Noise Absorption Coefficient Research Articles

Further study of spectral ripple with a laser scattering measurement apparatus

To study the spectral effect further, which exists in fold-type cavity ring-down or enhanced absorption spectrometer, a set of laser scattering measurement apparatus was presented and applied to evaluate the surface status of high-reflectivity coatings. Based on this apparatus and another set of optical-feedback cavity ring-down spectrometer with the equivalent noise absorption coefficient of 9.0×10−9cm−1, some experiments are carried out to inspect some analyses about the effect's mechanism. It is determined that the spectral ripple originates from the interference fringe's movement of resonance light beams along the surface of folding mirror, which causes the folding mirror's loss to change with the laser wavelength's scanning. Based on this conclusion, the potential application and the processing methods of spectral ripple are proposed and discussed.

Evaluation of Functionality in Porous Friction Courses

Porous friction courses (PFCs) are employed as surface layers primarily because of their functionality (i.e., their high permeability and noise absorption capability). In this study, the functionality of PFCs was evaluated through permeability and noise absorption tests in pavement structures of different thicknesses (i.e., 0.75, 1.5, and 2.5 in.), each at two air void contents (i.e., design and construction). Permeability was evaluated in the laboratory and the field with the Texas Department of Transportation (DOT) permeameter, the National Center of Asphalt Technology (NCAT) permeameter, and the Florida DOT falling head apparatus. Noise absorption was evaluated in the laboratory with the impedance tube. Results showed that, on average, the coefficient of permeability obtained with the Texas DOT permeameter was 67% higher than that obtained with the NCAT permeameter and 93% higher than that obtained with the Florida DOT falling head apparatus. In general, the coefficients of permeability increased with an increase in layer thickness. The noise absorption coefficient was not sensitive to thickness, but all values represented significant noise reduction. A permeability model on the basis of the Kozeny–Carman equation also was used to predict permeability values and relate them to the laboratory values obtained at different thicknesses. The equation accurately predicted the permeability coefficients of the thick PFC layers. The results suggested that minimum and maximum functionality requirements should be set to guarantee adequate performance.

An Investigation into Acoustic Properties of Lightly Needled Estabragh Nonwovens Using the Taguchi Method

Sound pollution has become an important issue that has been addressed by scientists of various disciplines. Control of sound in areas of transport and building industries is of paramount importance. Textiles are widely used as sound insulators. Among the broad spectrum of textiles, nonwoven fabrics due to their technical merits and wide acceptance are extensively used as sound controlling media. In this work, various blends of polypropylene and naturally grown hollow Estabragh fibers were used to prepare sound absorptive nonwoven layers. The fiber blends were fed to a laboratory scale carding machine. Carded webs were lightly needled on a laboratory scale needling machine. Acoustic properties of needled samples were evaluated using the Impedance tube method. The Taguchi method was used to analyze the effective parameters influencing fabric acoustic properties. Results show that the proportion of Estabragh fibers in the blends strongly affects the noise absorption coefficient (NAC). Frequency is the second effective factor and is followed by nonwoven layer mass (areal density) and punch density, respectively. It was concluded that layers comprised of 100% Estabragh fibers achieved the highest value of NAC. The results also pointed to existence of a direct correlation between noise absorption coefficient and the amount of punch density exerted during needling process. It was found that the increase in the amount punch density resulted in higher values of NAC.

Acoustic Noise Energy Scavenger with Noise Reduction Effect

A kind of novel acoustic noise energy scavenger with additional function of noise reduction is introduced in this article. The device consists of energy harvesting membranes and silicon micro perforated panel (MPP) resonant structure. Both of them are all fabricated with silicon wafer in use of MEMS technology. The maximum output power of the energy scavenger is about 14.1μW/m2, the maximum open circuit voltage achieves 69.41mV, and the average noise absorption coefficient of the device is higher than 0.7 in the bandwidth of 1500Hz to 6000Hz. The energy collected by the device could be used for driving micro and nano electronic systems.

Predicted sound absorption coefficients of absorber materials lined in a chamber

Aims: The present study was aimed to measurement of sound absorption coefficient of mineral wool and determination of their absorption ability. Materials and Methods: Mineral wool was used to find noise absorption coefficient. Random and normal sound absorption coefficient values were predicted. Then, the measures of transmission loss calculated as an overall value, for applied absorbent material and bare sheet metal. Results: The measured values of noise with one octave band frequency demonstrated an attenuation of 5.5-7 dB for these frequencies. The absorption coefficients of materials showed that mineral wool had more normal sound coefficients than its random sound absorption coefficient values. Conclusion: It can be concluded that predicted normal sound absorption coefficients of used mineral wool materials were near to the areas of standard line. It seems that the amount or thickness of absorbent lining was a main reason of noise reduction in low band frequencies. Mineral wool has a higher density and can provide better acoustical and insulating results than fiberglass. Besides, mineral wool doesn't lose its insulating value when wet and has an outstanding resistance to fire.

Evaluating the Acoustic Properties of Estabragh (Milkweed)/Hollow- Polyester Nonwovens vfor Automotive Applications

Reduction in noise pollution in general and particularly in automotive industry has become an important issue that has gained the interest of numerous researchers. The present research is aimed to introduce a new product which could serve as an alternative for other commonly used sound absorbing materials. It is expected that hollow fibers due to their structure can significantly influence noise insulation performance of materials in which these fibers are incorporated. Therefore sound absorbing material compose of blend of natural hollow fibers Milkweed or as it known Estabragh and hollow polyester fibers were used to produce needle-punched nonwoven samples. Eighteen different samples were produced by varying blend ratio, mass per unit area, punching density, and frequency of the incident sound. Noise Absorption Coefficient (NAC) of the samples was evaluated using impedance test method. The evaluation procedures were designed using Taguchi method. Results showed samples solely comprise of Estabragh fibers have attained the ability to absorb noise. It was observed that NAC decreases as the amount of Estabragh in the blend is reduced. Although the results showed an increase in punching density improves NAC of the samples, however statistical analysis dismissed the effectiveness of this parameter on sound absorption characteristics of the samples. It was found that NAC increases in direct relation to increase in mass per unit area (g/m2) of the samples. This was attributed to the increase in tortuosity of the samples and the energy losses due to frictional resistance. Finally it was observed that NAC peaks at frequency of 4000 Hz.

실내장식용 섬유소재의 흡음성에 관한 연구

The purpose of this study is to help to prevent daily noises by measuring the noise absorption coefficient of the non-woven fabrics and wallpapers which are commonly used in lining and noise absorption coefficient of lining curtain. Seven types of fiber materials for the interior decoration, one non-woven fabric for the wallpaper linings, and two types of textiles for curtain linings are used as the experimental materials in this study. The noise absorption coefficient of the noise absorbents were measured by using impedance tube. And the thermal transmittance were measured by using thermal transmittance tester. The results of this study are as follows; Observing the noise absorption efficiency of each experimental materials, the combination of fiber materials and linings, the noise absorption efficiency of cotton, polyester and silk were similar and for the experimental materials of flax, rayon, acrylic and nylon were resulted the similar noise absorption efficiency. The result of combination of fiber material and black fabric was highest among the combined linings. For the combination of fiber material and non-woven fabric, double layers of non-woven fabric resulted slightly higher noise absorption coefficient result than single layer of non-woven fabric. The thermal transmittance and the sound absorbents of experimental materials were affected by the thickness, density and layer of air of the experimental materials.

Fabrication and Sound Absorption Properties of Porous Fe<sub>0.2</sub>(Co<sub>20</sub>Ni<sub>80</sub>)<sub>0.8</sub> Alloy Microfibers

The porous nanocrystalline Fe0.2(Co20Ni80)0.8 alloy microfibers with diameters of 2-4 μm have been prepared by the citrate-gel and phase transformation process. The sound absorption coefficient for microfibers samples is measured by the standing wave tube method and it is is over 0.8 for the 15 mm thick sample at the frequency range of 2300-6000 Hz, which is extended to 600-6300 Hz for the 40 mm thick sample. The band width with the sound absorption coefficient above 0.6 is wider than 4300 Hz for the 15 mm thick sample and 5800 Hz for the 40 mm thick sample. For the 40 mm thick sample, the maximum absorption coefficient, noise absorption coefficient, noise reduction coefficient and half-width of the absorption peak are 0.99, 0.59, 0.64 and 5828 Hz, respectively. These microfibers are promising advanced acoustic absorbers.

Automotive Noise Insulation Composite Panel Using Natural Fibres with Different Perforation Areas

This research was carried out to study the acoustic properties of natural organic fibres; kenaf and coir fibres using impedance tube method. Kenaf fibre was used as noise absorber filler in an insulation panel while the coir fibre as reinforcement in the perforated composite panel. The perforated panel was made from coir fibre/polyester composites with coir fibre volume fraction of 10%, 20% and 30%. The perforation area of the perforated panel was also varied at 10%, 20% and 30%. During the processing stage, the kenaf fibre sheet has been treated with PVA and cut into 100 mm and 30 mm diameter sample for low and high frequency test. The density of the coir fibre is determined to be 32.2 g/cm3 while the density of the kenaf fibre is 42.6 g/cm3. The tests were carried out using impedance tube at acoustic lab, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia using ISO 10534-2 standard for noise absorption coefficients to determine their noise absorption coefficient. The results obtained show that the optimum noise absorption coefficient index for kenaf fibre is 0.8 with 10% fibre volume fraction of coir fibre/polyester perforated composites panel at 10% perforation areas.

Perforated Plate Backing with Coconut Fiber as Sound Absorber for Low and Mid Range Frequencies

A material’s sound absorbing properties can be described as a sound absorption coefficient in a particular frequency range. Porous absorbers are the most commonly used sound absorbing materials, however they do not have good absorption properties at low and mid frequencies. In order to increase the absorption properties at low and mid frequencies, porous absorber should have larger thickness and size. By using perforated plates backing with porous absorber, it is possible to gain higher noise absorption at low and mid frequencies. This study was conducted to investigate the effect of perforation size ratio, mass, air gap and holes diameter of perforated plates as backing plate for porous absorber. Coconut fibres were used as backing medium for perforated plates. Perforated plates were fabricated using two different materials; aluminium and zinc plates with perforation size ratio of 0.10, 0.15 and 0.22 at thickness of 1 mm. The holes diameters of the perforated plates are 2 mm, 3 mm, and 4 mm, respectively. The tests samples were tested at the acoustic lab of the Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia according to ASTM E 1050-98 international standards for noise absorption coefficient. The data presented the perforated plates backing with coconut fibers have perforation ratio less to 0.22 showed the significant absorption at 800 Hz – 1800 Hz frequency range. The perforated plates backing with coconut fibers that their holes diameters are 4 mm also showed a good absorption at 600 Hz – 1800 Hz frequency range. However, perforated plates backing with coconut fibers showed a decreasing absorption coefficient at frequencies more than 1800 Hz.

Sound Absorption Behavior of Knitted Spacer Fabrics

This paper presents an experimental investigation on the sound absorption behavior of knitted spacer fabrics. Both weft-knitted and warp-knitted fabrics were used in the study due to their different structure features. The weft-knitted spacer fabric used was composed of two varied plain knitted outer layers and one textured polyester multifilament spacer layer, and it was considered as a porous sound absorber. The warp-knitted spacer fabric used was made with mesh structure on the outer layers and monofilament yarn in the spacer layer, and it was considered as microperforated panel sound absorber. The noise absorption coefficients (NACs) of these two kinds of fabrics under both single and multilayer forms as well as their combinations were tested using a two-microphone impedance measurement tube. The results show that the fabric surface structure and thickness, spacer yarn type and their connecting ways, fabric combinations and their arrangement methods have significant effects on the sound absorbability. The results also demonstrate that good sound absorbability could be achieved by using knitted spacer fabrics if suitable fabric structures and combinations are used.

Noise Control Using Coconut Coir Fiber Sound Absorber with Porous Layer Backing and Perforated Panel

Problem statement: Noise control was one of the major requirements to improve the living environment. One of the methods to do that is provided by sound absorber. Commonly, multi-layer sound absorbers are applied to absorb broadband noise that was composed of perforated plates, air space and porous material. However, multi-layers sound absorbers effectiveness depends on their construction. This study was conducted to investigate the potential of using coconut coir fiber as sound absorber. The effects of porous layer backing and perforated plate on sound absorption coefficient of sound absorber using coconut coir fiber were studied. Approach: Car boot liners made from woven cotton cloth were used as type of porous layer in the study. This material has been used widely in automotive industry. Perforated plate used was machined with perforation ratio of 0.20, thickness of 1 mm and holed diameter of 2 mm. The samples were tested at the acoustic lab of the Faculty of Engineering and Built Environment, University Kebangsaan Malaysia, according to ASTM E 1050-98 international standards for noise absorption coefficient. Results: The experiment data indicates that porous layer backing can improve noise absorption coefficient at low and high frequencies with significant increasing. 20 mm thick layer coconut coir fiber with porous layer backing exhibit peak value at frequencies between 2750-2825 Hz with maximum value of 0.97. The experimental results also found that the coconut coir fiber with perforated plate gives higher value for lower frequencies range from 600- 2400 Hz. The optimum value for coconut coir fiber with perforated panel is around 0.94-0.95 for the frequency range 2600-2700 Hz. Conclusion: Noise absorption coefficient of coconut coir fiber was increased at all frequency when they were backing with Woven Cotton Cloth (WCC). At low frequency, the NAC have significant increasing. This is because WCC have higher flow resistivity than coconut coir fibers, so that sound can be dissipated as it travels through material significantly. The results from the experimental tests show that it has good acoustic properties at low and high frequencies and can used to be an alternative replacement of synthetic based commercial product. By using the porous layer and perforated plate backing to coconut coir fiber, the sound absorber panel shows a good potential to be an environmentally friendly product. This innovative sound absorption panel has a bright future because they are cheaper, lighter and environmentally compare to glass fiber and mineral based synthetic materials.

Acoustical evaluation of carbonized and activated cotton nonwovens

An activated carbon fiber nonwoven (ACF) was manufactured from a cotton nonwoven fabric. For the ACF acoustic application, a nonwoven composite of ACF with cotton nonwoven as a base layer was developed. Also produced were the composites of the cotton nonwoven base layer with a layer of glassfiber nonwoven, and the cotton nonwoven base layer with a layer of cotton fiber nonwoven. Their noise absorption coefficients and sound transmission loss were measured using the Brüel and Kjær impedance tube instrument. Statistical significance of the differences between the composites was tested using the method of Duncan’s grouping. The study concluded that the ACF composite exhibited a greater ability to absorb normal incidence sound waves than the composites with either glassfiber or cotton fiber. The analysis of sound transmission loss revealed that the three composites still obeyed the mass law of transmission loss. The composite with the surface layer of cotton fiber nonwoven possessed a higher fabric density and therefore showed a better sound insulation than the composites with glassfiber and ACF.

Carbonized and Activated Non-wovens as High-Performance Acoustic Materials: Part I Noise Absorption

Noise absorption is one method for noise reduction in engineering. The material acoustic property for noise absorption is principally based on the efficiency of material structures for damping sound wave reflections. In this paper, three non-woven composites with activated carbon fiber (rayon precursor) non-woven as a surface layer and cotton, ramie, and polypropylene fiber non-wovens as base layers were produced. Their noise absorption coefficients were measured using the Brüel and Kjær impedance tube instrument and compared to those of three glass-fiber-surfaced non-woven composites. Statistical significance of the differences between the activated carbon fiber composites and glass-fiber-surfaced composites was tested using the method of Duncan's grouping. The study concluded that the activated carbon fiber composites exhibited an exceptional ability to absorb normal incidence sound waves. The analysis also revealed that the activated carbon fiber composite with a cotton base layer would be 4.6 times lighter in weight, 14% higher in low-frequency absorption, and 7% higher in high-frequency absorption, in comparison with the composite having a glass fiber surface layer and a polypropylene fiber base layer.

Rapid thermal annealing effects on the optical properties in strained CdTe (100)/GaAs (100) heterostructures

The growth of CdTe thin films has attracted much interest because of their potential applications in the areas of solar energy conversion, gamma-ray detection, and electro-optic modulation due to their low thermal noise and large absorption coefficient [1–5]. CdTe epitaxial layers have been extensively grown because CdTe thin films can be useful buffer layers for the growth of Hg1−x Cdx Te epilayers [6–9]. However, since the growth of high-quality CdTe/GaAs heterostructures has inherent problems due to the large lattice mismatch ( a/a = 14.6% at 25 ◦C), studies of the structural and optical properties of CdTe/GaAs heterostructures are very important for achieving high-quality optoelectronic devices that can operate in the blue-green region of the spectrum [10]. In addition, studies of the effects of thermal annealing on the optical properties play a very important role in enhancing device efficiency, and systematic studies concerning rapid thermal annealing effects on the optical properties of the CdTe/GaAs heterostructures are still necessary if highquality heterostructures are to be obtained after thermal annealing [11]. This letter reports the dependence of the optical properties on the annealing treatment for CdTe thin films grown on GaAs (100) substrates by using molecular beam epitaxy. Photoluminescence (PL) measurements were carried out to investigate the optical properties and to determine the activation energy of the confined electrons existing at the donor state in the as-grown CdTe/GaAs heterostructures. Elemental Cd and Te with purities of 99.9999% were used as the source materials and were precleaned by repeated sublimation. Cr-doped and semi-insulating (100) GaAs substrates were degreased in warm trichloroethylene (TCE), rinsed thoroughly in deionized water, etched in a HF solution, and rinsed in TCE again. As soon as the chemical cleaning process was finished, the GaAs substrates were mounted onto a molybdenum susceptor. Prior to CdTe thin-film growth, the GaAs substrates were thermally cleaned at 600 ◦C for 5 min in situ in the growth chamber at a pressure of 10−10 Torr. The depositions of the CdTe epilayers were done on GaAs substrates by using the MBE technique

A reduced-scale railway noise barrier's insertion loss and absorption coefficients: comparison of field measurements and predictions

In situ testing determined the insertion loss ( IL) and absorption coefficients of a candidate absorptive noise barrier (soundwall) to abate railway noise for residents of Anaheim, CA. A 4000 m barrier is proposed south of the tracks, but residential areas to the north have expressed concerns that barrier reflections will increase their noise exposure. To address these concerns, a 3.66 m high by 14.6 m long demonstration barrier was built in the parking lot of Edison Field, Anaheim, as part of a public open house, thereby allowing for acoustical measurements. Insertion loss ( IL) was measured in third-octave bands assuming 1/2-scale construction. The IL for three, scaled railway noise sub-sources (rail/wheel interface, locomotive, and train horn) was measured at six, scaled distances. The highest total, A-weighted IL, after corrections for finite-barrier and point-source speaker effects was 22 dB(A) for rail/wheel noise, 18 dB(A) for locomotive noise, and 20 dB(A) for train horn noise. These results can be compared favourably to IL predictions made using algorithms from the US Federal Rail Administration (FRA) noise assessment guidelines. For the actual barrier installation, shielded residential receivers located south of the project are expected to see their future noise exposures reduced from an unmitigated 78 CNEL to 65 CNEL. Absorption coefficients were measured using time delay spectrometry. At lower frequencies, measured absorption coefficients were notably less than the reverberation room results advertised in the manufacturer's literature, but generally conformed with impedance tube results. At higher frequencies the correspondence between measured absorption coefficients and reverberation room results was much improved. For the actual barrier installation, unshielded residential receivers to the north are expected to experience noise exposure increases of less than 1 dB(A). This factor of increase is consistent with a finding of no impact when assessed using FRA guidelines for allowable increases of noise exposure.

Use of Nonwovens of Variable Porosity as Noise Control Elements

The optimal design of multilayer combinations of nonwoven fiberwebs is of prime importance in their applications as noise control elements. Such combinations can be simulated as a single layer with variable characteristics along the propagation of the sound wave. The paper examines the effect of variation of porosity on the sound absorption coefficients of three webs made of cotton, acrylic fibers and polyester. The numerical procedure is based upon our generalization of the C. Zwikker and C.W. Kosten theory on sound transfer through porous media reported elsewhere. We have considered four functional forms of the porosity: linear, quadratic, exponential and logarithmic. Our results clearly indicate that for all materials considered the logarithmic form yields the highest noise absorption coefficients for f=1000 Hz.

Development of recycled wiper blade rubber for vibration isolator and noise absorbing materials.

Rubber is a useful material but used rubber has disposal problems and continues to accumulate all over the world. In particular, wiper blade rubber is high quality natural rubber. We investigated the use of recycled wiper blade rubber for vibration isolator and noise absorbing materials. Wiper blade rubbers were cut or crushed to small sizes and molded. Dynamic and static properties as well as the noise absorption coefficient of recycled materials were measured. We found which cut or crushed size is better for vibration isolator and noise absorbing materials. We developed a method of wiper blade rubber recycling.

A Model for Calculating the Noise Absorption Capacity of Nonwoven Fiber Webs

Nonwovens can be used as noise control elements for a wide range of applications. This paper suggests a numerical method for calculating their acoustic performance using the theory of Zwikker and Kosten on sound propagation through porous media. The results for nonwovens made from acrylic fibers, cotton, and polyester are in fair agreement with experimental findings. The model yields the noise absorption coefficients of fiber webs as a function of their thickness and porosity. This relationship might serve as a guideline for the optimal design of passive control components made of nonwovens.

Effect of Pile Parameters on the Noise Absorption Capacity of Tufted Carpets

This paper examines the effect of pile characteristics on the noise absorption capacity of tufted carpets in the audible frequency range (125 < f < 4000 Hz.) The parameters considered are fiber content and fiber denier, pile density, and average pile height. The noise absorption coefficients of sample carpets differing from each other in only one pile parameter were measured using the impedance tube technique. Results of the experiments are displayed and evaluated in terms of a heuristic physical picture based on the theory of sound transfer through porous media. One of the interesting results is that when there is an air gap behind tufted carpet used as a wall covering, noise absorption capacity increases significantly in the low and medium frequency range, 250 < f < 1000 Hz.