponding weighted sound reduction indices (R_W) are 26.3, 32.2, and 34.6 dB, respectively, the rising trend tempered; the increase of density of Al–Si closed-cell aluminum foam can also increase the sound insulation property: when the densities of aluminum foam are 0.31, 0.51, and 0.67 g/cm³, the corresponding weighted sound reduction indices (R_W) are 28.9, 34.3, and 34.6 dB, the increasing value mitigating.     

Two-nozzle electrospinning of (MWNT/PU)/PU nanofibrous composite mat with improved mechanical and thermal properties

Composite nanofibrous mat composed of neat polyurethane (PU) and multiwalled carbon nanotubes/polyurethane (MWNT/PU) nanofibers have been fabricated by one-step angled two-nozzle electrospinning. The morphological, thermal, and mechanical properties of the electrospun nanofibers were evaluated. The diameters of electrospun neat PU and composite nanofibers ranged from 239 to 1058 nm. The two-nozzle electrospun (MWNT/PU)/PU composite nanofibers showed curly, and randomly-oriented fibers with interfiber bonding, and were generally bigger in size than single-nozzle electrospun nanofibers. The tensile strength of the neat PU composite nanofiber mat obtained from two-nozzle electrospinning was 25% higher than that obtained from neat PU single-nozzle electrospinning. The incorporation of MWNTs in the composite nanofiber increased the tensile strength by as much as 64% without reducing elongation, made the composite nanofiber more thermally stable, and improved the melting zone. The present results showed that side-by-side angled two-nozzle electrospinning can improve the quality of the electrospun nanofibers that could have potential application in different fields such as filtration, protective clothing and tissue engineering.     

The effects of post-thermal annealing on the optical parameters of indium-doped ZnO thin films

Indium-doped ZnO thin films are deposited on quartz glass slides by RF magnetron sputtering at ambient temper- ature. The as-deposited films are annealed at different temperatures from 400 C to 800 C in air for 1 h. Transmittance spectra are used to determine the optical parameters and the thicknesses of the films before and after annealing using a nonlinear programming method, and the effects of the annealing temperatures on the optical parameters and the thickness are investigated. The optical band gap is determined from the absorption coefficient. The calculated results show that the film thickness and optical parameters both increase first and then decrease with increasing annealing temperature from 400 C to 800 C. The band gap of the as-deposited ZnO:In thin film is 3.28 eV, and it decreases to 3.17 eV after annealing at 400 C. Then the band gap increases from 3.17 eV to 3.23 eV with increasing annealing temperature from 400 C to 800 C.     

NUMERICAL MODELLING OF THE SOUND FIELDS IN URBAN STREETS WITH DIFFUSELY REFLECTING BOUNDARIES

A radiosity-based theoretical/computer model has been developed to study the fundamental characteristics of the sound fields in urban streets resulting from diffusely reflecting boundaries, and to investigate the effectiveness of architectural changes and urban design options on noise reduction. Comparison between the theoretical prediction and the measurement in a scale model of an urban street shows very good agreement. Computations using the model in hypothetical rectangular streets demonstrate that though the boundaries are diffusely reflective, the sound attenuation along the length is significant, typically at 20-30 dB/100 m. The sound distribution in a cross-section is generally even unless the cross-section is very close to the source. In terms of the effectiveness of architectural changes and urban design options, it has been shown that over 2-4 dB extra attenuation can be obtained either by increasing boundary absorption evenly or by adding absorbent patches on the façades or the ground. Reducing building height has a similar effect. A gap between buildings can provide about 2-3 dB extra sound attenuation, especially in the vicinity of the gap. The effectiveness of air absorption on increasing sound attenuation along the length could be 3-9 dB at high frequencies. If a treatment is effective with a single source, it is also effective with multiple sources. In addition, it has been demonstrated that if the façades in a street are diffusely reflective, the sound field of the street does not change significantly whether the ground is diffusely or geometrically reflective.     

多层多孔吸声材料结构参数优化设计

高开孔率的发泡材料(如三聚氰胺、聚氨酯发泡材料)具有优良的吸声、隔热防火、防腐及环保性能,可以作为吸声、阻尼等材料应用于建筑、航空、交通工具等领域.该文基于Biot理论和多层介质声波传播理论(传递矩阵法),建立多层多孔吸声结构背衬刚性壁的理论模型,利用遗传算法优化多层结构厚度和质量.将理论模型计算结果与阻抗管测试结果进...     

Sound absorption and transmission through flexible micro-perforated panels backed by an air layer and a thin plate

This paper describes theoretical and experimental investigations into the sound absorption and transmission properties of micro-perforated panels (MPP) backed by an air cavity and a thin plate. A fully coupled modal approach is proposed to calculate the absorption coefficient and the transmission loss of finite-sized micro-perforated panels-cavity-panel (MPPCP) partitions with conservative boundary conditions. It is validated against infinite partition models and experimental data. A practical methodology is proposed using collocated pressure-velocity sensors to evaluate in an anechoic environment the transmission and absorption properties of conventional MPPCPs. Results show under which conditions edge scattering effects should be accounted for at low frequencies. Coupled mode analysis is also performed and analytical approximations are derived from the resonance frequencies and mode shapes of a flexible MPPCP. It is found that the Helmholtz-type resonance frequency is deduced from the one associated to the rigidly backed MPPCP absorber shifted up by the mass-air mass resonance of the flexible non-perforated double-panel. Moreover, it is shown analytically and experimentally that the absorption mechanisms at the resonances are governed by a large air-frame relative velocity over the MPP surface, with either in-phase or out-of-phase relationships, depending on the MPPCP parameters.     

Neural networks for prediction of acoustical properties of polyurethane foams

This paper presents a study of neural networks for prediction of acoustical properties of polyurethane foams. The proposed neural network model of the foam uses easily measured parameters such as frequency, airflow resistivity and density to predict multiple acoustical properties including the sound absorption coefficient and the surface impedance. Such a model is quite robust in the sense that it can be used to develop models for many different classes of materials with different sets of input and output parameters. The current neural network model of the foam is empirical and provides a useful complement to the existing analytical and numerical approaches.     

Optimal poroelastic layer sequencing for sound transmission loss maximization by topology optimization method

Optimal layer sequencing of a multilayered acoustical foam is solved to maximize its sound transmission loss. A foam consisting of air and poroelastic layers can be optimized when a limited amount of a poroelastic material is allowed. By formulating the sound transmission loss maximization problem as a one-dimensional topology optimization problem, optimal layer sequencing and thickness were systematically found for several single and ranges of frequencies. For optimization, the transmission losses of air and poroelastic layers were calculated by the transfer matrix derived from Biot's theory. By interpolating five intrinsic parameters among several poroelastic material parameters, distinct air-poroelastic layer distributions were obtained; no filtering or postprocessing was necessary. The optimized foam layouts by the proposed method were shown to differ depending on the frequency bands of interest.     

An evaluation method for measuring SPL and mode shape of tire cavity resonance by using multi-microphone system

This paper presents an evaluation method for measuring the sound pressure level and mode shapes of tire cavity resonance by using a multi-microphone system. Two commercial tires were evaluated to compare abilities of noise suppression by means of this method in the range of the first resonance from 200 to 260 Hz. One tire was a special tire that suppresses tire cavity resonance with polyurethane foam mounted on the tire’s inner liner. The other tire was a normal tire with no polyurethane foam. The mode shape change from vertical to horizontal direction in both tires. However, the sound pressure level of the special tire was lower than the normal tire at all frequencies.     

Oil absorption capacity of bare and PDMS-coated PET non-woven fabric; dependency of fiber strand thickness and oil viscosity

Water and oil absorption behaviors of the bare and PDMS (polydimethylsiloxane)-coated PET (polyethylene terephthalate) fabrics with two different fiber strand thicknesses were investigated. PET fabrics were superhydrophobically modified by PDMS-coating via a thermal vapor deposition process. The modification greatly enhanced the selectivity of PET fabric to oil absorption from oil/water mixture, making the PDMS-coated fabric a promising candidate as an absorbent of oil spills. We have also studied oil sorption behaviors of PDMS-coated PET fabric as a function of fiber strand thicknesses and oil viscosities. In general, fabric with a thinner fiber strand thickness showed a higher maximum oil absorption capacity, whereas uptake of the oil in thicker-fiber-absorbent was faster. Based on our results, we suggest that different absorbent structures with various fiber strand thicknesses can be chosen to maximize the efficiency of oil removal, depending on oil viscosity and available oil absorption time.     

Surface Characterization and Optical Study on Electrospun Nanofibers of PVDF/PAN Blends

Different concentrations of electrospun nanofibers of poly(vinylidene fluoride) (PVDF)/polyacrylonitrile (PAN) blends were prepared. The surface morphology of the nanofibers was studied by optical microscopy and scanning electron microscopy (SEM). Atomic force microscopy (AFM) was used to study the surface roughness. The change in the functional groups of PVDF after blending with PAN was investigated by Fourier transform infrared (FTIR). The wettability was examined to characterize the surface morphology of the blends. The effect of PAN content on the blend fiber was also studied for UV-vis absorption spectra before and after exposure to UV radiation.