Non-acoustical Properties Research Articles

Prediction of sound absorption performance of fibers coming from cigarette butts using a phenomenological model

This work deals with experimental and theoretical research about cellulose acetate fibers coming from cigarette butts. This waste, considered one of the most hazardous, numerous, and polluting waste all over the world, represents a high environmental risk. The possibility of using these fibers in sound-absorbing panels is evaluated. The acoustical properties are measured according to ISO 10534-2, and predicted using a phenomenological model over the frequency range from 100 Hz to 6400 Hz. This work includes the study of important non-acoustical properties for a better understanding of the porous structure. The Johnson-Champoux-Allard model provides good accuracy. The experimental results of the sound absorption spectrums show mean errors ranging from 1.1% to 9.0%. Moreover, it is possible to obtain non-acoustic properties of cellulose acetate fibers, which are difficult to obtain experimentally, using the Johnson-Champoux-Allard model and an inverse technique. The input parameters of this model are successfully obtained with low errors concerning the measurements (0.6–5.2% for porosity or 2.6–10.5% for flow resistivity). The results of the sound absorption spectrum at normal incidence of the cellulose acetate show very accurate predictions. The experimental results show values of sound-absorption coefficients close to 1.0 and sound absorption average ranging from 0.42 to 0.8. Therefore, these waste fibers are an alternative to the traditional porous absorbers, mainly composed of synthetic fibers or foams and petroleum-based resins.

Vrednovanje neakustičnih svojstava zidova za zaštitu od prometne buke

The presented research aims to reduce the uncertainty in the selection of noise wall panel materials, and thus to improve the traffic noise management process in the Republic of Croatia. An evaluation of concrete, metal, wood, and transparent panels is performed to improve the panel material selection procedure in the planning and design of traffic noise walls. Properties of panels made of these four basic materials are studied under the assumption that the acoustic requirements relating to the wall structure have been fully met. Thus, the research focuses on six non-acoustic properties of panels (criteria that must be considered by civil engineers): mechanical resistance and stability, service life, construction, maintenance, and replacement costs, service life costs, and recyclability. These properties are analysed using the publicly available data from secondary sources. The data are systematized and subjected to a multicriteria analysis, which has revealed that concrete panels have the highest (weighted and unweighted) mean score.

Acoustic Characterization of Some Steel Industry Waste Materials

From a circular economy perspective, the acoustic characterization of steelwork by-products is a topic worth investigating, especially because little or no literature can be found on this subject. The possibility to reuse and add value to a large amount of this kind of waste material can lead to significant economic and environmental benefits. Once properly analyzed and optimized, these by-products can become a valuable alternative to conventional materials for noise control applications. The main acoustic properties of these materials can be investigated by means of a four-microphone impedance tube. Through an inverse technique, it is then possible to derive some non-acoustic properties of interest, useful to physically characterize the structure of the materials. The inverse method adopted in this paper is founded on the Johnson–Champoux–Allard model and uses a standard minimization procedure based on the difference between the sound absorption coefficients obtained experimentally and predicted by the Johnson–Champoux–Allard model. The results obtained are consistent with other literature data for similar materials. The knowledge of the physical parameters retrieved applying this technique (porosity, airflow resistivity, tortuosity, viscous and thermal characteristic length) is fundamental for the acoustic optimization of the porous materials in the case of future applications.

Ultralight graphene oxide/polyvinyl alcohol aerogel for broadband and tuneable acoustic properties

An ultralight graphene oxide (GO)/polyvinyl alcohol (PVA) aerogel (GPA) is proposed as a new class of acoustic materials with tuneable and broadband sound absorption and sound transmission losses. The interaction between GO sheets and PVA molecules is exploited in our environmentally friendly manufacturing process to fabricate aerogels with hierarchical and tuneable porosity embedded in a honeycomb scaffold. The aerogels possess an enhanced ability to dissipate sound energy, with an extremely low density of 2.10 kg m−3, one of the lowest values ever reported for acoustic materials. We have first experimentally evaluated and optimised the effects of composition and thickness on the acoustic properties, namely sound absorption and sound transmission losses. Subsequently, we have employed a semi-analytical approach to evaluate the effect of different processing times on acoustic properties and assessed the relationships between the acoustic and non-acoustic properties of the materials. Over the 400–2500 Hz range, the reported average sound absorption coefficients are as high as 0.79, while the average sound transmission losses can reach 15.8 dB. We envisage that our subwavelength thin and light aerogel-based materials will possess other functional properties such as fire resistance and EMI shielding, and will prove to be novel acoustic materials for advanced engineering applications.

Characterizing and modelling the sound absorption of the cellulose acetate fibers coming from cigarette butts.

This work deals an experimental and theoretical research about the porous structure and the acoustic performance of cellulose acetate coming from the used cigarette filters. The porous structure was studied through the measurement and the prediction (using an inverse methodology) of some non-acoustic properties (bulk density, fiber size, porosity or flow resistivity and tortuosity). The sound absorption performance was evaluated by comparing experimental measurements using an impedance tube according to ISO 10534-2 with the best-fit approach obtained using some empirical models (Delany and Bazley, Garai-Pompoli, Komatsu and Miki). The accuracy of empirical models to obtain the flow resistivity was relatively high showing differences lower than 8% in the case of the Miki model or lower than 12% in the case of Delany & Bazley model. Regarding the absorption spectra, the errors found were lower than 9% for all the empirical models used in this work. These results showed that using relatively simple models such as empirical models were able to predict accurately the acoustic behaviour of the cellulose acetate. Finally, the sound absorption spectra obtained for cellulose acetate were compared with those obtained for fibrous materials currently used in building sector, suggesting that this fibrous waste could act as a possible substitute to traditional ones, due to the similar relatively high NRC values obtained for both type of fibrous absorber (NRC = 0.65).

The recycling of surgical face masks as sound porous absorbers: Preliminary evaluation

This paper presents the results of an experimental study on the acoustic efficiency of plastic surgical face masks. Since the very high number of disposable masks being used globally on a daily basis to face the Covid19 pandemic is posing new environmental risks, mainly connected to improper disposal, any possible improvements in the management of this waste stream is very important. In this work their potential use as sound porous absorber is discussed. Surgical face masks are mainly made of polypropylene fibers which show good acoustical properties. Their porous structure was studied through the measurement of some non-acoustic properties: bulk density, fiber diameter, porosity, flow resistivity and tortuosity. Moreover, the sound absorption performance of samples, made of scrapped face masks, with different thicknesses was evaluated using an impedance tube according to ISO 10534-2. The results obtained from the sound absorption spectra and two single indexes, Noise Reduction Coefficient and Sound Absorption Average showed a high sound absorption value over a frequency range of interest. Finally, the sound absorption spectra obtained for surgical face masks were compared with those obtained for fibrous materials currently used in building sector, suggesting that this fibrous waste could act as a possible substitute to traditional ones.

On the relationship of the observed acoustical and related non-acoustical behaviours of nanofibers membranes using Biot- and Darcy-type models

There is a general lack of publications on the acoustical and related non-acoustical properties of nanofibrous media. This work attempts to contribute to this gap and to highlight problems associated with acoustic and related non-acoustic characterisation of these materials. The work, presumably for the first time, applies Biot- and Darcy-type mathematical models to explain the observed acoustical and related non-acoustical behaviours of the nanofibres. It identifies theoretical gaps related to the physical phenomena which can be responsible for the observed acoustical behaviours of nanofibrous membranes and it presents recommendations to fill these gaps. The novelty of this work is in the use of a robust theoretical model to explain the measured acoustical behaviour of thin nanofibrous membranes placed on a foam substrate. With this model the actual flow resistivity of nanofibers is estimated from acoustical data. It is demonstrated that a classical model for the flow resistivity of fibrous media does not work when the Knudsen number becomes greater than 0.02, i.e. then the diameter of nanofibres becomes comparable with the mean free path.

Experimental study of a noise reducing barrier made of fly ash

Although fly ash is commonly used as an additive to cement, large amounts of this material are disposed in landfills. To mitigate, it would be interesting to develop new products in which fly ash can be easily used and required in large quantities. In this work, fly ash is added to manufacture eco-friendly materials with acceptable acoustic and non-acoustic properties and a low cost. We built a barrier composed of fly ash (60 wt.%), type II Portland cement (25 wt.%), vermiculite (14.5 wt.%) and polypropylene fibers (0.5 wt.%). The barrier complied with the mechanical requirements of European standards. The sound absorption coefficient and the airborne sound insulation were determined in a reverberation room, and the barrier was classified as A2 and B3. No leaching problems were observed.

Wool waste used as sustainable nonwoven for building applications

Reusing textile waste in building applications has the potential to reduce the environmental impact of two sectors considered the main sources of environmental pollution: the textile and the construction industries. Thus, the main goal of the present research study is to assess the potential conversion of wool waste into new raw materials suitable for building components. Hence, hygrothermal, acoustic and non-acoustic properties of nonwovens consisting of 100% wool waste fibers thermally bonded with polyester/copolyester bi-component fibers were explored. Five different density values (51, 90, 115, 136 and 167 kg/m3) were examined. Absorption coefficients ranging from 0.7 to almost 1 were measured above 1 kHz using 50 mm thick samples; thermal conductivity values from 0.044 to 0.057 W/(m·K) were obtained and a water vapour permeability close to 2·10−11 kg/(m·s·Pa) was found. Furthermore, a comparison between nonwovens under test and other previously experimented materials was carried out. Measurement results showed that the manufacturing processes mainly affected the sound absorption coefficients and the hygric properties of the fibrous nonwovens. Comparison between tested materials and those currently available on the market allows to state that the tested nonwovens may represent a valid alternative for building applications, thus opening a new research area.

Composite Eco-Friendly Sound Absorbing Materials Made of Recycled Textile Waste and Biopolymers.

In recent years, the interest in reusing recycled fibers as building materials has been growing as a consequence of their ability to reduce the production of waste and the use of virgin resources, taking advantage of the potential that fibrous materials may offer to improve thermal and acoustic comfort. Composite panels, made of 100% wool waste fibers and bound by means of either a chitosan solution and a gum Arabic solution, were tested and characterized in terms of acoustic and non-acoustic properties. Samples with a 5 cm thickness and different density values were made to investigate the influence of flow resistivity on the final performance. Experimental results demonstrated that the samples had thermal conductivity ranging between 0.049 and 0.060 W/(m K), well comparable to conventional building materials. Similarly, acoustic results were very promising, showing absorption coefficients that, for the given thickness, were generally higher than 0.5 from 500 Hz on, and higher than 0.9 from 1 kHz on. Finally, the effects of the non-acoustic properties and of the air gap behind the samples on the acoustic behavior were also analyzed, proving that the agreement with absorption values predicted by empirical models was also very good.

A three-parameter analytical model for the acoustical properties of porous media.

Many models for the prediction of the acoustical properties of porous media require non-acoustical parameters few of which are directly measurable. One popular prediction model by Johnson, Champoux, Allard, and Lafarge [J. Appl. Phys. 70(4), 1975-1979 (1991)] (459 citations, Scopus, April 2019) requires six non-acoustical parameters. This paper proves that the use of more than three parameters in the Johnson-Champoux-Allard-Lafarge model is not necessary at all. Here the authors present theoretical and experimental evidence that the acoustical impedance of a range of porous media with pore size distribution close to log-normal (granular, fibrous, and foams) can be predicted through the knowledge of the porosity, median pore size, and standard deviation in the pore size only. A unique feature of this paper is that it effectively halves the number of parameters required to predict the acoustical properties of porous media very accurately. The significance of this paper is that it proposes an unambiguous relationship between the pore microstructure and key acoustical properties of porous media with log-normal pore size distribution. This unique model is well suited for using acoustical data for measuring and inverting key non-acoustical properties of a wider range of porous media used in a range of applications which are not necessarily acoustic.

Are nano-fibers an emerging noise control solution?

This paper presents experimental evidence that laboratory measurements of the acoustical and related non-acoustical data is not a trivial task. This paper also shows that the prediction of the acoustical properties of this seemingly simple material is far from easy. First, there is a problem of characterising the non-acoustical properties of nano-porous membrane with standard laboratory methods. Second, there is a high uncertainty in the value of the membrane thickness which can lead to a substantial change in its predicted acoustical and related non-acoustical properties. Thirdly, there is an argument whether or not sound propagation in a nano-porous membrane can actually be treated as that in a classical porous media. The paper attempts to respond to these questions and to present new evidence that we may not know well yet the true acoustical properties of these fibres. Therefore, the key question is Are nano-fibres still an emerging noise control solution?

Improved acoustic damping in polyurethane foams by the inclusion of silicon dioxide nanoparticles

AbstractIn this research, flexible semi‐open cell polyurethane (PU) foams incorporating silicon dioxide (SiO2) nanoparticles are produced to have improved acoustic damping. Nanoparticles of different loading fractions and various sizes are employed to achieve improvement in sound absorption of foams. The effect of SiO2 nanoparticles’ size on the acoustic damping of PU foams, which is investigated in this study, has not been studied before. The mechanical, nonacoustical, and acoustical properties of the foams are measured by experiments or calculated by theoretical models. The results show that a PU foam sample incorporating only 0.05 wt% of nanoparticles has a great improvement (more than 90%) in sound absorption ability over the frequency range of 500–1,000 Hz. By the increase in the loading fraction of nanoparticles, a gradual decrease in acoustic damping is observed. Moreover, regarding size of nanoparticles, those with bigger nanometer sizes bring about a better sound absorption ability in PU foams. Besides acoustical properties, microcellular characteristics, nonacoustic parameters, and mechanical properties are also presented which are in agreement with the improved acoustical ability of foams quite well.

Acoustic performances of silicone foams for sound absorption

The aim of this study is to investigate the link between the elaboration process, the microstructure and the acoustic behaviour of silicone foams obtained using a two-component silicone. Different parameters such as the ratio of components, the addition of a thinning agent and the curing temperature are varied, with the objective of understanding the influence of each parameter in the foam’s acoustic absorption. The microstructure is analysed using scanning electron microscopy and acoustic properties are measured. Two non-acoustical properties of the porous material are also investigated, namely the porosity and the flow resistivity. Pore cell size and interconnected porosity have great impact on acoustical properties. Significant enhancements of the absorption properties could be obtained in the low-frequency band by increasing the rate of agent B through an increase in the amount of interconnected porous cells. An improvement in absorption is observed in the higher frequency range when a thinning agent is added to the mixture. Representative models of the foam for acoustic simulations are obtained allowing estimation of the tortuosity, viscous and thermal characteristic length from acoustic measurements. These models are able to simulate the acoustic behaviour of the silicone foams when embedded in sound packages.

Corn Husk Fiber-Polyester Composites as Sound Absorber: Nonacoustical and Acoustical Properties

This study investigates the acoustical and nonacoustical properties of composites using corn husk fiber (CHF) and unsaturated polyester as the sound-absorbing materials. The influence of the volume fraction of CHF on acoustic performance was experimentally investigated. In addition, the nonacoustical properties, such as air-flow resistivity, porosity, and mechanical properties of composites have been analyzed. The results show that the sound absorptions at low frequencies are determined by the number of lumens in fiber, particularly the absorption coefficient, which increases the amount of fiber. For high-frequency sound, the absorption coefficient is determined by the arrangement of fibers in the composite. An absorption coefficient is close to zero when the fibers are arranged in a conventional pattern; however, when they are arranged in a random pattern, a high absorption coefficient can be obtained. The bond interface between the fiber and resin enhances its mechanical properties, which increases the longevity of the composite panel.

Toward mechanistic understanding of the relationship between the sound absorption and the natural and resonant frequencies of porous media.

In this paper, the natural and resonant frequencies of porous media are studied based on Biot's equations. The governing equations of porous media are analytically solved by using the recent developed potential function method. Based on the obtained results, the natural and resonant frequencies of the porous medium can be investigated. In this research, several foams with different acoustical and non-acoustical properties are considered and the natural and resonant frequencies are studied. In addition, for a better understanding of the natural and variation of resonant frequencies of the considered foams, various damping gains are defined and by changing them gradually, the variations of absorption coefficient and field variables are studied. The results show that the trends of absorption coefficient versus frequency for porous media can be predicted by considering the arrangement of structural and fluid natural frequencies. Also, around the structural natural frequencies two types of variations in absorption coefficient occur (i.e., maximum-minimum or maximum variations). Additionally, after computing the corresponding results of rigid frame and Biot's models it can be seen that the rigid frame theory cannot correctly predict the sound absorption coefficient in the vicinity of structural natural frequencies.

Acoustic modeling of perforated concrete using the dual porosity theory

Perforated concrete shows nowadays a high potential for many construction and building engineering applications. This work is devoted to the analysis of the acoustic properties of perforated concrete made from arlite lightweight aggregates. Concrete produced from these materials is an environmentally friendly alternative to traditional materials and offers a higher durability, excellent strength-to-weight ratio and low cost. In particular, it is shown that the acoustic behavior of perforated concrete can be modeled using a dual porosity approach based on the knowledge of the non-acoustic properties of the matrix granular material and geometrical data. To this end, various non-perforated and perforated samples were prepared and characterized in an experimental test facility, their acoustic properties being determined through the transfer function impedance tube method. Experimental and estimated results related to the acoustic properties of a number of prepared specimens are presented, showing a good agreement. Results suggest that this approach is suitable for practical design of such materials as part of noise control systems.

Influence of the Microstructure in the Acoustical Performance of Consolidated Lightweight Granular Materials

This paper reports an investigation of the acoustical performance of consolidated lightweight granular materials, made of perlite, arlite and vermiculite mixed with polyurethane resin. These materials could help combat the noise control and replace some products that lack the sufficient structural strength and require expensive protection when exposed to the elements. The advantages of using these composites in acoustic applications, like noise barriers, compared with commercial materials, are their very light mass combined with a relatively high structural strength, high physical–chemical stability and low cost. Besides, these materials could be considered more acceptable, from the health point of view, and better suited to operate in an aggressive environment. Acoustical properties were assessed according to ISO 10534–2: 1998. The microstructure, through non-acoustical properties (porosity, tortuosity and flow resistivity), and the acoustical performance (sound absorption spectrum and normalized acoustic impedance) were studied, showing the influence of the non-acoustical properties in the acoustical performance.

Effects of Silicone Surfactant on the Properties of Open‐Cell Flexible Polyurethane Foams

ABSTRACTIn this paper, the effects of silicone surfactant on the mechanical, thermal, and acoustical properties of flexible polyurethane foams are studied. At first, by introducing the reactions of the foam production and expressing the stoichiometric relationships, a method is provided to select the amounts of the basic compounds for getting the desirable properties. By knowing the OH number of polyol and the NCO content of isocyanate after selecting an amount of a blowing agent for the reaction, the quantity of each compound to produce the desirable foam can be found. Then different foams with various amounts of surfactant are produced. By preparing proper samples, the mechanical, thermal, nonacoustical, and acoustical properties of the foams are measured or calculated by using different instruments. The nonacoustical properties are obtained based on two distinct methods, i.e., a semiphenomenological method and an indirect method. The obtained properties are validated by comparing the calculated theoretical sound absorption coefficients with the measured ones. The results show that with a small increase of surfactant, the acoustical properties of foam samples improve by about 50% without any significant changes in other mechanical and thermal properties of the foams. This improvement results from the variations of reticulation rate and cell size. In addition, the initial desirable porosity and density considered in the stoichiometric relations are fairly achieved in produced samples. These results validate the accuracy of the stoichiometric relations.

Acoustical characterization of nano-porous carbons

The acoustical and related non-acoustical properties of activated carbon were studied to understand better the effect of added catalyst and acid treatment on the nano- and micro-structure of activated, porous nano-carbon. The acoustical impedance was measured in a 45 mm diameter impedance tube with a special adaptor to accommodate a very small material quantity available for this experiment. The presence of the adaptor was compensated using the procedure detailed in Dupont et al. [POMA 19, 065008 (2013); http://dx.doi.org/10.1121/1.4799701]. The acoustic impedance of activated carbon was predicted using the model proposed by Venegas [Section 6.1, Ph.D. thesis, University of Salford, 2011]. The micro- and nano-scale porosities and pore sizes were determined by fitting the model to the acoustic impedance data in the frequency range between 50 and 1000 Hz. It is shown that the acid treatment and addition of catalyst result in the reduced radius of nano-pores and reduced nano-porosity. These effects are small but measurable acoustically. These results are consistent with the results of the BET experiment. This work provides the foundation for the development of acoustical methods for nano-porous material characterization which are rapid and non-invasive.